Tricarboxylic Acid Cycle and Pyruvate Dehydrogenase Complex

Pyruvate is oxidatively decarboxylated by the pyruvate dehydrogenase complex (PDHC), producing acetyl coenzyme A (CoA), which is the major fuel for the tricarboxylic acid (TCA) cycle (Fig. 1). The multienzyme PDHC requires five coenzymes: thiamine pyrophosphate, lipoic acid, flavin adenine dinucleotide (FAD), nicotinamide adenine dinucleotide (NAD+), and CoA. The PDHC is regulated by covalent modification of E1 (pyruvate decarboxylase) by PDH kinase and PDH phosphatase: Phosphorylation inhibits E1. PDH kinase is allosterically activated by ATP, acetyl CoA, and NADH and inhibited by pyruvate. The phosphatase is activated by calcium (Ca2+). E1 deficiency is the most common biochemical cause of congenital lactic acidosis. The brain is particularly affected in this X-linked dominant disorder. Arsenic poisoning causes inactivation of the PDHC by binding to lipoic acid. In the TCA cycle, citrate is synthesized from oxaloacetate (OAA) and acetyl CoA by citrate synthase, which is inhibited by product. Citrate is isomerized to isocitrate by aconitase (aconitate hydratase). Isocitrate is oxidatively decarboxylated by isocitrate dehydrogenase to α-ketoglutarate, producing carbon dioxide (CO2) and NADH. The enzyme is inhibited by ATP and NADH and activated by adenosine diphosphate (ADP) and Ca2+. α-Ketoglutarate is oxidatively decarboxylated to succinyl CoA by the α-ketoglutarate dehydrogenase complex, producing CO2 and NADH. The enzyme is very similar to the PDHC and uses the same coenzymes. The α-ketoglutarate dehydrogenase complex is activated by Ca2+ and inhibited by NADH and succinyl CoA but is not covalently regulated. Succinyl CoA is cleaved by succinate thiokinase (also called succinyl CoA synthetase), producing succinate and guanosine triphosphate (GTP). This is an example of substrate-level phosphorylation. Succinate is oxidized to fumarate by succinate dehydrogenase, producing FADH2. Fumarate is hydrated to malate by fumarase (fumarate hydratase), and malate is oxidized to OAA by malate dehydrogenase, producing NADH.

Three NADH and one FADH2 are produced by one round of the TCA cycle. The generation of acetyl CoA by the oxidation of pyruvate via the PDHC also produces an NADH. Oxidation of the NADH and FADH2 by the ETC yields 14 ATP. The terminal phosphate of the GTP produced by substrate-level phosphorylation in the TCA cycle can be transferred to ADP by nucleoside diphosphate kinase, yielding another ATP. Therefore, a total of 15 ATP are produced from the complete mitochondrial oxidation of pyruvate to CO2.

Figure 1: Key concept map for the tricarboxylic acid (TCA) cycle. PDHC =pyruvate dehydrogenase complex; CoA = coenzyme A; CO2 = carbon dioxide; NAD(H) = nicotinamide adenine dinucleotide; FAD(H2) = flavin adenine dinucleotide; GDP and GTP = guanosine di- and triphosphates; ADP = adenosine diphosphate; Pi = inorganic phosphate.