Pyruvate dehydrogenase complex from baker's yeast. 2. Molecular structure, dissociation, and implications for the origin of mitochondria

Integrative structure of a 10-megadalton eukaryotic pyruvate dehydrogenase complex from native cell extracts

The pyruvate dehydrogenase complex (PDHc) is a giant enzymatic assembly involved in pyruvate oxidation. PDHc components have been characterized in isolation, but the complex's quaternary structure has remained elusive due to sheer size, heterogeneity, and plasticity. Here, we identify fully assembled Chaetomium thermophilum α-keto acid dehydrogenase complexes in native cell extracts and characterize their domain arrangements utilizing mass spectrometry, activity assays, crosslinking, electron microscopy (EM), and computational modeling. We report the cryo-EM structure of the PDHc core and observe unique features of the previously unknown native state. The asymmetric reconstruction of the 10-MDa PDHc resolves spatial proximity of its components, agrees with stoichiometric data (60 E2p:12 E3BP:∼20 E1p: ≤ 12 E3), and proposes a minimum reaction path among component enzymes. The PDHc shows the presence of a dynamic pyruvate oxidation compartment, organized by core and peripheral protein species. Our data provide a framework for further understanding PDHc and α-keto acid dehydrogenase complex structure and function.

An Asymptotic Analysis of the Malonyl-CoA Route to 3-Hydroxypropionic Acid in Genetically Engineered Microbes

There has been recent interest in creating an efficient microbial production route for 3-hydroxypropionic acid, an important platform chemical. We develop and solve a mathematical model for the time-dependent metabolite concentrations in the malonyl-CoA pathway for 3-hydroxypropionic acid production in microbes, using a combination of numerical and asymptotic methods. This allows us to identify the most important targets for enzyme regulation therein under conditions of plentiful and sparse pyruvate, and to quantify their relative importance. In our model, we account for sinks of acetyl-CoA and malonyl-CoA to, for example, the citric acid cycle and fatty acid biosynthesis, respectively. Notably, in the plentiful pyruvate case we determine that there is a bifurcation in the asymptotic structure of the system, the crossing of which corresponds to a significant increase in 3-hydroxypropionic acid production. Moreover, we deduce that the most significant increases to 3-hydroxypropionic acid production can be obtained by up-regulating two specific enzymes in tandem, as the inherent nonlinearity of the system means that a solo up-regulation of either does not result in large increases in production. The types of issue arising here are prevalent in synthetic biology applications, and it is hoped that the system considered provides an instructive exemplar for broader applications.

Applying asymptotic methods to synthetic biology: Modelling the reaction kinetics of the mevalonate pathway

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We investigate a kinetic model for the mevalonate pathway which includes inhibition effects and a sink of acetyl-CoA.

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Of the enzymes in the pathway, upregulating HMG-CoA reductase has the most significant positive effect on improving pathway efficiency.

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Upregulating pyruvate dehydrogenase complex and HMG-CoA synthase can also help, but only in conjunction with the upregulation of HMG-CoA reductase.

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We confirm our theoretical predictions by introducing the mevalonate pathway into Cupriavidus necator.

The mevalonate pathway is normally found in eukaryotes, and allows for the production of isoprenoids, a useful class of organic compounds. This pathway has been successfully introduced to Escherichia coli, enabling a biosynthetic production route for many isoprenoids. In this paper, we develop and solve a mathematical model for the concentration of metabolites in the mevalonate pathway over time, accounting for the loss of acetyl-CoA to other metabolic pathways. Additionally, we successfully test our theoretical predictions experimentally by introducing part of the pathway into Cupriavidus necator. In our model, we exploit the natural separation of time scales as well as of metabolite concentrations to make significant asymptotic progress in understanding the system. We confirm that our asymptotic results agree well with numerical simulations, the former enabling us to predict the most important reactions to increase isopentenyl diphosphate production whilst minimizing the levels of HMG-CoA, which inhibits cell growth. Thus, our mathematical model allows us to recommend the upregulation of certain combinations of enzymes to improve production through the mevalonate pathway.

Strep-tag II and Twin-Strep based cassettes for protein tagging by homologous recombination and characterization of endogenous macromolecular assemblies in Saccharomyces cerevisiae

Peptide sequences fused to a gene of interest facilitate the isolation of proteins or protein complexes from cell extracts. In the case of fluorescent protein tags, the tagged protein can be visually localized in living cells. To tag endogenous genes, PCR-based homologous recombination is a powerful approach used in the yeast Saccharomyces cerevisiae. This approach uses short, homologous DNA sequences that flank the tagging cassette to direct recombination. Here, we constructed a set of plasmids, whose sequences were optimized for codon usage in yeast, for Strep-tag II and Twin-Strep tagging in S. cerevisiae. Some plasmids also contain sequences encoding for a fluorescent protein followed by the purification tag. We demonstrate using the yeast pyruvate dehydrogenase (PDH) complex that these plasmids can be used to purify large protein complexes efficiently. We furthermore demonstrate that purification from the endogenous pool using the Strep-tag system results in functionally active complexes. Finally, using the fluorescent tags, we show that a kinase and a phosphatase involved in regulating the activity of the PDH complex localize in the cells' mitochondria. In conclusion, our cassettes can be used as tools for biochemical, functional, and structural analyses of endogenous multi-protein assemblies in yeast.

Peroxisomes are juxtaposed to strategic sites on mitochondria

Peroxisomes are ubiquitous and dynamic organelles that house many important pathways of cellular metabolism. In recent years it has been demonstrated that mitochondria are tightly connected with peroxisomes and are defective in several peroxisomal diseases. Indeed, these two organelles share metabolic routes as well as resident proteins and, at least in mammals, are connected via a vesicular transport pathway. However the exact extent of cross-talk between peroxisomes and mitochondria remains unclear. Here we used a combination of high throughput genetic manipulations of yeast libraries alongside high content screens to systematically unravel proteins that affect the transport of peroxisomal proteins and peroxisome biogenesis. Follow up work on the effector proteins that were identified revealed that peroxisomes are not randomly distributed in cells but are rather localized to specific mitochondrial subdomains such as mitochondria-ER junctions and sites of acetyl-CoA synthesis. Our approach highlights the intricate geography of the cell and suggests an additional layer of organization as a possible way to enable efficient metabolism. Our findings pave the way for further studying the machinery aligning mitochondria and peroxisomes, the role of the juxtaposition, as well as its regulation during various metabolic conditions. More broadly, the approaches used here can be easily applied to study any organelle of choice, facilitating the discovery of new aspects in cell biology.

Participation of acetaldehyde dehydrogenases in ethanol and pyruvate metabolism of the yeast Saccharomyces cerevisiae

This work was undertaken to clarify the role of acetaldehyde dehydrogenases in Saccharomyces cerevisiae metabolism during growth on respiratory substrates. Until now, there has been little agreement concerning the ability of mutants deleted in gene ALD4, encoding mitochondrial acetaldehyde dehydrogenase, to grow on ethanol. Therefore we constructed mutants in two parental strains (YPH499 and W303-1a). Some differences appeared in the growth characteristics of mutants obtained from these two parental strains. For these experiments we used ethanol, pyruvate or lactate as substrates. Mitochondria can oxidize lactate into pyruvate using an ATP synthesis-coupled pathway. The ald4Delta mutant derived from the YPH499 strain failed to grow on ethanol, but growth was possible for the ald4Delta mutant derived from the W303-1a strain. The co-disruption of ALD4 and PDA1 (encoding subunit E1alpha of pyruvate dehydrogenase) prevented the growth on pyruvate for both strains but prevented growth on lactate only in the double mutant derived from the YPH499 strain, indicating that the mutation effects are strain-dependent. To understand these differences, we measured the enzyme content of these different strains. We found the following: (a) the activity of cytosolic acetaldehyde dehydrogenase in YPH499 was relatively low compared to the W303-1a strain; (b) it was possible to restore the growth of the mutant derived from YPH499 either by addition of acetate in the media or by introduction into this mutant of a multicopy plasmid carrying the ALD6 gene encoding cytosolic acetaldehyde dehydrogenase. Therefore, the lack of growth of the mutant derived from the YPH499 strain seemed to be related to the low activity of acetaldehyde oxidation. Therefore, when cultured on ethanol, the cytosolic acetaldehyde dehydrogenase can partially compensate for the lack of mitochondrial acetaldehyde dehydrogenase only when the activity of the cytosolic enzyme is sufficient. However, when cultured on pyruvate and in the absence of pyruvate dehydrogenase, the cytosolic acetaldehyde dehydrogenase cannot compensate for the lack of the mitochondrial enzyme because the mitochondrial form produces intramitochondrial NADH and consequently ATP through oxidative phosphorylation.

Plant mitochondrial 2-oxoglutarate dehydrogenase complex: purification and characterization in potato

The 2-oxoglutarate dehydrogenase complex (OGDC) in potato (Solanum tuberosum cv. Romano) tuber mitochondria is largely associated with the membrane fraction of osmotically ruptured organelles, whereas most of the other tricarboxylic acid cycle enzymes are found in the soluble matrix fraction. The purification of OGDC from either membrane or soluble matrix fractions resulted in the increasing dependence of its activity on the addition of dihydrolipoamide dehydrogenase (E3). A 30-fold purification of OGDC to apparent homogeneity and with a specific activity of 4.6 micromol/min per mg of protein in the presence of exogenously added E3 was obtained. SDS/PAGE revealed that the purified complex consisted of three major polypeptides with apparent molecular masses of 48, 50 and 105 kDa. Before the gel-filtration purification step, E3 polypeptides of 57 and 58 kDa were identified by immunoreaction as minor proteins associated with OGDC. The N-terminal sequence of the 57 kDa protein was identical with that previously purified as the E3 component of the pyruvate dehydrogenase complex from potato. The 105 kDa protein was identified as the 2-oxoglutarate dehydrogenase subunit of OGDC by N-terminal sequencing. The N-terminal sequences of the 50 and 48 kDa proteins shared 90-95% identity over 20 residues and were identified by sequence similarity as dihydrolipoamide succinyltransferases (OGDC-E2). The incubation of OGDC with [U-(14)C]2-oxoglutarate resulted in the reversible succinylation of both the 48 and the 50 kDa protein bands. Proteins previously reported as subunits of complex I of the respiratory chain from Vicia faba and Solanum tuberosum are proposed to be OGDC-E2 and the possible basis of this association is discussed.

Characterization of the dihydrolipoamide acetyltransferase of the mitochondrial pyruvate dehydrogenase complex from potato and comparisons with similar enzymes in diverse plant species

The pyruvate dehydrogenase complex (mPDC) from potato (Solanum tuberosum cv. Romano) can be disassociated in 1 M NaCl and 0.1 M glycine into a large dihydrolipoamide acetyltransferase (E2) complex and smaller pyruvate dehydrogenase (E1) and dihydrolipoamide dehydrogenase (E3) complexes. The E2 complex consists of 55 and 78-kDa polypeptides which are reversibly radiolabelled to a similar degree in the intact mPDC by [2-14C]pyruvate. Affinity-purified antibodies against the 55-kDa protein do not cross-react with the 78-kDa protein and the two proteins show different peptide patterns following partial proteolysis. The 78 and 55-kDa proteins are present in approximately equal abundance in the E2 complex and incorporate a similar amount of [14C] on incubation with [2-14C]pyruvate. Native mPDC and the E2 complex have sedimentation coefficients of 50S and 30S, respectively. Titration of electro-eluted polypeptides against the intact mPDC and E2 complex revealed that each mg of mPDC contains 0.4 mg of E1, 0.4 mg of E2 and 0.2 mg of E3. Labelling of partially purified mPDC from potato, pea, cauliflower, maize and barley, with [2-14C]pyruvate, suggest that a 78-kDa acetylatable protein is only found in the dicotyledonous species, while all plant species tested contained a smaller 52-60 kDa acetylatable protein.

Plant mitochondrial pyruvate dehydrogenase complex: purification and identification of catalytic components in potato

The pyruvate dehydrogenase complex (mPDC) from potato (Solanum tuberosum cv. Romano) tuber mitochondria was purified 40-fold to a specific activity of 5.60 micromol/min per mg of protein. The activity of the complex depended on pyruvate, divalent cations, NAD+ and CoA and was competitively inhibited by both NADH and acetyl-CoA. SDS/PAGE revealed the complex consisted of seven polypeptide bands with apparent molecular masses of 78, 60, 58, 55, 43, 41 and 37 kDa. N-terminal sequencing revealed that the 78 kDa protein was dihydrolipoamide transacetylase (E2), the 58 kDa protein was dihydrolipoamide dehydrogenase (E3), the 43 and 41 kDa proteins were alpha subunits of pyruvate dehydrogenase, and the 37 kDa protein was the beta subunit of pyruvate dehydrogenase. N-terminal sequencing of the 55 kDa protein band yielded two protein sequences: one was another E3; the other was similar to the sequence of E2 from plant and yeast sources but was distinctly different from the sequence of the 78 kDa protein. Incubation of the mPDC with [2-14C]pyruvate resulted in the acetylation of both the 78 and 55 kDa proteins.

Pyruvate metabolism in Saccharomyces cerevisiae

In yeasts, pyruvate is located at a major junction of assimilatory and dissimilatory reactions as well as at the branch-point between respiratory dissimilation of sugars and alcoholic fermentation. This review deals with the enzymology, physiological function and regulation of three key reactions occurring at the pyruvate branch-point in the yeast Saccharomyces cerevisiae: (i) the direct oxidative decarboxylation of pyruvate to acetyl-CoA, catalysed by the pyruvate dehydrogenase complex, (ii) decarboxylation of pyruvate to acetaldehyde, catalysed by pyruvate decarboxylase, and (iii) the anaplerotic carboxylation of pyruvate to oxaloacetate, catalysed by pyruvate carboxylase. Special attention is devoted to physiological studies on S. cerevisiae strains in which structural genes encoding these key enzymes have been inactivated by gene disruption.

Pyruvate dehydrogenase complex from the primitive insect trypanosomatid, Crithidia fasciculata: dihydrolipoyl dehydrogenase-binding protein has multiple lipoyl domains

The pyruvate dehydrogenase complex (PDC) has been purified to apparent homogeneity from the insect trypanosomatid, Crithidia fasciculata, a member of the most primitive eukaryotic group to contain mitochondria. Separation of the purified PDC by SDS-PAGE yielded five bands of 70 (p70), 60 (p60), 55, 46 and 36.5 kDa, which appeared to correspond to dihydrolipoyl dehydrogenase binding protein (E3BP), dihydrolipoyl transacetylase (E2), E3, E1 alpha and E1 beta, respectively. The purified complex did not exhibit endogenous PDHa kinase activity. p70 was much less abundant than p60. Polyclonal antisera raised against p70 did not cross-react with p60, and antisera raised against p60 did not cross-react with p70, suggesting that p60 did not arise from p70 by proteolysis. Both p70 and p60 contained similar amino terminal sequences. Both sequences contained the MPALSP motif similar to sequences present in both E3BP and E2 from other sources. Incubation of the purified PDC with [2-14C]pyruvate in the absence of CoA resulted in the acetylation of both p70 and p60, suggesting that both proteins contained lipoyl domains, but the specific incorporation of label into p70 was significantly greater than for p60. Limited proteolysis of the acetylated complex with trypsin yielded two major fragments derived from p60 of 35 and 30 kDa, corresponding to E2L and E2I, and one major acetylated fragment of 58 kDa derived from p70. Therefore, these results suggest that p70 is an E3BP and given its apparent M(r) and degree of acetylation, it contains multiple lipoyl domains.

The pyruvate dehydrogenase complex of Saccharomyces cerevisiae is regulated by phosphorylation

Mitochondria were isolated from Saccharomyces cerevisiae grown on different carbon sources prior to incubation with [gamma-32P]ATP. A major 46,000-M(r) phosphoprotein, corresponding in M(r) value to the E1 alpha subunit of the yeast pyruvate dehydrogenase complex (PDC), was detected only in mitochondria isolated from cells grown on a fermentable carbon source such as galactose. Immunoprecipitation with subunit-specific antiserum to the E1 component of mammalian or yeast PDC confirmed the identity of this polypeptide. PDC activity in isolated yeast mitochondria could be inactivated in an ATP-dependent fashion and reactivated in the presence of Ca2+ ions.

Isolation and characterisation of the pyruvate dehydrogenase complex of anaerobically grown Enterococcus faecalis NCTC 775

In this contribution the isolation and some of the structural and kinetic properties of the pyruvate dehydrogenase complex (PDC) of anaerobically grown Enterococcus faecalis are described. The complex closely resembles the PDC of other Gram-positive bacteria and eukaryotes. It consists of four polypeptide chains with apparent molecular masses on SDS/PAGE of 97, 55, 42 and 36 kDa, and these polypeptides could be assigned to dihydrolipoyl transacetylase (E2), lipoamide dehydrogenase (E3) and the two subunits of pyruvate dehydrogenase (E1 alpha and E1 beta), respectively. The E2 core has an icosahedral symmetry. The apparent molecular mass on SDS/PAGE of 97 kDa of the E2 chain is extremely high in comparison with other Gram-positive organisms (and eukaryotes) and probably due to several lipoyl domains associated with the E2 chain. NADH inhibition is mediated via E3. The mechanism of inhibition is discussed in view of the high PDC activities in vivo that are found in E. faecalis, grown under anaerobic conditions.

Cellular concentrations of enzymes and their substrates

The activity of crude and pure enzyme preparations as well as the molecular weight of these enzymes were obtained from the literature for several organisms. From these data enzyme concentrations were calculated and compared to the concentration(s) of their substrates in the same organism. The data are expressed as molar ratios of metabolite concentration to enzyme site concentration. Of the 140 ratios calculated, 88% were one or greater, indicating that in general substrates exceed their cognate enzyme concentrations. Of the 17 cases where enzyme exceeds metabolite concentration, 16 were in glycolysis. The data in general justify the use of enzyme kinetic mechanisms determined in vitro in the construction of dynamic models which simulate in vivo metabolism.

Dihydrolipoamide dehydrogenase: functional similarities and divergent evolution of the pyridine nucleotide-disulfide oxidoreductases

Dihydrolipoamide dehydrogenase (E3) is the common component of the three alpha-ketoacid dehydrogenase complexes oxidizing pyruvate, alpha-ketoglutarate, and the branched-chain alpha-ketoacids. E3 also participates in the glycine cleavage system. E3 belongs to the enzyme family called pyridine nucleotide-disulfide oxidoreductases, catalyzing the electron transfer between pyridine nucleotides and disulfide compounds. This review summarizes the information available for E3 from a variety of species, from a halophilic archaebacterium which has E3 but no alpha-ketoacid dehydrogenase complexes, to mammalian species. Evidence is reviewed for the existence of two E3 isozymes (one for pyruvate dehydrogenase complex and alpha-ketoglutarate dehydrogenase complex and the other for branched-chain alpha-ketoacid dehydrogenase complex) in Pseudomonas species and for possible mammalian isozymes of E3, one associated with the three alpha-ketoacid dehydrogenase complexes and one for the glycine cleavage system. The comparison of the complete amino acid sequences of E3 from Escherichia coli, yeast, pig, and human shows considerable homologies of certain amino acid residues or short stretches of sequences, especially in the specific catalytic and structural domains. Similar homology is found with the limited available amino acid sequence information on E3 from several other species. Sequence comparison is also presented for other member flavoproteins [e.g., glutathione reductase and mercury(II) reductase] of the pyridine nucleotide-disulfide oxidoreductase family. Based on the known tertiary structure of human glutathione reductase it may be possible to predict the domain structures of E3. Additionally, the sequence information may help to better understand a divergent evolutionary relationship among these flavoproteins in different species.

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.

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.

Lipoic acid is the site of substrate-dependent acetylation of component X in ox heart pyruvate dehydrogenase multienzyme complex

The recently characterized Mr-50000 polypeptide associated with mammalian pyruvate dehydrogenase complex, referred to as component or protein X, was shown to incorporate N-ethylmaleimide only in the presence of pyruvate or NADH. Component X, modified with N-ethyl[2,3-14C]maleimide in the presence of pyruvate, was isolated and subjected to acid hydrolysis. The radioactive products were resolved on an amino acid analyser and these coeluted with products from similarly modified and hydrolysed lipoate acetyltransferase. Preincubation of pyruvate dehydrogenase complex with pyruvate or NADH and acetyl-CoA resulted in a time-dependent diminution of incorporation of radiolabelled N-ethylmaleimide into component X and lipoate acetyltransferase and, correspondingly, in the extent of inhibition of overall complex activity by N-ethylmaleimide.

The size of the pyruvate dehydrogenase complex of Azotobacter vinelandii. Association phenomena

Sedimentation analysis and light-scattering studies indicate that the aggregation state of the pyruvate dehydrogenase complex of Azotobacter vinelandii in 50 mM potassium phosphate (pH 7.0) can be described in terms of a monomer-dimer equilibrium with a dissociation constant of 6.8 microM. The apparent molecular mass of the monomeric particle is 750 000-850 000 Da. The equilibrium is shifted to the monomeric species when pressure is applied on the system. Pressure-jump experiments yielded a relaxation time of about 70 ms. In the presence of 3% poly(ethylene glycol) 6000 and 10 mM MgCl2, further association takes place to a system that can be described in terms of dimer-tetramer-octamer equilibria. Upon applying a pressure of 80 MPa to this system these equilibria are shifted to the dimeric state but some monomer formation cannot be excluded. Release of pressure shows that the relaxation time of the dimer-tetramer equilibrium is less than 5 s, that of tetramer-octamer equilibrium is of the order of minutes. The isolated E2 component has a molecular mass of 2 000 000 +/- 100 000 Da; and thus consists of about 30 E2 peptide chains. Electron micrographs are similar to those of the E2 component of the Escherichia coli complex, which were interpreted as cubic structures with an octagonal symmetry. Upon addition of E1 to the pure E2 component, changes in the assembly occur and mixtures of large (E. coli-like, 22-45 S) and small (A. vinelandii-like, 11-18 S) subcomplexes are obtained. The two forms of the subcomplexes are in slow equilibrium (relaxation time 10-30 min). It is proposed that the E2 tetramer of the intact pyruvate dehydrogenase complex of A. vinelandii is represented by the corner structures of the isolated E2 component.

The synthesis of yeast pyruvate decarboxylase is regulated by large variations in the messenger RNA level

The yeast PDC1 gene coding for the fermentative enzyme pyruvate decarboxylase was isolated. This DNA sequence was used to identify the corresponding messenger RNA by hybridization. It could be shown that the synthesis of pyruvate decarboxylase is efficiently regulated by variations in the amount of PDC1 mRNA. Very low levels of PDC1 mRNA were found in cells growing in a medium containing ethanol. Glucose addition to these cells leads to a rapid accumulation of PDC1 mRNA. The PDC1 mRNA levels found in different mutants and in cells growing in media containing carbon sources other than glucose or ethanol suggest that the amount of PDC1 mRNA in yeast cells is affected by a number of different factors.

Pyruvate dehydrogenase complex from baker's yeast. 1. Purification and some kinetic and regulatory properties

Pyruvate dehydrogenase complex, for the first time, was highly purified from commercial baker's yeast (saccharomyces cerevisiae). Proteolytic degradation was prevented by the inclusion of the protease inhibitors pepstatin A, leupeptin, and phenylmethanesulfonyl fluoride during the enzyme purification. The yield from 1 kg of pressed yeast was about 15--20 mg enzyme with a specific activity of 17--30 U/mg. Most of the kinetic and regulatory properties of the yeast enzyme were found similar to those of the mammalian mitochondrial pyruvate dehydrogenase complexes except that Km for pyruvate, when assayed at the pH optimum, was much higher than in the mammalian complexes and resembled the values reported for the complexes of gram-negative bacteria. Furthermore, neither in yeast homogenates nor in the isolated yeast pyruvate dehydrogenase complex, was any evidence found for regulation by interconversion (phosphorylation-dephosphorylation) as occurs in mammals, plants, and Neurospora crassa.