A computer model analysis of the active-site coupling mechanism in the pyruvate dehydrogenase multienzyme complex of Escherichia coli.

A computer modeling system developed to analyze experimental data for inactivation of the Escherichia coli alpha-ketoglutarate dehydrogenase complex (KGDC) accompanying release of lipoyl moieties by lipoamidase and by trypsin [Hackert, M.L., Oliver, R.M. & Reed, L.J. (1983) Proc. Natl. Acad. Sci. USA 80, 2226-2230] was used to analyze analogous data for the E. coli pyruvate dehydrogenase complex (PDC). The model studies indicate that the activity of PDC, as found for KGDC, is influenced by redundancies and random processes, which we describe as a multiple random coupling mechanism. In both complexes more than one lipoyl moiety services each pyruvate dehydrogenase (EC 1.2.4.1) or alpha-ketoglutarate dehydrogenase (EC 1.2.4.2) (E1) subunit, and an extensive lipoyl-lipoyl interaction network for exchange of electrons and possibly acyl groups must also be present. The best fit between computed and experimental data for PDC was obtained with a model that has four lipoyl domains with four or, more probably, eight lipoyl moieties servicing each E1 subunit. The lipoyl-lipoyl interaction network for PDC has lipoyl domain interactions similar to those found for KGDC plus the additional possibility of interaction of a lipoyl moiety and its paired mate on each dihydrolipoamide acetyltransferase (EC 2.3.1.12) (E2) subunit. The two lipoyl moieties on an E2 subunit in PDC appear to be functionally indistinguishable, each servicing the acetyltransferase site of that E2 subunit and a dihydrolipoamide dehydrogenase (EC 1.6.4.3) (E3) subunit if the latter is bound to that particular E2 subunit. The observed difference between inactivation of PDC by lipoamidase and by trypsin appears to be due to dead-end competitive inhibition by lipoyl domains that have been modified by excision of lipoyl moieties by lipoamidase.     

Acyl group and electron pair relay system: A network of interacting lipoyl moieties in the pyruvate and α-ketoglutarate dehydrogenase complexes from Escherichia coli

The dihydrolipoyl transacetylase component of the Escherichia coli pyruvate dehydrogenase complex [pyruvate:lipoate oxidoreductase (decarboxylating and acceptor-acetylating), EC 1.2.4.1] bears two sites on each of its 24 polypeptide chains that undergo reductive acetylation by [2-(14)C]pyruvate and thiamin pyrophosphate, acetylation by [1-(14)C]acetyl-CoA in the presence of DPNH, and reaction with N-ethyl[2,3-(14)C]maleimide in the presence of pyruvate and thiamin pyrophosphate. The data strongly imply that these sites are covalently bound lipoyl moieties. The results of similar experiments with the E. coli alpha-ketoglutarate dehydrogenase complex [2-oxoglutarate:lipoate oxidoreductase (decarboxylating and acceptor-succinylating), EC 1.2.4.2] indicate that its dihydrolipoyl transsuccinylase component bears only one lipoyl moiety on each of its 24 chains. Charging of the 48 acetyl acceptor sites on the transacetylase or the 24 succinyl acceptor sites on the transsuccinylase by pyruvate or alpha-ketoglutarate, respectively, and thiamin pyrophosphate was observed in the presence of only a few functionally active pyruvate dehydrogenase or alpha-ketoglutarate dehydrogenase chains. Extensive crosslinking of the transacetylase chains was observed when the pyruvate dehydrogenase complex was treated with pyruvate and thiamin pyrophosphate or with DPNH in the presence of N,N'-o- or N,N'-p-phenylenedimaleimide, respectively. When the alpha-ketoglutarate dehydrogenase complex was treated with DPNH in the presence of N,N'-p-phenylenedimaleimide, only transsuccinylase monomers and crosslinked transsuccinylase dimers were detected. It appears that the 48 lipoyl moieties in the transacetylase and the 24 lipoyl moieties in the transsuccinylase comprise an interacting network that functions as an acyl group and electron pair relay system through thiol-disulfide and acyl-transfer reactions among all of the lipoyl moieties.     

Inhibition of alpha-ketoglutarate dehydrogenase activity by a distinct population of autoantibodies recognizing dihydrolipoamide succinyltransferase in primary biliary cirrhosis

Sera from patients with primary biliary cirrhosis contain autoantibodies that recognize mitochondrial proteins. Five of the target autoantigens have now been identified as enzymes of three related multienzyme complexes: the pyruvate dehydrogenase complex, the branched chain alpha-ketoacid dehydrogenase complex and the alpha-ketoglutarate dehydrogenase complex. Each complex consists of component enzymes designated E1, E2 and E3. In this report, we confirm that primary biliary cirrhosis sera react with dihydrolipoamide succinyltransferase, the E2 component of alpha-ketoglutarate dehydrogenase complex. Seventy-three of 188 (39%) primary biliary cirrhosis sera reacted with alpha-ketoglutarate dehydrogenase complex-E2 when immunoblotted against purified alpha-ketoglutarate dehydrogenase complex; one of these sera also reacted with the E1 component. In addition, primary biliary cirrhosis sera possessing alpha-ketoglutarate dehydrogenase complex-E2 reactivity specifically inhibited enzyme function of alpha-ketoglutarate dehydrogenase complex. Enzyme activity was not affected by primary biliary cirrhosis sera that contained autoantibodies to pyruvate dehydrogenase complex-E2 and/or branched chain alpha-ketoacid dehydrogenase complex-E2, which lacked alpha-ketoglutarate dehydrogenase complex-E2 reactivity. Furthermore, affinity-purified primary biliary cirrhosis sera against alpha-ketoglutarate dehydrogenase complex-E2 inhibited only alpha-ketoglutarate dehydrogenase complex activity but did not alter enzyme activity of either pyruvate dehydrogenase complex or branched chain alpha-ketoacid dehydrogenase complex. Finally, alpha-ketoglutarate dehydrogenase complex-E2 specific affinity-purified antisera did not react on immunoblot with any component enzymes of pyruvate dehydrogenase complex or branched chain alpha-ketoacid dehydrogenase complex.(ABSTRACT TRUNCATED AT 250 WORDS)     

The remarkable structural and functional organization of the eukaryotic pyruvate dehydrogenase complexes

The three-dimensional reconstruction of the bovine kidney pyruvate dehydrogenase complex (M(r) approximately 7.8 x 10(6)) comprising about 22 molecules of pyruvate dehydrogenase (E(1)) and about 6 molecules of dihydrolipoamide dehydrogenase (E(3)) with its binding protein associated with the 60-subunit dihydrolipoamide acetyltransferase (E(2)) core provides considerable insight into the structural and functional organization of the largest multienzyme complex known. The structure shows that potentially 60 centers for acetyl-CoA synthesis are organized in sets of three at each of the 20 vertices of the pentagonal dodecahedral core. These centers consist of three E(1) molecules bound to one E(2) trimer adjacent to an E(3) molecule in each of 12 pentagonal openings. The E(1) components are anchored to the E(1)-binding domain of the E(2) subunits through an approximately 50-A-long linker. Three of these linkers emanate from the outside edges of the triangular base of the E(2) trimer and form a cage around its base that may shelter the lipoyl domains and the E(1) and E(2) active sites. The docking of the atomic structures of E(1) and the E(1) binding and lipoyl domains of E(2) in the electron microscopy map gives a good fit and indicates that the E(1) active site is approximately 95 A above the base of the trimer. We propose that the lipoyl domains and its tether (swinging arm) rotate about the E(1)-binding domain of E(2,) which is centrally located 45-50 A from the E(1), E(2), and E(3) active sites, and that the highly flexible breathing core augments the transfer of intermediates between active sites.     

Nucleotide sequence for yeast dihydrolipoamide dehydrogenase.

Rabbit antiserum to the dihydrolipoamide dehydrogenase (dihydrolipoamide:NAD+ oxidoreductase, EC 1.8.1.4) component of the pyruvate dehydrogenase complex from bakers' yeast was used to screen plaques produced by a lambda gt11 yeast cDNA library. A 2.1-kilobase insert was isolated that also hybridized to a 17-base mixed oligonucleotide probe corresponding to the amino-terminal sequence of the yeast dihydrolipoamide dehydrogenase. The cDNA has a coding sequence of 499 amino acids that corresponds to a 21-residue signal peptide and a 478-residue mature protein (Mr = 51,558). Computer analysis shows that yeast dihydrolipoamide dehydrogenase has about 41% amino acid identity with Escherichia coli dihydrolipoamide dehydrogenase. Particularly striking is the conservation of sequence in the active site region of the dihydrolipoamide dehydrogenases from E. coli, yeast, and pig heart.     

Reactivity of primary biliary cirrhosis sera with Escherichia coli dihydrolipoamide acetyltransferase (E2p): characterization of the main immunogenic region.

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial autoantibodies in the serum. The major antigens recognized by the antibodies are the E2 components of the 2-oxo acid dehydrogenase complexes, all of which possess covalently attached lipoic acid cofactors. A bacterial etiology has been proposed for the disease, and patients' antibodies are known to recognize the E2 subunits (E2p) of both mammalian and bacterial pyruvate dehydrogenase complexes. Immunoblotting and ELISA inhibition techniques using extracts of Escherichia coli deletion strains, genetically restructured E2 polypeptides, and isolated lipoyl domains demonstrate that (i) the E2o subunit of the E. coli 2-oxoglutarate dehydrogenase complex is recognized by patients' antibodies; (ii) the main immunogenic region of E2p lies within the lipoly domains; (iii) the presence of a lipoly residue within the domain is crucial for effective recognition by the antibodies; and (iv) octanoylated E2p, octanoylated E2o, and octanoylated lipoyl domain, produced by a mutant deficient in lipoate biosynthesis, are recognized by patients' antibodies but not as effectively as their lipoylated counterparts. These findings indicate that antibodies in PBC patients' sera bind to a unique peptide-cofactor conformation within the lipoyl domains of the E2 polypeptides and that this epitope is partially mimicked by substituting the lipoyl cofactor with an octanoyl group.     

Subunit structure of dihydrolipoyl transacetylase component of pyruvate dehydrogenase complex from Escherichia coli

Limited tryptic digestion of the pyruvate dehydrogenase complex of Escherichia coli or its dihydrolipoyl transacetylase core cleaves the trypsin-sensitive transacetylase subunits into two large fragments, A (lipoyl domain) and D (subunit binding domain). Release of fragments A from the complex does not significantly affect its sedimentation coefficient or its appearance in the electron microscope. Fragment A contains the lipoyl moieties ((3)H-labeled), is acidic with an apparent isoelectric point of about 4.0, has a M(r) of 31,600 as determined by sedimentation equilibrium analysis, and has a swollen or extended structure (f/f(o) = 1.78). Fragment A exhibits anomalous properties, probably due to its acidic nature. It is resistant to staining with Coomassie blue and it migrates on sodium dodecyl sulfate/polyacrylamide gels as if it had a M(r) of 46,000-48,000. Further tryptic digestion converts fragment A into a lipoyl-containing fragment of M(r) 20,000 (fragment B) and eventually into an apparently stable product of estimated M(r) about 10,000 (fragment C). Fragment D has a compact structure of M(r) about 29,600 as determined by sedimentation equilibrium analysis in 6 M guanidinium chloride, and it possesses the intersubunit binding sites of the transacetylase, the binding sites for pyruvate dehydrogenase and dihydrolipoyl dehydrogenase, and the catalytic site for transacetylation. The assemblage of fragments D is responsible for the cube-like appearance of the transacetylase in the electron microscope. High-resolution electron micrographs of the transacetylase show fiber-like extensions, apparently corresponding to tryptic fragment A, surrounding the cube-like core.     

Autoepitope mapping and reactivity of autoantibodies to the dihydrolipoamide dehydrogenase-binding protein (E3BP) and the glycine cleavage proteins in primary biliary cirrhosis

Primary biliary cirrhosis (PBC) is an autoimmune liver disease characterized by the presence of antimitochondrial antibodies (AMA) directed primarily against the E2 subunits of the pyruvate dehydrogenase complex, the branched chain 2-oxo-acid dehydrogenase complex, the 2-oxoglutarate dehydrogenase complex, as well as the dihydrolipoamide dehydrogenase-binding protein (E3BP) of pyruvate dehydrogenase complex. The autoantibody response to each E2 subunit is directed to the lipoic acid binding domain. However, hitherto, the epitope recognized by autoantibodies to E3BP has not been mapped. In this study, we have taken advantage of the recently available full-length human E3BP complementary DNA (cDNA) to map this epitope. In addition, another lipoic binding protein, the H-protein of the glycine cleavage complex, was also studied as a potential autoantigen recognized by AMA. Firstly, the sequence corresponding to the lipoic domain of E3BP (E3BP-LD) was amplified by polymerase chain reaction and recombinant protein and then purified. Immunoreactivity of 45 PBC sera (and 52 control sera) against the purified recombinant E3BP-LD was analyzed by enzyme-linked immunosorbent assay (ELISA) and immunoblotting. Secondly, reactivity of PBC sera was similarly analyzed by immunoblotting against H-protein. It is interesting that preabsorption of patient sera with the lipoic acid binding domain of E3BP completely removed all reactivity with the entire protein by immunoblotting analysis, suggesting that autoantibodies to E3BP are directed solely to its lipoic acid binding domain. Fifty-three percent of PBC sera reacted with E3BP-LD, with the majority of the response being of the immunoglobulin G (IgG) isotype (95%). Surprisingly, there was little IgM response to the E3BP-LD suggesting that the immune response was secondary because of determinant spreading. In contrast, H-protein does not appear to possess (or expose) autoepitopes recognized by PBC sera. This observation is consistent with structural data on this moiety.     

Molecular Structure of the Pyruvate Dehydrogenase Complex from Escherichia coli K-12

The pyruvate dehydrogenase core complex from E. coli K-12, defined as the multienzyme complex that can be obtained with a unique polypeptide chain composition, has a molecular weight of 3.75 x 10(6). All results obtained agree with the following numerology. The core complex consists of 48 polypeptide chains. There are 16 chains (molecular weight = 100,000) of the pyruvate dehydrogenase component, 16 chains (molecular weight = 80,000) of the dihydrolipoamide dehydrogenase component, and 16 chains (molecular weight = 56,000) of the dihydrolipoamide dehydrogenase component. Usually, but not always, pyruvate dehydrogenase complex is produced in vivo containing at least 2-3 mol more of dimers of the pyruvate dehydrogenase component than the stoichiometric ratio with respect to the core complex. This "excess" component is bound differently than are the eight dimers in the core complex.     

Cloning and nucleotide sequence of the gene for dihydrolipoamide acetyltransferase from Saccharomyces cerevisiae.

A 537-base cDNA encoding a portion of Saccharomyces cerevisiae dihydrolipoamide acetyltransferase (acetyl-CoA:dihydrolipoamide S-acetyltransferase, EC 2.3.1.12) was isolated from a lambda gt11 yeast cDNA library by immunoscreening. This cDNA was subcloned and used as a probe to screen a lambda gt11 yeast genomic DNA library. Two overlapping clones were used to determine the complete sequence of the acetyltransferase gene. The composite sequence has an open reading frame of 1446 nucleotides encoding a presequence of 28 amino acids and a mature protein of 454 amino acids (Mr = 48,546). The deduced amino acid sequence contains the experimentally determined amino acid sequences of the amino terminus and two internal peptide fragments of the acetyltransferase. Hybridization analysis of yeast genomic DNA showed that the gene has a single copy. A 915-base segment of the acetyltransferase gene hybridized to a yeast mRNA of approximately equal to 1.6 kilobases. Analysis of the deduced amino acid sequence of the dihydrolipoamide acetyltransferase revealed a multidomain structure similar to those reported for the corresponding acetyltransferases from Escherichia coli and rat liver, and extensive sequence similarity among the three enzymes. However, the yeast enzyme contains only one lipoyl domain, in contrast to three lipoyl domains reported for the E. coli enzyme and apparently two for the rat liver enzyme.     

Identification of two missense mutations in a dihydrolipoamide dehydrogenase-deficient patient.

The molecular basis of dihydrolipoamide dehydrogenase (E3; dihydrolipoamide:NAD+ oxidoreductase, EC 1.8.1.4) deficiency in an E3-deficient patient was studied. Fibroblasts cultured from the patient contained only approximately 6% of the E3 activity of cells from a normal subject. Western and Northern blot analyses indicated that, compared to control cells, the patient's cells had a reduced amount of protein but normal amounts of E3 mRNA. Direct sequencing of E3 cDNA derived from the patient's RNA as well as each of the subclones of the cDNA revealed that the patient had two substitution mutations in the E3 coding region. One mutation changed a single nucleotide from A to G, resulting in substitution of Glu (GAA) for Lys-37 (AAA). The other point mutation was a nucleotide change from C to T, resulting in the substitution of Leu (CTG) for Pro-453 (CCG). These mutations appear to be significant in that they alter the active site and possibly the binding of FAD.     

Autoantibodies in primary biliary cirrhosis: analysis of reactivity against eukaryotic and prokaryotic 2-oxo acid dehydrogenase complexes

Six components of the mammalian 2-oxo acid dehydrogenase complexes have previously been identified as M2 autoantigens in primary biliary cirrhosis. In this report, we present data showing that both polypeptide-specific and cross-reacting antibodies are present in patients' sera. Antibodies reacting with E2 of the pyruvate dehydrogenase complex cross-react with protein X but not with any other mammalian antigen. The main immunogenic region on protein X has been localized to within its single lipoyl domain. Polypeptide-specific antibodies bind to E1 alpha and E1 beta of the pyruvate dehydrogenase complex. Antibodies reacting with the E2 polypeptides of the 2-oxoglutarate dehydrogenase complex and branched-chain 2-oxo acid dehydrogenase complex show some cross-reactivity but do not recognize any of the antigens of the pyruvate dehydrogenase complex. Antibodies against the E2 component of the mammalian pyruvate dehydrogenase complex cross-react effectively with the corresponding protein from yeast but not with E2 from Escherichia coli. Antibody titer against mammalian antigens is significantly higher than against the bacterial antigens, arguing against a bacterial origin for primary biliary cirrhosis.     

Selective identification of new therapeutic targets of Mycobacterium tuberculosis by IVIAT approach

The in vivo induced antigen technology (IVIAT)(1) has been used for the identification of open reading frames (ORFs) which could be possible therapeutic targets. A recombinant lambdagt11:: Mycobacterium tuberculosis H37Rv expression library was screened with pooled TB patient sera preabsorbed with in vitro grown M. tuberculosis H37Rv. Preabsorption of pooled TB patient sera allowed identification of antigens specifically expressed or upregulated during infection and growth in vivo. Six ORFs were identified, of which four (rv0287, rv2402, rv3878 and rv1045) were of hypothetical functions. Rv0287 is a probable regulatory protein. Rv3878 is present uniquely in M. tuberculosis H37Rv and is a part of RDI deletion region of M. bovis BCG, which includes esat 6 region. This could be exploited as a tool for diagnosis. Two ORFs were assigned function solely on the basis of homology, dnaQ (rv3711c) and lpdA (rv3303c). dnaQ codes for the epsilon subunit of DNA polymerase III, which is responsible for the proofreading activity of the complex. lpdA codes for dihydrolipoamide dehydrogenase, which is a part of many multienzyme complexes such as pyruvate dehydrogenase, keto-acid dehydrogenase and alpha-ketoglutarate dehydrogenase. These two enzymes appear to be potential targets for drug development.     

Variant tricarboxylic acid cycle in Mycobacterium tuberculosis: identification of alpha-ketoglutarate decarboxylase

Mycobacterium tuberculosis (Mtb) has adapted its metabolism for persistence in the human macrophage. The adaptations are likely to involve Mtb's core intermediary metabolism, whose enzymes have been little studied. The tricarboxylic acid cycle is expected to yield precursors for energy, lipids, amino acids, and heme. The genome sequence of Mtb H37Rv predicts the presence of a complete tricarboxylic acid cycle, but we recently found that alpha-ketoglutarate dehydrogenase (KDH) activity is lacking in Mtb lysates. Here we showed that citrate synthase, aconitase, isocitrate dehydrogenase, fumarase, malate dehydrogenase, and succinate dehydrogenase, but not KDH, are present, raising the possibility of separate oxidative and reductive half-cycles. As a potential link between the half-cycles, we found that Rv1248c, annotated as encoding SucA, the putative E1 component of KDH, instead encodes alpha-ketoglutarate decarboxylase (Kgd) and produces succinic semialdehyde. Succinic semialdehyde dehydrogenase activity was detected in Mtb lysates and recapitulated with recombinant proteins GabD1 (encoded by Rv0234c) and GabD2 (encoded by Rv1731). Kgd and GabD1 or GabD2 form an alternative pathway from alpha-ketoglutarate to succinate. Rv1248c, which is essential or required for normal growth of Mtb [Sassetti, C., Boyd, D. H. & Rubin, E. J. (2003) Mol. Microbiol 48, 77-84] is the first gene shown to encode a Kgd. Kgd is lacking in humans and may represent a potential target for chemotherapy of tuberculosis.     

Mitochondrial stress protein recognition of inactivated dehydrogenases during mammalian cell death

The mammalian renal toxicant tetrafluoroethylcysteine (TFEC) is metabolized to a reactive intermediate that covalently modifies the lysine residues of a select group of mitochondrial proteins, forming difluorothioamidyl lysine protein adducts. Cellular damage is initiated by this process and cell death ensues. NH₂-terminal sequence analysis of purified mitochondrial proteins containing difluorothioamidyl lysine adducts identified the lipoamide succinyltransferase and dihydrolipoamide dehydrogenase subunits of the α-ketoglutarate dehydrogenase complex (αKGDH), a key regulatory component of oxidative metabolism, as targets for TFEC action. Adduct formation resulted in marked inhibition of αKGDH enzymatic activity, whereas the related pyruvate dehydrogenase complex was unmodified by TFEC and its activity was not inhibited in vivo. Covalent modification of αKGDH subunits also resulted in interactions with mitochondrial chaperonin HSP60 in vivo and with HSP60 and mitochondrial HSP70 in vitro. These observations confirm the role of mammalian stress proteins in the recognition of abnormal proteins and provide supporting evidence for reactive metabolite-induced cell death by modification of critical protein targets.     

Identification and analysis of the major M2 autoantigens in primary biliary cirrhosis.

Primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by the presence of antimitochondrial antibodies in the serum. It is possible that the PBC-specific immunoreactive trypsin-sensitive antigens on the inner mitochondrial membrane, termed M2, are important in the pathogenesis of this autoimmune disease. We have previously shown that a major M2"a" antigen is the E2 component of the pyruvate dehydrogenase multienzyme complex located within mitochondria. Analysis of the primary structure of the E2 components of all three 2-oxo acid dehydrogenase complexes reveals a high degree of homology with a similar highly segmented structure including lipoyl domains, E3-binding domains, C-terminal catalytic domains, and interdomain linker sequences. Immunoblotting of PBC patients' sera against purified E2 protein from 2-oxoglutarate dehydrogenase complex and branched-chain 2-oxo acid dehydrogenase complex reveals that these polypeptides are also autoantigens in this disease. Sera from 29 of 40 (72.5%) PBC patients gave a positive response against bovine 2-oxoglutarate dehydrogenase complex E2 and from 25 of 40 (62.5%) PBC patients gave a positive response against bovine branched-chain 2-oxo acid dehydrogenase complex E2. All 40 PBC patients (100%) have autoantibodies directed against at least one of the E2 components of the family of 2-oxo acid dehydrogenase complexes. Identification of these M2 mitochondrial autoantigens and detailed knowledge of their structure will allow important questions concerning this autoimmune disease to be addressed.     

Rapid intramolecular coupling of active sites in the pyruvate dehydrogenase complex of Escherichia coli: Mechanism for rate enhancement in a multimeric structure

In the absence of CoA and presence of pyruvate, the lipoic acid residues covalently bound to the lipoate acetyltransferase core component (acetyl-CoA:dihydrolipoate S-acetyltransferase, EC 2.3.1.12) of the pyruvate dehydrogenase multienzyme complex of Escherichia coli become reductively acetylated. A study of a series of reassembled complexes varying only in their content of pyruvate decarboxylase [pyruvate:lipoate-oxidoreductase (decarboxylating and acceptor-acetylating) EC 1.2.4.1] showed that the initial direct reductive acetylation of lipoic acid residues can be followed by extensive intramolecular transacetylation reaction between lipoic acid residues on neighboring polypeptide chains of the lipoate acetyltransferase core [Bates, D. L., Danson, M. J., Hale, G., Hooper, E. A. & Perham, R. N. (1977) Nature (London) 268, 313-316]. Pulsed-quenched-flow measurements of the rates of the acetylation reactions in the various complexes now demonstrate that the intramolecular transacetylation reactions are not rate-determining in the normal reaction mechanism of the enzyme. There is therefore the potential for rapid multiple coupling of active sites in the lipoate acetyltransferase core. The rate constant for the overall complex reaction, measured by stopped-flow fluorimetry, is found to be approximately twice that for the reductive acetylation reaction measured by pulsed-quenched flow. This result could mean that CoA is an allosteric stimulator of the reductive acetylation part of the overall reaction or that there are two active sites on each chain of the lipoate acetyltransferase component working in parallel. A system of rapid functional connection of active sites in a multienzyme complex ensures that sequential reactions can be successfully coupled even under conditions of low substrate concentrations for the different steps. The substantial rate enhancement thus achieved offers a plausible explanation for the unusual complexity of the quaternary structure of the enzyme.     

Antibodies against mitochondrial dehydrogenase complexes in primary biliary cirrhosis

Antimitochondrial antibodies, serological hallmarks of primary biliary cirrhosis, recently were found to be directed against the E2 subunits of mitochondrial dehydrogenase complexes (pyruvate, branched-chain ketoacid, and alpha-ketoglutarate dehydrogenases). The objectives of this study were to extend these findings and to determine whether purified immunoglobulin from the sera of patients with primary biliary cirrhosis inhibit activity of these dehydrogenase complexes in vitro. Sera were examined from 14 patients with primary biliary cirrhosis (13 mitochondrial antibody positive), 23 with rheumatic diseases and 30 with chronic active hepatitis (all 53 positive for mitochondrial antibodies by indirect immunofluorescence), 10 with alcoholic liver disease, and 5 normal controls. Antibodies against pyruvate dehydrogenase, branched-chain alpha-ketoacid dehydrogenase and alpha-ketoglutarate dehydrogenase complexes were detected by immunoblot and quantified by enzyme-linked immunosorbent assay. Of the 14 serum samples obtained from patients with primary biliary cirrhosis, 13, 11, and 2 samples tested positive by immunoblot for the E2 subunits of pyruvate, branched-chain ketoacid, and alpha-ketoglutarate dehydrogenase, respectively. In contrast, samples from subjects with rheumatic diseases, chronic active hepatitis, and alcoholic liver disease and control subjects tested negative for these antibodies. Serum immunoglobulin G with high titers of mitochondrial antibodies showed concentration-dependent inhibition of activity of the dehydrogenase complexes, and close correlation (r = 0.917, n = 13) was observed between inhibitory activity against pyruvate dehydrogenase complex and the reciprocal titer of immunoglobulin against this complex. These data suggest that such autoantibodies, besides serving as diagnostic markers for primary biliary cirrhosis, may have a pathogenic role by their ability to inhibit important mitochondrial enzymes.     

Epitope mapping on E1α subunit of pyruvate dehydrogenase complex with autoantibodies of patients with primary biliary cirrhosis

Background: A major mitochondrial autoantigen recognized by sera of patients with primary biliary cirrhosis (PBC) is dihydrolipoamide acetyltransferase (E2) of the pyruvate dehydrogenase complex (PDH). The α subunit of pyruvate decarboxylase (E1α) of PDH is also recognized in some E2‐reactive PBC sera, suggesting that the occurrence of autoimmunity against E1α is subsequent to that against E2. Methods: To investigate the mechanism inducing autoimmunity against E1α, we surveyed immunoreactive sequences of E1α by ELISA with synthesized oligopeptides, and determined minimum amino acid residues for each determinant. Results: The major determinants of E1α appeared to reside in its N‐terminal region, apparently forming ‘nested epitopes’, and all E1α‐reactive PBC sera tested recognized these regions. Minor epitopes were also found scattered throughout the entire sequence. The reactivities of these minor epitopes to individual PBC sera were proportional to those of the major epitopes. All the epitopes were located in hydrophilic regions of E1α, and many of them were out of the known functional domains (TPP‐binding domain, subunit interaction site, and phosphorylation sites) whose structures are phylogenically well conserved. Furthermore, the sequences of many epitopes appeared to be specific to humans. Conclusion: These observations suggest that determinant spreading might underlie the autoimmunity against E1α.     

A zebrafish model for pyruvate dehydrogenase deficiency: rescue of neurological dysfunction and embryonic lethality using a ketogenic diet

Defects in the pyruvate dehydrogenase (PDH) complex result in severe neurological dysfunction, congenital lactic acidosis, growth retardation, and early death. Current treatments for PDH deficiency are administered postnatally and are generally unsuccessful. Because many patients with this disease are born with irreversible defects, a model system for the development of effective pre- and postnatal therapies would be of great value. In a behavioral genetic screen aimed to identify zebrafish with visual function defects, we previously isolated two alleles of the recessive lethal mutant no optokinetic response a (noa). Here we report that noa is deficient for dihydrolipoamide S-acetyltransferase (Dlat), the PDH E2 subunit, and exhibits phenotypes similar to human patients with PDH deficiency. To rescue the deficiency, we added ketogenic substrates to the water in which the embryos develop. This treatment successfully restored vision, promoted feeding behavior, reduced lactic acidosis, and increased survival. Our study demonstrates an approach for establishing effective therapies for PDH deficiency and other congenital diseases that affect early embryonic development.