Lecture 29 Summary: Bridging glycolysis and the citric acid cycle

Pyruvate, the end product of glycolysis under aerobic conditions, is a metabolic branch point. As a preliminary to following the central path of aerobic metabolism from glycolysis to the citric acid cycle, we put pyruvate in perspective by considering its various possible fates.

The most important fate of pyruvate - at least for our present purposes - is its oxidative decarboxylation to acetyl CoA. This reaction is catalyzed by complex, multisubunit enzyme called the pyruvate dehydrogenase complex (PDH complex, or often simply "PDH"). This large supramolecular assembly contains multiple copies of three different types of subunit. These subunits catalyze different steps of the overall reaction. Central to the operation of the PDH complex is a key catalytic cofactor, thiamine pyrophosphate (TPP). We will examine closely the chemistry of this extraordinary and important cofactor.

The pyruvate dehydrogenase (PDH) complex

The PDH complex carries out the oxidative decarboxylation process that generates acetyl CoA from pyruvate. The PDH complex serves as the link between glycolysis and the citric acid cycle, and is required for oxidative metabolism. The activity of PDH involves three distinct enzymes, four activities, and five different cofactors (see BTS, pp.467-472).

Steps of the PDH complex:

(1) decarboxylation (E1, formation of hydroxyethyl-TPP)

(2) oxidation (transfer of acetyl group to lipoamide)

(3) transfer of acetyl group from acetyllipoamide to CoA)

(4) oxidation of dihydrolipoamide to lipoamide (E3, FAD, NAD⁺)

The α-ketoglutarate dehydrogenase complex, which participates in the citric acid cycle, shows a close resemblance to PDH complex.

The biochemistry of thiamine

Thiamine pyrophosphate (TPP), a key catalytic cofactor of the PDH complex, assists in decarboxylations of α-keto acids, and is a carrier of activated aldehyde moeities.

A hydrogen attached to the C2 carbon of the thiazole ring of TPP shows an unusually low pK_a.

Thiamine deficiency underlies the disorder beriberi.

Learning objectives

• Draw the structure of the key metabolic intermediate pyruvate, recognize it as an α-keto acid.
• List the possible metabolic fates of pyruvate.
• Explain the concept of redox balance and describe how the difference between aerobic and anaerobic conditions affects the fate of pyruvate generated by glycolysis.
• Describe the energetics and mechanism of biotin-dependent carboxylases, such as pyruvate kinase.
• Distinguish mechanistically between decarboxylation of an α- and a β-keto acid.
• Distinguish between nonoxidative and oxidative decarboxylation.
• Illustrate the mechanistic role of thiamine pyrophosphate (TPP) in decarboxylation of an α-keto acid.
• Write a chemical equation representing the net reaction catalyzed by pyruvate dehydrogenase (PDH) complex.
• Describe the components (enzyme subunits and cofactors) of PDH complex and their roles.

Page updated 12-02-09

References

1. Staunton J. Primary Metabolism: A Mechanistic Approach (1978, Oxford University Press)
2. Silverman RB. The Organic Chemistry of Enzyme-Catalyzed Reactions (Revised ed., 2002, Academic Press).
3. Knowles JR. (1989) The mechanism of biotin-dependent enzymes. Ann Rev Biochem 58: 195-221.