Oxidative Decarboxylation (Conversion of Pyruvate To Acetyl CoA)

Oxidative Decarboxylation

(Conversion of Pyruvate To Acetyl CoA)

Transport of Pyruvate:

• To enter the matrix from the cytosol, pyruvate first diffuses through large openings in the outer mitochondrial membrane.
• Then is transported across the inner membrane by the mitochondrial pyruvate carrier (MPC), a passive transporter specific for pyruvate.
• MPC is encoded by two genes, MPC1 and MPC2
• Pyruvate in the mitochondrial matrix is oxidized to acetyl-CoA and CO₂ by the pyruvate dehydrogenase (PDH) complex.
• This highly ordered cluster of enzymes
  • multiple copies of each of three enzymes
  • located in the mitochondria of all eukaryotic cells and in the cytosol of bacteria.

Conversion  of Pyruvate to Acetyl CoA

• Pyruvate is converted to acetyl  CoA by oxidative decarboxylation. 
• This is an irreversible reaction, catalyzed by a multienzyme complex, known as pyruvate dehydrogenase complex.
• High activities of  PDH  are found in cardiac muscle and kidney. 
• The enzyme  PDH requires five cofactors  (coenzymes),  namely-TPP i.e, Thiamine pyrophosphate (TPP or ThPP), lipoamide, FAD,  coenzyme A, and  NAD⁺. 
• The overall  reaction  of PDH is

Pyruvate Dehydrogenase Complex(PDC)

• PDC is a large multienzyme composed of:
  • Pyruvate dehydrogenase or Pyruvate decarboxylase (E1),
  • Dihydrolipoyl transacetylase (E₂),
  • Dihydrolipoyl dehydrogenase (E₃)
• The enzyme also consists of 5 coenzymes
  • Thiamine pyrophosphate (TPP),
  • Lipoic acid (LA),
  • Flavin adenine dinucleotide (FAD),
  • Coenzyme A (CoA), and
  • Nicotinamide adenine dinucleotide (NAD⁺).

• All these enzymes and coenzymes are organized into a cluster to keep the prosthetic groups close together, thus allowing the reaction intermediates to react quickly with each other.
• These 3 enzyme components associate by the noncovalent bond to form the pyruvate dehydrogenase complex when they are mixed at neutral pH in the absence of urea.
• The organization of the PDH complex is very similar to that of the enzyme complexes that catalyze the oxidation of α-ketoglutarate and the branched-chain α-keto acids.

Working mechanism of PDC:

The enzyme carries out the five consecutive reactions in the decarboxylation and dehydrogenation of pyruvate.

1. Pyruvate reacts with the bound thiamine pyrophosphate (TPP) of pyruvate dehydrogenase (E₁), undergoing decarboxylation to from hydroxyethyl derivative of thiazole ring of TPP.
2. Pyruvate dehydrogenase transfers two electrons and the acetyl group from TPP to the oxidized form of the lipoyllysyl group of the core enzyme, dihydrolipoyl transacetylase (E₂), to form the acetyl thioester of the reduced lipoyl group.
3. It is a transesterification process in which the —SH group of CoA replaces the—SH group of E₂ to yield acetyl-CoA and the fully reduced (dithiol) form of the lipoyl group.
4. Dihydrolipoyl dehydrogenase (E₃) promotes transfer of two hydrogen atoms from the reduced lipoyl groups of E₂ to the FAD prosthetic group of E₃, restoring the oxidized form of the lipoyllysyl group of E₂.
5. The reduced FADH₂ of E₃ transfers a hydride ion to NAD⁺, forming NADH. The enzyme complex is now ready for another catalytic cycle.

• Then, NADH + H⁺ can pass through the respiratory chain to give ATP by oxidative phosphorylation.
• The intermediates of PDH catalysed reaction are not free but bound with enzyme complex.
• A comparable enzyme with PDH is α-ketoglutarate dehydrogenase complex of citric acid cycle which catalyses the oxidative decarboxylation of α-ketoglutarate to succinyl CoA.

Regulation of PDH:

The conversion of pyruvate into acetyl-CoA is a key irreversible step in the metabolism of animals because the animals cannot convert acetyl-CoA into glucose.PDC which catalyzes the oxidative decarboxylation of pyruvate is regulated in 3 ways:

• End-product inhibition:
  • Pyruvate dehydrogenase is a good example for end product (acetyl CoA, NADH) inhibition.
  • Acetyl-CoA and NADH, both end products of the pyruvate dehydrogenase reaction, are potent allosteric inhibitors of the enzyme.
  • The inhibitory effects are reversed on the addition of coenzyme A and NAD⁺ respectively.
• Feedback regulation:
  • The activity of PDC is controlled by the energy charge.
  • The pyruvate dehydrogenase component is specifically inhibited by GTP or ATP and activated by AMP.
• Covalent modification.
  • Under conditions of high concentrations of ATP, acetyl-CoA, and those of the intermediates of TCA cycle, further formation of acetyl-CoA is slowed down.
  • This is accomplished by covalent modification.
  • PDH is regulated by phosphorylation and dephosphorylation.
  • PDH is active as a dephosphoenzyme while it is inactive as a phosphoenzyme.
  • PDH kinase (responsible to form inactive PDH) is promoted by ATP, NADH, and acetyl CoA, while it is inhibited by NAD+, CoA, and pyruvate.

• PDH activity is promoted by Ca²⁺ ,Mg²⁺ and insulin(in adipose tissue).
• Calcium released during muscle contraction stimulates PDH (by increasing phosphatase activity)for energy Production.
• The net result is that in the presence of high energy signals (ATP, NADH), the PDH is turned off.

 References:

•  Lehninger, A., Nelson, D. and Cox, M., 2013. Principles Of Biochemistry. 6th ed. New York: W.H. Freeman.
• Satyanarayana, U., 2014. Biochemistry. Elsevier Health Sciences APAC.
• Champe, P., Harvey, R. and Ferrier, D., 2008. Biochemistry. Philadelphia, Pa.: Wolters Kluwer / Lippincott Williams & Wilkins.
• Murray, R., Bender, D., Botham, K., Kennelly, P., Rodwell, V. and Weil, P., 2012. Harpers Illustrated Biochemistry. 29th ed. Blacklick: McGraw-Hill Publishing.