Talk:Oxoglutarate dehydrogenase complex

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what does this mean? "A high energy charge in the cell will also be inhibitive." —Preceding unsigned comment added by 71.173.145.119 (talk) 07:25, 21 January 2008 (UTC)

The following is a dump of some text that I think could be very useful in cleaning up this article and the article about the pyruvate dehydrogenase multi-enzyme complex... It is dumped here for reference, and is for my colleagues only...

Crystal Structure of the Truncated Cubic Core Component of the Escherichia coli 2-Oxoglutarate Dehydrogenase Multienzyme Complex

James E. Knapp, David T. Mitchell, Mohammad A. Yazdi, Stephen R. Ernst, Lester J. Reed and Marvin L. Hackert*

Department of Chemistry and Biochemistry, The University of Texas at Austin, Austin TX 78712, USA

The dihydrolipoamide succinyltransferase (E2o) component of the 2-oxo- glutarate dehydrogenase multienzyme complex is composed of 24 sub- units arranged with 432 point group symmetry. The catalytic domain (CD) of the E2o component catalyzes the transfer of a succinyl group from the S-succinyldihydrolipoyl moiety to coenzyme A. The crystal structure of the Escherichia coli E2oCD has been solved to 3.0 AE^ resolution using molecular replacement phases derived from the structure of the catalytic domain from the Azotobacter vinelandii dihydrolipoamide acetyl-

transferase (E2pCD). The reo""ned model of the E. coli E2oCD consists of residues 172 to 404 and has an R-factor of 0.205 (Rfree L' 0.249) for 9696 rer'ections between 20.0 and 3.0 AE^ resolution. Although both E2oCD and E2pCD form 24mers, subtle changes in the orientations of two helices in E2oCD increase the stability of the E2oCD 24mer in comparison to the less stable A. vinelandii E2pCD 24mer. Like E2pCD and chloramphenicol acetyltransferase (CAT), the active site of E2oCD is located in the middle of a channel formed at the interface between two 3-fold related subunits. Two of the active-site residues (His375 and Thr323) have a similar orien- tation to their counterparts in E2pCD and CAT. A third catalytic residue (Asp379) assumes a conformation similar to the corresponding residue in E2pCD (Asn614), but different from its counterpart in CAT (Asp199). Binding of the substrates to E2oCD is proposed to induce a change in the conformation of Asp379, allowing this residue to form a salt bridge with Arg184 that is analogous to that formed between Asp199 and Arg18 in CAT. Computer models of the active site of E2o complexed with dihy- drolipoamide and with coenzyme A led to the identio""cation of the prob- able succinyl-binding pocket. The residues which form this pocket (Ser330, Ser333, and His348) are probably responsible for E2o's substrate specio""city.

1. 1998 Academic Press

Keywords: dihydrolipoamide succinyltransferase; crystal structure; X-ray crystallography; multienzyme complexes*Corresponding author

Introduction

2-Oxo acid dehydrogenase multienzyme systems have been isolated as functional units with molecu- lar weights in the millions (Reed & Hackert, 1990; Patel & Roche, 1990; Perham, 1991). Three classes of these multienzyme complexes have been charac- terized, one specio""c for pyruvate (PDH complex), a second specio""c for 2-oxoglutarate (OGDH com- plex), and a third specio""c for branched-chaina

-keto acids (BCKDH complex). Each complex is

Present address: D. T. Mitchell, Department of Biochemistry, 4-225 Millard Hall, University of Minnesota, 435 Delaware Street SE, Minneapolis, MN 55455-0326, USA.

Abbreviations used: CD, catalytic domain; CAT, chloramphenicol acetyltransferase; RMS, root-mean- square; GST, glutathione-S-transferase.

E-mail address of the corresponding author: m.hackert@mail.utexas.edu

Article No. mb981924 J. Mol. Biol. (1998) 280, 655s'668 0022s'2836/98/290655s'14 $30.00/0 # 1998 Academic Press� composed of multiple copies of three enzymes: a substrate-specio""c decarboxylase-dehydrogenase (E1), a dihydrolipoamide acyltransferase (E2) specio""c for each type of complex, and dihydroli- poamide dehydrogenase (E3). The complexes are organized about a core consisting of the oligomeric E2 to which multiple copies of E1 and E3 are bound by non-covalent bonds. In mammalian and yeast PDH complexes, E3 is anchored to the E2 core by an E3-binding protein (Maeng et al., 1994). The E2 subunits have multidomain structures con- sisting of one, two, or three amino-terminal lipoyl domains, followed by an E1- and/or E3-binding domain, and then by a carboxyl-terminal catalytic domain (Reed & Hackert, 1990; Perham, 1991; Roche & Cox, 1996). The domains are linked to each other by r'exible segments.

An assemblage of catalytic domains comprises the inner core of E2. Two polyhedral forms, the cube and the pentagonal dodecahedron, have been shown by electron microscopy (Oliver & Reed, 1982) and X-ray crystallography (DeRosier et al., 1971; Fuller et al., 1979; Mattevi et al., 1992), to be the fundamental morphologies of the E2 cores. The cubic structure has 24 subunits and the pentagonal dodecahedron has 60 subunits, each grouped as trimers positioned at the 8 or 20 vertices of the two structures, respectively. The former organization, which exhibits octahedral symmetry (point group 432) is characteristic of the E2 components of the Escherichia coli PDH and OGDH complexes (E2p and E2o, respectively). The latter structure, which exhibits icosahedral symmetry (point group 532), is characteristic of the E2p cores from eukaryotes and some Gram-positive bacteria (Perham, 1991).

Attempts to crystallize the intact dihydrolipoa- mide acyltransferases (E2) have been unsuccessful, probably due to the presence of the r'exible linker segments. Nevertheless, much is known of the individual domain structures. NMR structures have been reported for the lipoyl domains of the E2p from Bacillus stearotheromophilus (Dardel et al., 1993) and E. coli (Green et al., 1995), the lipoyl domains of the E2o from Azotobacter vinelandii (Berg et al., 1996) and E. coli (Ricaud et al., 1996), and the E3-binding domain from the E. coli E2o (Robien et al., 1992) and B. stearothermophilus E2p (Kalia et al., 1993). Crystal structures of truncated cubic forms of E2 (tE2), lacking most or all of the N-terminal, peripheral domains, have been reported. These include low resolution (15 to 18 AE^ ) structures of truncated forms of dihydrolipoamide succinyltransferase (tE2o; DeRosier et al., 1971) and the acetyltransferase (tE2p; Fuller et al., 1979) from E. coli, and a high resolution (2.6 AE^ ) structure of a tE2p from A. vinelandii (Mattevi et al., 1992, 1993a). The tE2s used in the earlier investigations were pro- teolytically derived, whereas the tE2p used in the latter investigation was a recombinant protein.

This paper reports the crystal structure at 3.0 AE^ resolution of a truncated form of the cubic E. coli dihydrolipoamide succinyltransferase (tE2o) and comparisons of this structure with that of a trun-

cated form of the cubic A. vinelandii dihydrolipoa- mide acetyltransferase (tE2p). This structure provides additional insights into the organization, substrate specio""city, and catalytic mechanism of the acyltransferase component of a-keto acid dehy- drogenase multienzyme complexes.

Results

The reo""ned model of E. coli E2oCD consists of a total of 230 amino acid residues (residues 172 to 404 with surface loop residues Pro275, Arg276, and Arg307 omitted due to poor density), 16 water molecules and one sulfate ion. Due to the lack of observed side-chain electron density, the side- chains of two residues were truncated to glycine (Arg183 and Asp258), the side-chains of 11 resi- dues were truncated to alanine, and the side-chains of seven additional residues were truncated by one or more atoms by setting their occupancies to zero. The model consists of all residues which comprise the catalytic domain plus eight residues of the linker region. This model has been reo""ned to an R-factor of 0.205 and a Rfree of 0.249 for 9696 rer'ections between 20.0 and 3.0 AE^ resolution. The stereochemistry of the reo""ned model is excellent (Table 1) when compared to values derived from the Cambridge database of small molecule struc- tures (Engh & Huber, 1991). The Ramachandran plot indicates that 93.5% of the non-glycine, non- proline residues have dihedral angles within the most favored regions and the remaining residues have psi-phi angles which lie in the additionally allowed regions. Like the catalytic domain (CD) of the A. vinelandii E2p, the E. coli E2oCD contains a single cis peptide bond (Pro340) located at the third position of a type VIa b-turn. The confor- mation of this residue was verio""ed by o""tting the entire turn into a 2Fo ss Fc simulated annealing omit map.

Quaternary structure

Both in solution and in the crystalline state, E2oCD assembles into a 24-subunit oligomer with

Table 1. Reo""nement statistics for the model of E2oCD Resolution range (AE^ ) 20.0-3.0 Number of reflections (F ? 0.0) 9696 Number of non-hydrogen protein atoms 1722 Number of non-hydrogen water atoms 16 Number of non-hydrogen sulfate atoms 5 R-factor 0.205 Rfree 0.249 rms deviation from ideal values

Bond lengths (AE^ ) 0.010 Bond angles (ffi) 1.48 Dihedral angles (ffi) 24.7 Improper angles (ffi) 1.37 Mean B-factors (AE^ 2)

Main-chain atoms 28.6 Side-chain atoms 36.3 Water atoms 36.1 Sulfate atoms 65.0

656 The E2o Catalytic Domain� 432 point group symmetry (Figure 1). The 3-fold symmetry generates tightly associated trimers which are arranged at the corners of a cube. Neigh- boring trimers, which are related by 2-fold sym- metry, interact along each of the edges of the cube. This arrangement of the E2oCD creates a cube-like structure with edges varying between 110 and 138 AE^ in length. The lack of interactions around the 4-fold axis results in the creation of a large opening which leads to a cavity in the center of the cube. The openings to the central cavity have dimensions which vary between 24 and 43 AE^ in length.

The overall fold

The catalytic domain of the E. coli E2o (E2oCD) belongs to the mixed ab class of protein folds with a total of 79 residues forming o""ve a-helical seg- ments and 71 residues comprising ten b-strands (Table 2). The topology of the E2oCD (Figure 2) is similar to that of both CAT (Leslie et al., 1988; Leslie, 1990) and the A. vinelandii E2pCD (Mattevi et al., 1992, 1993a). All three acyltransferases fold into a two-layer, open-faced sandwich motif (Richardson, 1981) in which one layer is composed of a large, central b-sheet (F#E"G"I"J#B"H0#) and

the second, peripheral layer is formed from a small, antiparallel b-sheet (A0#D"C#) combined with the helical segments. The H0 b-strand of the central b-sheet refers to the H strand from a second subunit related to the o""rst by a 3-fold rotation. The similarity between the seven-stranded, centralb

-sheet of all three acyltransferases is indicated by rms difference values of 0.86 AE^ and 1.3 AE^ when all main-chain atoms of the central b-sheet of E2pCD and CAT, respectively, are superimposed onto the corresponding main-chain atoms of E2oCD.

The superposition of the A. vinelandii E2pCD onto the E2oCD (Figure 3) gives a rms difference of 1.8 AE^ for 225 of 230 Ca atoms (the amino acid sequences of the two acyltransferases are 32% iden- tical for the regions used in this superposition calculation). In both E2 structures, the peripheralb

-sheets are composed of the antiparallel b-strands C and D from one subunit and the antiparallelb

-strand A0 (residues 174 to 179) from a second subunit related to the o""rst by a 3-fold rotation. The peripheral b-sheets of the two E2s differ in two respects. First, the loop connecting b-strands C and D in the E2oCD contains a two amino acid del- etion. Second, the peripheral b-sheet and helix H1

Figure 1. Twelve subunits from the E2oCD cube. A Ca trace of 12 of the 24 E2oCD subunits illustrates the two types of interactions which generate the octahedral core of the E. coli OGDH complex. The subunits of each tri- mer are colored green, red, blue, with the green subunit pointing towards the missing back half of the cube. Each trimer interacts with neighboring trimers via a 2-fold along each edge.

Figure 2. The subunit fold of E2oCD. The fold is illus- trated as a ribbon diagram (Kraulis, 1991) with each of the elements of the secondary structure labeled. The three surface residues which were omitted from the reo""ned model are represented by a thin black line. The side-chains of two active site residues, His375 and Ser323 are shown in black.

Table 2. Assignment of the secondary structural elements

a-Helixes b-Strands H1 182-197 A 175-179 H2 210-227 B 200-208 H3 234-247 C 253-256 H4 291-305 D 259-263 H6 380-395 E 268-273

F 278-283 310 Helixes G 320-323

H 332-333 H5 326-328 I 343-357 H7 398-402 J 360-374

The E2o Catalytic Domain 657�

of the E. coli E2oCD differ from those of the A. vinelandii E2pCD by a 14ffi rotation around an axis near the center of helix H1. These changes are particularly interesting because helix H1 forms part of the active site and probably participates in the binding of the lipoyl domain/dihydrolipoa- mide substrate. The two-amino acid deletion may be responsible for the 14ffi rotation of both the per- ipheral b-sheet and helix H1. Note, however, that

these two regions are involved in crystal contacts, and the differences between E2oCD and E2pCD in these two regions might result from crystal pack- ing forces.

In addition to the peripheral b-sheets and the H1 helices, the helical segments H2, H3, H4, and H7 of E2oCD show some small differences from the cor- responding segments in E2pCD. In both E2 struc- tures, helix H2 is bent so that this helix can be divided into N-terminal and C-terminal segments. In the E. coli E2oCD, the two segments of helix H2 form an angle of 109ffi, which is 32ffi greater than that observed in the catalytic domain of E2pCD. This change in the bending angle results in an average rms difference of 2.6 AE^ for the six Ca

atoms located in the C-terminal half of helix H2. In addition to the change in the bending angle, the C-terminal half of helix H2 of E2oCD includes a single amino acid insertion (His227) at its C-term- inal end. The orientations of helices H3 and H4 from both E2 structures are similar. The C-terminal end of E2oCD is only slightly altered by the addition of two residues (Asp403 and Val404) after helix H7. The C-terminal segment (helix H7 and the two additional residues) of E2oCD is stabilized by an intramolecular salt bridge formed between the C terminus (Val404) and the side-chain of Arg399. Residues from each of the helical segments (H2, H3, H4, and H7) participate in the formation of the 432 symmetry of the octahedral cube. Most

of the changes in the orientation of these segments in the E2oCD versus E2pCD, particularly those involving helices H2 and H7, tend to increase the stability of the E2oCD cube.

Comparison with CAT

The superposition of CAT onto E2oCD (not shown) gives a rms difference of 1.8 AE^ for 121 out of 212 Ca atoms. The most striking difference between CAT and E2oCD involves the location of the o""rst 25 residues. When compared to the E2oCD and E2pCD structures, the o""rst 25 residues of CAT are rotated by 120ffi around an axis which is paral- lel to the 3-fold rotation axis. This rotation pos- itions b-strand A so that it forms the antiparallel, peripheral b-sheet with b-strands C and D, all of which are located on the same subunit. In contrast, the peripheral b-sheets in both E2 structures include b-strand A0 from a 3-fold related subunit. Because of the differences between CAT and E2pCD in the regions of the C-terminal end of helix H1 and the residues which link helix H1 withb

-strand B, this region of the E2o structure (resi- dues 191 to 202) was modeled into a simulated annealing omit map to reduce model bias (Figure 4). From this density, the E2oCD clearly resembles the fold of the E2pCD in this region.

Trimer contacts

E. coli E2oCDs form very stable, disk-shaped tri- mers with a diameter of 70 AE^ and a thickness of 50 AE^ (Figure 5). Applying the scheme used to label the A. vinelandii E2pCD trimers, subunits of the E2oCD trimers are labeled A, B , and C. Subunit B is related to subunit A by a clockwise 3-fold sym- metry operation and subunit C is related to A by a counterclockwise 3-fold symmetry operation when the trimer is viewed along the 3-fold axis from out- side the 24mer toward the inner cavity. The large number of interactions involved in the formation of the trimer suggests that trimerization represents the o""rst step in the assembly of the core of all 2-oxo acid dehydrogenase complexes. A total of 56 residues from each subunit participate in the trimer stabilization, resulting in the burial of 2740 AE^ 2 of surface area from each monomer. Each subunit contributes 14 hydrogen bonds and two salt bridges to each subunit interface so that a total of 52 H-bonds and six salt bridges stabilize each trimer.

The 3-fold interface is arranged so that most of the intersubunit contacts result from four different sets of interactions, with each set triplicated by the 3-fold symmetry. The o""rst set of intersubunit inter- actions is centered around strand bB and helix H6 from subunit A and the residues 328 to 334 and residue 351 (strand bH, parts of strand bI and helix H5) from subunit C. Three H-bonds are formed as strand bB from subunit A interacts with strand bH from subunit C so that the main sheet is extended by a single, antiparallel strand. The second set of

Figure 3. The superposition of E2pCD onto E2oCD. A stereo Ca trace illustrates the superposition of 225 Ca atoms of the A. vinelandii E2pCD (black) onto those of E2oCD (green), giving a RMS difference of 1.8 AE^ . Note that the two acyltransferases differ in the orientations of their N-terminal segment (residues 172 to 195) and helix H2 (residues 210 to 227).

658 The E2o Catalytic Domain�

intra-trimer contacts involves the addition of strandb

A from subunit A to strands bC and bD to form the peripheral b sheet. This interaction results in the formation of eight H-bonds, seven of which are formed between the main-chain atoms from both interacting b-strands. The third set of interactions is centered around helix H1, which is oriented so that one side of the helix is exposed to solvent whereas the other side forms a number of contacts with both symmetry-related subunits. The last set of 3-fold contacts results in the burial of 427 AE^ 2 of surface area as strands bI and bJ from each subunit form one side of a three-sided barrel structure.

2-Fold contacts

The E2 core of the A. vinelandii PDC readily dis- sociates into trimers when either the E1 or E3 com-

ponents binds to E2 (Bosma et al., 1984). Dissociation of the E2p is due to its weak inter- actions around the 2-fold axes. These interactions result in the burial of only 780 AE^ 2 of monomeric surface area and the formation of only two intersubunit hydrogen bonds. In contrast to the E2pCD, the core formed by the E. coli E2oCD is stable, even in the presence of E1 and E3. The increased stability is due to formation of stronger interactions between 2-fold related subunits. The 2- fold interaction results in the burial of 882 AE^ 2 of surface area from each monomeric E2oCD, an increase of 100 AE^ 2 of surface area buried for each of the 24 subunits. Although the non-polar inter- actions are very similar, the polar interactions are signio""cantly different. The two hydrogen bonds formed between the 2-fold related subunits in the A. vinelandii E2pCD are replaced by four different H-bonds, one of which is involved in a salt-bridge.

The 2-fold interface is centered around the last nine residues which participate in two types of interactions (Figure 6). Hydrophobic contacts occur between Pro397, Leu400, and Leu401 located on both 2-fold related subunits, resulting in the pack- ing of helix H7 against its symmetry-related counterpart (Figure 7). This part of the interface is stabilized by a pair of hydrogen bonds which are formed between the hydroxyl side-chain of Thr398 and the hydroxyl group of its symmetry-related counterpart (Thr3980).

The second type of 2-fold interaction occurs as the C-terminal segment o""ts into a hydrophobic pocket composed of residues located on helix H20 (Leu2150, Tyr2190, and Phe2230), helix H30 (Phe2360), and helix H40 (Met2910) from a 2-fold related subunit. Five of the last nine residues (Thr398, Arg399, Leu401, Leu402, and Val404) par- ticipate in this interaction by forming van der Waals contacts with the hydrophobic pocket. The

Figure 4. The electron density in the region which links helix H1 to b-strand B. Residues 181 to 199 of subunit C and residues 192 to 195 of subunit A from the reo""ned model are superimposed upon a sA-weighted 2Fo ss Fc simulated annealing omit map contoured at 1 s. The Ca trace of CAT (residues 20 to 32) is shown in red. After a subunit of CAT is superimposed upon E2oCD, b-strand B from CAT (residues 32 to 40) is positioned on top of b-strand B from subunit A of E2oCD (residues 200 to 208), whereas CAT0s H1 helix is positioned on top of that from subunit C. Unlike CAT, the fold of E2oCD does not cross into a neighboring subunit. Note that the sulfate ion pictured above is sitting on the 3-fold axis so that the main-chain of E2oCD cannot cross into a 3-fold related subunit through this density.

Figure 5. The E2oCD trimer. A ribbon diagram illustrat- ing the interactions which form the trimer. Subunits A, B, and C are shown in green, blue, and red, respectively. The E2oCD trimer is viewed down the 3-fold axis, look- ing into the trimer from the outside of the cubic com- plex. The positions of His375 and Ser323 are represented by a H and S, respectively.

The E2o Catalytic Domain 659� orientation of the C-terminal segment within the pocket is maintained by three polar interactions involving two residues from the C-terminal region and three residues from helix H20 (Figure 8). One of these interactions results in the formation of a salt bridge between Asp403 and Arg2260. The side- chains for both of these residues are oriented so that only one hydrogen bond is formed. In addition to the salt bridge, this part of the 2-fold interface is stabilized by two additional hydrogen bonds, one between the Od2 atom of Asp396 and

the hydroxyl of Tyr2190 and the second between the main-chain nitrogen of Asp403 and the Ne2 atom of His2270.

Substrate-binding channel

The substrate-binding sites of CAT, E2pCD, and E2oCD are located in a channel formed by the interactions between two of the 3-fold related sub- units. A total of three channels are formed in each trimer. Each of the substrate-binding channels runs nearly parallel to the 3-fold rotation axis so that one end of the channel is exposed to the outside of the cube whereas the other end leads to the central cavity (Figure 9). The channel is approximately 30 AE^ in length and varies between 6 and 9 AE^ in diameter.

In the E2oCD, one side of the channel is formed by b-strand B, the N-terminal end of helix H6, and the loop segment connecting b-strand J and helix H6. The remainder of the channel is formed from helix H1, the b-turn connecting b-strands E and F, and the b-strands G, H, I, and J which are located on a 3-fold related subunit. In contrast to the sub- strate-binding channels of E2pCD and CAT, one side of the channel in the E2oCD is exposed to sol- vent. The rearrangement of helix H1 results in a large gap between the N-terminal end of helix H1 and the EF b-turn (residues 274 to 277). The lack of contacts between these two parts of the E2oCD structure increases the mobility of the EF b-turn region and allows solvent molecules to enter the active-site channel.

Active site

The dihydrolipoamide acyltransferases are thought to catalyze reactions by a mechanism ana- logous to that employed by chloramphenicol acet- yltransferase (CAT; Guest, 1987; Shaw & Leslie, 1991). The proposed catalytic mechanism of dihy-

drolipoamide acyltransferases involves a histidine that removes a proton from the thiol group of coenzyme A. The CoASss then attacks the carbonyl carbon of the acylated lipoyl moiety to form a tet- rahedral intermediate. The collapse of this inter- mediate results in the transfer of the acyl group to

Figure 7. The interface between helix H7 and its 2-fold symmetry- related counterpart. This interface is stabilized by hydrophobic con- tacts between Pro397, Leu400, and Leu401 from both subunits. A hy- drogen bond (shown as a broken line) is formed between Thr398 and its symmetry-related counterpart.

Figure 6. A stereo ribbon diagram illustrating 2-fold related subunits interacting along each edge of a cube. Residues forming the 2-fold interface are located on helices H2 (blue) and H7 (purple).

660 The E2o Catalytic Domain� CoASss. The tetrahedral intermediate is stabilized by either a serine residue as in the A. vinelandii E2p or a threonine residue as in the E. coli E2o. In addition, either an asparagine residue (A. vinelandii E2p) or an aspartic acid residue (CAT and E. coli E2o) helps in the stabilization of the correct tauto- mer of the catalytically essential histidine. Whether or not this residue has an additional catalytic role remains to be established (Hendle et al., 1995).

The catalytic residues of E2oCD are located in the middle of the channel. The active site histidine (His375) is located on one subunit and the active site threonine (Thr3230) is located on a 3-fold related subunit (Figure 10). In the E. coli E2oCD,

the side-chain of the catalytic histidine (His375) is oriented with a w1 angle of ss169ffi and a w2 angle of 20ffi, indicating only a slight change in the con- formation when compared to that of the A. vinelan- dii E2pCD (w1 L' ss156, w2 L' ss7; Mattevi et al., 1992). This conformation positions the imidazole ring further from Thr3230. The hydroxyl side-chain of Thr323 assumes a similar position to that of Ser558 of the A. vinelandii E2pCD and Ser148 of CAT, except that the entire b-strand G, including Thr3230, is moved 1.0 AE^ away from His375. The combined effects of the movement of this b-strand and the orientation of His375 result in a distance of 8.7 AE^ between the Ne2 atom of His375 and the Og1

Figure 8. The interaction between helix H2 (left) and helix H70 (right) further aids the stabilization of the octahedral cube. The contacts between these two helices in E2oCD are signio""cantly different from those of E2pCD. Two inter- molecular H-bonds present in the A. vinelandii E2pCD are replaced by four different hydrogen bonds, three of which occur at this part of the interface. The hydrogen bonds are shown as broken lines.

Figure 9. Dihydrolipoamide and coenzyme A are modeled into the substrate-binding site based on the A. vinelandii E2pCD-substrate com- plexes (Mattevi et al., 1993b). When viewed down the 3-fold axis (a), each active-site channel is approximately 30 AE^ in length and varies between 6 and 9 AE^ in diam- eter. When viewed perpendicular to the 3-fold axis (b), it can be seen that dihydrolipoamide enters its binding site from outside the cube (right) whereas coenzyme A enters its binding site from inside the cube (left).

The E2o Catalytic Domain 661� atom of Thr3230, thus these two residues in the E2oCD are 2.0 AE^ further apart than those in E2pCD and 1.6 AE^ further apart than those in CAT.

The catalytic histidine is located on a loop which is stabilized by a salt bridge between Asp374 and Arg376 (Figure 10). The side-chain of Arg376 is positioned so that a single hydrogen bond is formed between the NH1 atom of Arg376 and the Od1 atom of Asp374. This interaction is similar to that observed between Asp609 and Arg611 of the A. vinelandii E2pCD except that the guanidino side- chain of Arg611 is oriented so that an additional hydrogen bond is formed. Based on the alignment of 33 known E2 sequences, this salt bridge is con- served in all but one E2 sequence, the exception being the E2p from Clostridium magnum. In this E2p, the residue which corresponds to Asp373 is replaced by an asparagine. This salt bridge is not found in any of the known sequences of CAT (Mattevi et al., 1992).

A third residue (Asp or Asn) is thought to par- ticipate in the catalytic mechanism of this family of acyltranferases by forming an electrostatic inter- action with the catalytic histidine (Hendle et al., 1995). In the E. coli E2oCD, Asp379 is oriented with a w1 angle of 64ffi and a w2 angle of ss45ffi so that the carboxyl side-chain of Asp379 is positioned away form His375 (Figure 10). This conformation differs from that observed for Asp199 in the liganded structures of CAT (w1 L' ss63ffi,w2 L' 147ffi), but is similar to the confor-

mation of Asn614 found in the unliganded struc- ture of the A. vinelandii E2pCD (w1 L' 48ffi,w2 L' 51ffi). Asp199 forms an important salt bridge in CAT (Lewendon et al., 1988) whereas the side-chain of Asp379 does not interact with another protein atom.

Discussion Role of Asp379 during substrate binding

The catalytic activity of CAT is dependent upon a salt bridge that is formed between Arg18 and Asp199. Based on a combination of mutagen- esis, kinetic, and crystallographic experiments, Lewendon et al. (1988) and Gibbs et al. (1990) pro- posed that this salt bridge maintains the structural

integrity of the active site. In addition to the struc- tural role, this salt bridge is positioned near the catalytic histidine (His195), suggesting that Asp199 might interact electrostatically with the catalytic histidine (His195 in CAT; Hendle et al., 1995). Replacement of the corresponding aspartic acid residue in the E2 component of the E. coli PDC (Russell & Guest, 1991a) or yeast PDC (Niu et al., 1990), results in a signio""cant decrease in catalytic activity, suggesting that this residue is also import- ant for the catalytic mechanism of this family of acyltransferases. The corresponding aspartic acid residue in E2oCD (Asp379) does not form this important salt bridge in the unliganded structure.

The A. vinelandii E2pCD, unlike CAT and E2oCD, has an asparagine residue (Asn614) in the position corresponding to Asp199 in CAT. Substi- tution of this residue with an Asp results in a decrease in kcat (Hendle et al., 1995). In the A. vine- landii E2pCD, the binding of coenzyme A to the E2pCD results in the movement of the side-chain of Asn614 so that the carboxamide group of its side-chain is now pointing towards the catalytic histidine (Mattevi et al., 1993b), allowing the for- mation of a long hydrogen bond (3.4 AE^ ) between the Od1 atom of Asn614 and the Nd1 atom of His610. This conformation of Asn614 is analogous to that of Asp199 in CAT, which in the liganded structure forms a long hydrogen bond with the cat- alytic histidine and forms an important salt bridge with Arg18. By analogy to both of these acetyl- transferases, it is likely that the binding of coenzyme A to the E. coli E2oCD initiates confor- mational changes allowing Asp379 to interact with His375 and possibly to form a salt bridge with Arg1840 (Figure 11).

The role of Arg184

The basic residue that interacts with Asp379 (the analog of Arg18 in CAT) cannot be identio""ed by sequence alignments alone. The lack of sequence similarity between the E2 family and CAT in this region is probably due to the 120ffi difference in their orientations of the N-terminal segment. The orientation of helix H1 in CAT positions Arg18 so that an intramolecular salt bridge is formed with Asp199. Within the active site of the E. coli E2o,

Figure 10. Model of the active site of E2oCD superimposed upon a sA weighted 2Fo ss Fc acalc map contoured at 1.0s. The catalytic histidine (His375) and threonine (Thr3230) are located on opposite sides of the substrate-binding channel and the catalytically important Asp379 is pointing away from His374 in the native structure.

662 The E2o Catalytic Domain�

there are three arginine residues (Arg184, Arg276, and Arg381) nearby which might form a salt bridge with Asp379. Unfortunately, the side-chains of all three residues are disordered in the native structure. Thus, a preferred side-chain confor- mation for each arginine residue was modeled into the structure. Although the side-chains of Arg381 and Arg1840 (Arg184 from a 3-fold related subunit) can easily be modeled so that a salt bridge is formed with Asp379, Arg184 occupies a similar, but not identical, position to that of Arg18 in CAT. Furthermore, Arg184 is conserved in all 13 known E2oCD sequences, whereas basic residues corresponding to Arg381 and Arg276 are present in 12 and 11 E2oCD sequences, respectively (Figure 12). Because E2oCD and CAT differ in the orientation of the N-terminal segment, Arg1840 in the E2oCD is donated from a 3-fold related subunit whereas Arg18 in CAT is on the same subunit as Asp199. If the Arg1840 s'Asp379 salt bridge does form in the liganded state, it would add emphasis to the remarkable differ- ences between the E2 and CAT structures in that both acyltransferases form an equivalent active site even though part of the active site of CAT

(helix H1) is swapped with that of an E2oCD subunit which is related to the core of CAT by a 3-fold symmetry operation.

Modeling the two substrates into the active site of E2oCD

Attempts to soak in coenzyme A and dihydroli- poamide failed to produce diffraction quality crys- tals. Therefore, these substrates were modeled into the substrate-binding site of E2oCD (Figure 9) based on the crystallographic analysis of the A. vinelandii E2pCD complexed with its substrates (Mattevi et al., 1993b). Mattevi et al. (1993b) reported that coenzyme A binds in ``Out or ``In conformations depending on the position of the pantetheine arm of coenzyme A. The coenzyme A-binding site consists of a hydrophobic pocket which binds the adenine ring and a patch of con- served bases (Arg216 and Arg230) which interacts with the diphosphate moiety of coenzyme A. The pantetheine arm of coenzyme A binds to one side of the active-site channel and dihydrolipoamide binds to the other side. Modeling the In confor- mational form of coenzyme A into the E2oCD

Figure 11. A probable E2oCD- coenzyme A complex. The hypo- thetical model of this interaction suggests that upon coenzyme A binding (shown in green), Asp379 changes conformation to form a salt bridge with Arg1840 (Od1s'NH2 distance of 2.5 AE^ , Od2s'NH2 dis- tance of 3.5 AE^ ). The new confor- mation of Asp379 allows its side- chain to form an electrostatic interaction with His375 (Od1s'Nd1 distance of 3.4 AE^ ).

Figure 12. The alignment of the active site regions from selected E2 catalytic domains. This alignment includes the E2oCD sequences from E. coli, A. vinelandii (Avine), Alcaligenes eutrophus (Aeutr), Rhodobacter capsulatus (Rcaps), Bacil- lus subtilis (Bsubt), S. cerevisiae (Scerv), rat, and human. The E2oCD sequences from Haemophilus inr'uenzae, Neisseria gonorrhoeae, Coxiella burnetii, Caenorhabditis elegans, and Fugu rubripes have also been aligned, but are not presented in this Figure. The E2oCD active site is lined with several conserved bases (Arg184, Arg276, and Arg381) which could perform the same function as Arg18 in CAT. The sequences alignment was made with Macaw version 2.0.5 (Schuler et al., 1991).

The E2o Catalytic Domain 663� active site required a slight rearrangement of the panthetheine arm so that its thiol group occupied a position similar to its counterpart in the CoA- E2pCD liganded structure (Figure 11). The result- ing model is stabilized by eight hydrogen bonds, seven of which are identical to those found in the CoA-E2pCD complex. The only unique hydrogen bond found in the E2oCD model is formed between the Nd2 of Asn324 and the OP2 of the diphosphate moiety of coenzyme A (using the nomenclature of Mattevi et al., 1993b). The prob- able dihydrolipoamide binding site of the E. coli E2oCD is similar to that of E2pCD with seven out of the nine residues contacting dihydrolipoamide being identical. The only noticeable difference is the replacement of Asn425 in E2pCD with Arg190 in E2oCD. However, the location of Arg190 suggests that it does not participate in the binding of the succinyl part of the substrate.

Substrate specificity of the E2o component

E2oCD catalyzes a succinyl transfer from the S-succinyldihydrolipoyl moiety and coenzyme A whereas both CAT and E2pCD catalyze an acetyl transfer reaction. The crystal structure of E2pCD liganded with acetyl-coenzyme A indicated that Leu560 and Phe568 form van der Waals contacts with the acetyl group. This o""nding suggested that the replacement of Leu560 and Phe568 with a smaller residue (glycine or serine) could provide a binding pocket for the larger succinyl group. Based on the alignments of all known E2o, E2p, and E2b sequences, Russell & Guest (1991b) proposed that Gly325, Ser333, and Arg381 contribute to the sub- strate specio""city observed in the E2oCD. The side- chain of Arg381 is near the active site, suggesting that it might form a salt bridge with the carboxyl group of succinyldihydrolipoamide (Mattevi et al., 1993b).

To analyze the roles these residues might play in the substrate specio""city of the E2o family, succinyl-

dihydrolipoamide was modeled into the active site of the E2oCD. This model was based in part on the model of dihydrolipoamide binding in the active site of the E2pCD, but the model also took into account the likely position of the tetrahedral inter- mediate and the orientation of coenzyme A with respect to this intermediate. In the proposed model, the succinyl group o""ts into a pocket (Figure 13) formed from four hydrophobic residues (Leu278, Pro335, Tyr373, and Val384) and six hydrophilic residues (Asn324, Gly325, Ser330, Ser333, His348, and Gly380). The lack of acidic side-chains near this site should be advantageous because a negative charge would repel the carbox- ylate ion of the succinyldihydrolipoamide. The size and shape of the proposed succinyl-binding site is large enough for the succinyl group to assume sev- eral conformations. In most conformations, the car- boxyl group should be able to form a hydrogen bond with one of the hydroxyl groups (Ser330, Ser333, Tyr373) that line the binding pocket. In the proposed model, the side-chain of Ser333 is posi- tioned so that it can form a hydrogen bond with the carboxyl group of succinyldihydrolipoamide.

In the absence of large structural changes follow- ing substrate binding, there is only one base (His348) which is located near the proposed succi- nyl-binding site. In this model, the Ne2 of this histi- dine would be 3.0 AE^ from a carboxylate oxygen of succinyldihydrolipoamide. His348 is conserved in all 13 known E2o sequences, but it is not found in any of the E2b or E2p sequences. A second base (Arg381) was suggested previously (Mattevi et al., 1993b) to interact with the carboxyl group of the S-succinyllipoyl moiety. In the proposed model, the side-chain of this residue is pointing away from the succinyl-binding site. Although it is still possible that Arg381 might interact with the sub- strate, the rearrangement of the side-chain of this residue would be hindered by the side-chains of Asp379 and Phe328, and the pantetheine arm of the coenzyme A substrate. Because of these steric

Figure 13. A ball and stick rep- resentation of succinyldihydrolipo- amide modeled into the active site of E2oCD. The succinyl-binding site (drawn as a stick represen- tation) contains several hydroxyl groups (Ser330, Ser333, and Tyr373). Of the two bases near the succinyl group, His348 appears to be oriented so that it could interact with the substrate whereas Arg381 would point away from the succi- nyl-binding pocket.

664 The E2o Catalytic Domain� hindrances, it seems more reasonable that the succinyl group is recognized by either His348 or by one (or more) of the hydroxyl groups which line the binding pocket.

Materials and Methods Materials

Plasmid pGS490, containing nucleotides 3392 to 4360 of the E. coli sucB gene (Spencer et al., 1984), was kindly provided by Drs John R. Guest and George C. Russell (University of Shefo""eld, United Kingdom), succinyl-Co A synthetase was from Dr Jonathan S. Nishimura (The University of Texas Health Science Center at San Anto- nio), and plasmid pGro ESL was from Dr George Lori- mer (DuPont Experimental Station). Plasmid pGEX-2T and glutathione-Sepharose 4B were obtained from Phar- macia, and heparin-agarose and thrombin were from Sigma.

Vector construction and expression

A DNA fragment encoding amino acid residues 93 to 404 of E2o (tE2o, lacking the lipoyl domain; Spencer et al., 1984) was amplio""ed by the polymerase chain reaction from pGS490 using the primers 50 TCTGGATC- CAAAGCGGCGTCCACTCCG 30 and 50 TAAAG- GATCCCTACTACACGTCCAGCA 30. These primers introduced a BamHI site at the two ends of the subgene. The DNA fragment was sequenced to cono""rm its iden- tity. The restriction sites were used to subclone the DNA fragment into pGEX-2T in-frame with the glutathione S-transferase (GST) gene to generate pGEX-2T-E2oK93 for expression in E. coli. Strain JM101 was cotransformed with the latter plasmid plus pGroESL, which encodes the chaperonin proteins groEL and groES. Double transfor- mants were selected on media containing 50 mg/ml ampicillin and chloramphenicol. Cultures of transformed cells were grown under conditions optimal for expression of soluble active GST-tE2o fusion protein (Mr,

60,000) as determined by SDS-PAGE and by assay of dihydrolipoamide succinyltransferase (E2o) activity (Knight & Gunsalus, 1962).

Purification and cleavage of fusion protein, and purification of tE2o

Transformed cells were grown on LB medium contain- ing 50 mg/ml ampicillin and chloramphenicol at 28ffiC to an A600 of 1.0. Expression was induced by addition of isopropyl b-thiogalactoside to a o""nal concentration of 50 mM. Incubation was continued overnight at 28ffiC before harvesting. The cells were disrupted in a French pressure cell. The GST-tE2o fusion protein was purio""ed by afo""nity chromatography on glutathione-Sepharose 4B beads, cleaved by treatment with thrombin, and the tE2o was further purio""ed by chromatography on heparin-agarose by procedures described previously (Maeng et al., 1994). Analysis of tE2o by fast protein liquid chromatography (FPLC) with a Superose 6 col- umn showed that the recombinant protein eluted before thyroglobulin (Mr L' 640,000) and after Blue Dex- tran (2,000,000; data not shown). This observation indi- cated the recombinant tE2o is a large oligomer, consistent with a calculated molecular mass of 816,000 for the 24-mer. When analyzed by SDS-PAGE (data not shown), the purio""ed tE2o showed a major band

with an anticipated apparent molecular mass of,

34,000. Its identity was cono""rmed by amino-terminal sequence analysis.

Crystallization and data collection

Using the hanging drop method (McPherson, 1990), the tE2o crystallized when 1.2 M ammonium sulfate and 1% (v/v) ethanol were used as precipitants. The hanging drop consisted of 5 ml of a 20 mg/ml solution of tE2o (in 50 mM potassium phosphate buffer (pH 7.3), 1 mM dithiothreitol, 1 mM EDTA, 1 mM benzamidine) and 5 ml of the precipitant solution. Large octahedral crystals (0.6 to 1.2 mm) grew over a six-month period. The crys- tals diffracted to 3.6 AE^ resolution when analyzed on a San Diego Multi-Wire area detector equipped with a RIGAKU RU-200 rotating anode generator operated at 50 kV and 110 mA. Crystals of E. coli tE2o belong to the cubic space group F432 with a cell parameter a L' 222.8 AE^ , in agreement with that reported by DeRosier et al. (1971).

Higher resolution data were collected on a single crys- tal ( 1.2 mm \Theta 1.2 mm) using a MAR image plate detec- tor at the Stanford Synchrotron Radiation Laboratory (beamline 7.1). This data set was collected at room tem- perature using a crystal to o""lm distance of 220 mm, an X-ray wavelength of 1.08 AE^ , an oscillation angle of 0.75ffi, and an exposure time of 30 seconds. The diffraction data were indexed using the program Reo""x (Kabsch, 1988), reduced using the program MOSFLM (Leslie, 1993), and merged using the programs AGROVATA and ROTOVA- TA (CCP4, 1994). The o""nal data set omitted all partially recorded rer'ections. The merged data set is 99% com- plete to 3.0 AE^ resolution with an overall multiplicity of 4.1 and an overall Rsym of 4.6%. The data in the highest resolution shell (3.16 to 3.0 AE^ ) is almost 100 % complete, with an average multiplicity of 4.5, a Rsym of 14.5%, and an average I/sig(I) of 4.9. At this stage, 1019 rer'ections or 10% of the total rer'ections were set aside to be used in the calculation of Rfree (Brunger, 1992b).

Amino-terminal sequence analysis

The N-terminal sequence was determined at the Uni- versity of Texas at Austin Protein Microanalysis Facility. A single crystal (0.5 mm \Theta 0.5 mm \Theta 0.3 mm) was washed with 1.2 M ammonium sulfate before it was dissolved in a 5 % aqueous solution of acetonitrile. The protein solution was applied to a polyvinylidene dir'uoride membrane and the amino-terminal sequence was determined using an Applied Biosystems gas- phase sequencer (model 477A). This analysis detected the following o""ve sequences, which are listed in the order of decreasing quantity of the peptide present in the crystal.

(1) 166AQPALAARSEKRVFM (2) 174SEKRVFMTRLRKRVAL (3) 152APAKESAPAAAPQPAL (4) GS93KASTPA-R (5) 163APAAQAPAL

Although one of the N-terminal sequences (peptide (4)) corresponds to the sequence of the genetically engin- eered tE2o sequence, with two residues remaining from the thrombin cleavage site, this peptide was present in the crystal at a very low concentration. The majority of the protein from the dissolved crystal lacks both the E3-binding domain and most of the adjacent linker

The E2o Catalytic Domain 665� region. Apparently, proteolysis occurred during the rela- tively long time frame required to produce the crystals. It is possible that the thrombin used to cleave the GST- tE2o fusion protein was not removed completely during the purio""cation procedure.

Structure determination and refinement

The initial phases were obtained using the Molecular Replacement method with the catalytic domain of the A. vinelandii E2p (Mattevi et al., 1993a) serving as the initial model. The amino acid sequence of the E. coli E2oCD is 32% identical to that of the E2pCD for those residues included in the initial model (residues 185 to 402). Non-identical residues were truncated to alanine before reo""nement. The model was further adjusted to include a single amino acid insertion after helix 2 (resi- due 227) and a two amino acid deletion resulting in a b- turn deo""ned by residues 256 to 259. Because the crystals of E2oCD are isomorphous to those of the E2pCD, the E2oCD model was simply positioned in the unit cell using the rigid-body reo""nement protocol of X-PLOR (BruE` nger, 1992a). Following rigid-body reo""nement, the model had an initial R-factor of 0.492 (Rfree L' 0.491) for all measured rer'ections between 10.0 and 4.0 AE^ resolution.

The crystallographic reo""nement of the E2oCD model involved a total of 13 rounds of reo""nement with X-PLOR version 3.1 and one round of reo""nement with X-PLOR version 3.85. The initial round of reo""nement optimized the model using the rer'ections between 10.0 and 4.0 AE^ resolution. During the next three rounds of X-PLOR reo""nement, the resolution of the data was extended to 3.0 AE^ resolution. During the last round of reo""nement, the low resolution terms were included in the reo""nement protocols, thereby using all recorded rer'ections between 20.0 and 3.0 AE^ resolution. A typical round of reo""nement involved a conjugate gradient energy minimization step followed by a grouped B-factor reo""nement step in which main-chain and side-chain B-factors were both opti- mized. During the o""rst and second rounds of reo""nement, rigid body reo""nement was used to optimize the position of each element of the secondary structure before the model was subjected to energy minimization. During the second, third, fourth, seventh, eighth, and eleventh rounds of X-PLOR reo""nement, a simulated annealing step (BruE` nger et al., 1987, 1990) was included in the reo""nement protocol. The last round of reo""nement included a bulk solvent correction step (Jiang & BruE` nger, 1994) followed by a grouped B-factor reo""nement step implemented with X-PLOR version 3.85. During this round of reo""nement, the B-factors for protein atoms were not allowed to reo""ne to values greater than 65 AE^ 2, and the B-factors for solvent molecules were not allowed to exceed 50 AE^ 2.

Between each round of X-PLOR reo""nement, the model was manually adjusted using the program ``O (Jones et al., 1991) running on a Silicon Graphics Crimson work- station. The model was o""tted to 2Fo ss Fc, Fo ss Fc, and non-existence density (NED) omit 2Fo ss Fc maps (Kolatkar et al., 1992). Regions of the structure with poor electron density were built into simulated annealing omit maps (Hodel et al., 1992). After the eighth round of X-PLOR reo""nement, one-third of the model was built into simulated annealing omit maps. Side-chains from the truncated residues were only added to the model if adequate density was present in the 2Fo ss Fc electron density maps. Residues or side-chains with poor density

were removed from the model. After the tenth round of reo""nement, solvent molecules were added to the model. The CCP4 programs PEAKMAX and WATPEAK were used to locate water molecules in a Fo ss Fc electron den- sity map contoured at 2.3s. Water molecules were removed from the model if either the 2Fo ss Fc electron density was poor or the water failed to form at least one hydrogen bond with either a protein or another water atom.

Following the o""rst round of reo""nement, the R-factor for the E2oCD model decreased to 0.342 (Rfree L' 0.469) for all data between 10.0 and 4.0 AE^ resolution. The qual- ity of the 2Fo ss Fc electron density map allowed 93 side- chains and two additional residues (Asp403 and Val404) to be added to the model. Due to the initial poor quality of the electron density around the N-terminal loop, resi- dues 161 to 185 were removed from the model. After the next three rounds of reo""nement, 87.8% of the side-chains had been added to the model. At this stage, the N-term- inal residues (172 to 185) were incorporated into the model based on NED omit 2Fo ss Fc maps. The model was subjected to nine additional rounds of X-PLOR reo""nement followed by manual intervention. Before the addition of water molecules, the model had a R-factor of 0.212 and a Rfree of 0.275 for all measured rer'ections between 8.0 and 3.0 AE^ resolution. At this stage, 24 water molecules were incorporated into the model. One sulfate ion and several additional water molecules were added to the model during the next three rounds of reo""nement. Following the bulk solvent correction, several of the water molecules were removed from the o""nal model due to the poor density present in the 2Fo ss Fc sA weighted maps (Read, 1986), leaving a total of 16 water molecules and one sulfate ion.

Analysis of the refined model

The superposition calculations were performed using LSQMAN (Kleywegt, 1996). The solvent accessi- ble surface area was calculated using the algorithm of Lee & Richards (1971) as implemented in SURFACE (CCP4, 1994). The CCP4 program CONTACT was used to list the intersubunit contacts (CCP4, 1994). Figures 1, 2, 3, 5, 6, 9, and 13 were prepared using the program MOLSCRIPT (Kraulis, 1991). Figures 4, 7, 8, 10, and 11 were prepared using the program MOL- VIEW (Smith, 1992). The stereochemical quality of the model was evaluated using the program PROCHECK (Laskowski et al., 1993). The E2o sequences were obtained from GENBANK (Benson et al., 1994) and aligned with Macaw version 2.0.5 (Schuler et al., 1991). The coordinates of E2oCD have been deposited in the Brookhaven Protein Data Bank under the acces- sion number 1e20.

Acknowledgments

Were are grateful to Dr Klause Linsey for performing the N-terminal sequence analysis, to the staff at SSRL for helpful assistance with the data collection, and to Dr Diane McCarthy for helpful comments on the prep- aration of the manuscript. This work was supported by Texas Advanced Research Program no. 003658-189 (M.L.H. and L.J.R.), Robert A. Welch Foundation grant no. 1219 (M.L.H.), and US Public Health Service grant GM06590 (L.J.R.).

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Edited by D. Rees (Received 24 December 1997; received in revised form 17 April 1998; accepted 20 April 1998)

668 The E2o Catalytic Domain�

Disambiguation/duplication

There is a separate Wikipedia entry for "OGDH" (https://en.wikipedia.org/wiki/OGDH). Neuroglider (talk) 03:48, 25 November 2013 (UTC)