Pyruvate dehydrogenase complex
Pyruvate dehydrogenase complex (PDC) is a complex of three enzymes that transform pyruvate into acetyl-CoA by a process called pyruvate decarboxylation. Acetyl-CoA may then be used in the citric acid cycle to carry out cellular respiration, and this complex links the glycolysis metabolic pathway to the citric acid cycle. Pyruvate decarboxylation is also known as the "pyruvate dehydrogenase reaction" because it also involves the oxidation of pyruvate.
This multi-enzyme complex is related structurally and functionally to the oxoglutarate dehydrogenase and branched-chain oxo-acid dehydrogenase multi-enzyme complexes.
- 1 Reaction
- 2 Structure & function in eukaryotes
- 3 Regulation
- 4 Localization of pyruvate decarboxylation
- 5 Structural differerences between species
- 6 See also
- 7 References
- 8 Related links
The reaction catalysed by pyruvate dehydrogenase complex is:
pyruvate pyruvate dehydrogenase complex acetyl CoA CoA-SH + NAD+ CO2 + NADH + H+
Structure & function in eukaryotes
Pyruvate dehydrogenase complex is organized in cubic symmetry in prokaryotes, having 60 subunits in three functional proteins. Pyruvate dehydrogenase complex is located in the mitochondrial matrix of eukaryotes. It is organized in dodecahedral symmetry, and consists of a total of 96 subunits, organized into three functional proteins in the human enzyme:
Enzyme Abbrev. Cofactor(s) # subunits prokaryotes # subunits eukaryotes pyruvate dehydrogenase
E1 TPP (thiamine pyrophosphate) 24 30 dihydrolipoyl transacetylase
24 60 dihydrolipoyl dehydrogenase
Initially, pyruvate and thiamine pyrophosphate (TPP or vitamin B1) are bound by Pyruvate dehydrogenase subunits. The thiazolium ring of TPP is in a zwitterionic form, and the anionic C2 carbon performs a nucleophilic attack on the C2 (ketone) carbonyl of pyruvate. The resulting hemithioacetal undergoes decarboxylation to produce an acyl anion equivalent (see cyanohydrin or aldehyde-dithiane umpolung chemistry, as well as benzoin condensation). This anion attacks S1 of an oxidized lipoate species that is attached to a lysine residue. In a ring-opening SN2-like mechanism, S2 is displaced as a sulfide or sulfhydryl moiety. Subsequent collapse of the tetrahedral hemithioacetal ejects thiazole, releasing the TPP cofactor and generating a thioacetate on S1 of lipoate. The E1-catalyzed process is the rate-limiting step of the whole pyruvate dehydrogenase complex.
Dihydrolipoyl transacetylase (E2)
At this point, the lipoate-thioester functionality is translocated into the dihydrolipoyl transacetylase (E2) active site, where a transacylation reaction transfers the acetyl from the "swinging arm" of lipoyl to the thiol of coenzyme A. This produces acetyl-CoA, which is released from the enzyme complex and subsequently enters the citric acid cycle. E2 can also be known as lipoamide reductase-transacetylase.
Dihydrolipoyl dehydrogenase (E3)
The dihydrolipoate, still bound to a lysine residue of the complex, then migrates to the dihydrolipoyl dehydrogenase (E3) active site where it undergoes a flavin-mediated oxidation, identical in chemistry to disulfide isomerase. First, FAD oxidizes dihydrolipoate back to its lipoate resting state, producing FADH2. Then, a NAD+ cofactor oxidizes FADH2 back to its FAD resting state, producing NADH.
- PDK phosphorylates three specific serine residues on E1 with different affinities. Phosphorylation of any one of them renders E1 (and in consequence the entire complex) inactive.
- Dephosphorylation of E1 by PDP reinstates complex activity.
Products of the reaction act as allosteric inhibitors of the PDC, because they activate PDK. Substrates in turn inhibit PDK, and thus, reactivating PDC.
During starvation, PDK increases in amount in most tissues, including skeletal muscle, via increased gene transcription. Under the same conditions, the amount of PDP decreases. The resulting inhibition of PDC prevents muscle and other tissues from catabolizing glucose and gluconeogenesis precursors. Metabolism shifts toward fat utilization, while muscle protein breakdown to supply gluconeogenesis precursors is minimized, and available glucose is spared for use by the brain.
Localization of pyruvate decarboxylation
In eukaryotic cells the Swanson conversion occurs inside the mitochondria, after transport of the substrate, pyruvate, from the cytosol. The transport of pyruvate into the mitochondria is via a transport protein and is active, consuming energy. Passive diffusion of pyruvate into the mitochondria is impossible because it carries a negative charge.
On entry to the mitochondria the pyruvate decarboxylation occurs, producing acetyl CoA. This irreversible reaction traps the acetyl CoA within the mitochondria (the acetyl-CoA can only be transported out of the mitochondrial matrix under conditions of high oxaloacetate via the citrate shuttle, a TCA intermediate that is normally sparse). The carbon dioxide produced by this reaction is nonpolar and small, and can diffuse out of the mitochondria and out of the cell.
In prokaryotes, which have no mitochondria, this reaction is either carried out in the cytosol, or not at all.
Structural differerences between species
PDC is a large complex composed of multiple copies of 3 or 4 subunits depending on species.
Up to 24 copies of Pyruvate dehydrogenase (E1) and 12 molecules of dihydrolipoyl dehydrogenase (E3) bind to the outside of the E2 core.
Gram-positive bacteria and eukaryotes
In contrast, in Gram-positive bacteria (e.g. Bacillus stearothermophilus) and eukaryotes the central PDC core contains 60 E2 molecules arranged into an icosahedron.
Eukaryotes also contain 12 copies of an additional core protein, E3 binding protein (E3BP). The exact location of E3BP is not completely clear. Cryo-electron microscopy has established that E3BP binds to each of the icosahedral faces in yeast. However, it has been suggested that it replaces an equivalent number of E2 molecules in the bovine PDC core.
Up to 60 E1 or E3 molecules can associate with the E2 core from Gram-positive bacteria - binding is mutually exclusive. In eukaryotes E1 is specifically bound by E2, while E3 associates with E3BP. It is thought that up to 30 E1 and 6 E3 enzymes are present, although the exact number of molecules can vary in vivo and often reflects the metabolic requirements of the tissue in question.
- ^ Mattevi, A., Obmolova, G., Schulze, E., Kalk, K.H., Westphal, A.H., de Kok, A., Hol, W.G. (1992) Atomic structure of the cubic core of the pyruvate dehydrogenase multienzyme complex. Science 255, 1544-1550.
- ^ Smolle, M., Prior, A.E., Brown, A.E., Cooper, A., Byron, O., Lindsay, J.G. (2006) A new level of architectural complexity in the human pyruvate dehydrogenase complex. J. Biol. Chem. 281, 19772-19780.
- ^ Stoops, J.K., Cheng, R.H., Yazdi, M.A., Maeng, C.Y., Schroeter, J.P., Klueppelberg, U., Kolodziej, S.J., Baker, T.S., Reed, L.J. (1997) On the unique structural organization of the Saccharomyces cerevisiae pyruvate dehydrogenase complex. J. Biol. Chem. 272, 5757-5764.
- http://www.dentistry.leeds.ac.uk/biochem/MBWeb/mb1/part2/krebs.htm#animat1 - animation of the general mechanism of the PDC (link on upper right) at University of Leeds
- MeSH Pyruvate+Dehydrogenase+Complex
- Zhou, H. .; Mccarthy, B. .; O'Connor, M. .; Reed, J. .; Stoops, K. . (Dec 2001). "The remarkable structural and functional organization of the eukaryotic pyruvate dehydrogenase complexes" (Free full text). Proceedings of the National Academy of Sciences of the United States of America 98 (26): 14802–14807. Bibcode 2001PNAS...9814802Z. doi:10.1073/pnas.011597698. ISSN 0027-8424. PMC 64939. PMID 11752427. http://www.pnas.org/cgi/pmidlookup?view=long&pmid=11752427. , bovine kidney pyruvate dehydrogenase complex
- Yu, X.; Hiromasa, Y.; Tsen, H.; Stoops, K.; Roche, E.; Zhou, H. (Jan 2008). "Structures of the Human Pyruvate Dehydrogenase Complex Cores: A Highly Conserved Catalytic Center with Flexible N-Terminal Domains". Structure 16 (1): 104–114. doi:10.1016/j.str.2007.10.024. ISSN 0969-2126. PMID 18184588. , human full-length and truncated E2 (tE2) cores of PDC, expressed in E. coli
Mitochondrial proteins Outer membrane Intermembrane space Inner membraneGlutamate aspartate transporter, Glycerol-3-phosphate dehydrogenase, ATP synthase, Carnitine palmitoyltransferase II, Uncoupling protein) Matrix
alcohol metabolism (ALDH2)PMPCB
Other/to be sorted Mitochondrial DNAtRNA (MT-TA, MT-TC, MT-TD, MT-TE, MT-TF, MT-TG, MT-TH, MT-TI, MT-TK, MT-TL1, MT-TL2, MT-TM, MT-TN, MT-TP, MT-TQ, MT-TR, MT-TS1, MT-TS2, MT-TT, MT-TV, MT-TW, MT-TY) Metabolism: Citric acid cycle enzymes Cycle Anapleroticto acetyl-CoAto succinyl-CoAto oxaloacetate Mitochondrial
electron transport chain/
Glycolysis Metabolic Pathway Glucose Hexokinase Glucose 6-phosphate Glucose-6-phosphate isomerase Fructose 6-phosphate phosphofructokinase-1 Fructose 1,6-bisphosphate Fructose bisphosphate aldolase ATP ADP ATP ADP Dihydroxyacetone phosphate Glyceraldehyde 3-phosphate Triosephosphate isomerase Glyceraldehyde 3-phosphate Glyceraldehyde-3-phosphate dehydrogenase 1,3-Bisphosphoglycerate NAD+ + Pi NADH + H+ + 2 2 Phosphoglycerate kinase 3-Phosphoglycerate Phosphoglycerate mutase 2-Phosphoglycerate Phosphopyruvate hydratase(Enolase) Phosphoenolpyruvate Pyruvate kinase Pyruvate ADP ATP H2O ADP ATP 2 2 2 2
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