Pyruvate dehydrogenase and the citric acid cycle Michael

Pyruvate dehydrogenase and the citric acid cycle © Michael Palmer 2019 1

Pyruvate degradation occurs in the mitochondria layout: title_6_inches image -format: high margin_right: 1. 2 margin_top: 0. 5 output_max_height: 6. 75 © Michael Palmer 2019 2

The PDH reaction occurs in three successive steps that are catalyzed by three different subunits © Michael Palmer 2019 3

The structural organization of the E. coli PDH complex © Michael Palmer 2019 4

A lipoamide tether guides the substrate from one active site to the next © Michael Palmer 2019 5

The pyruvate dehydrogenase reaction involves multiple coenzymes Coenzyme © Michael Palmer 2019 Subunit Role in catalysis thiamine pyrophosphate E 1 provides a carbanion for nucleophilic attack on the substrate lipoamide E 2 transfers substrate to coenzyme A, retains hydrogen flavin adenine dinucleotide (FAD) E 3 transfers H 2 from lipoamide to NAD+ 6

Thiamine pyrophosphate forms a carbanion © Michael Palmer 2019 7

Decarboxylation of pyruvate by E 1 © Michael Palmer 2019 8

Release of acetyl-Co. A and disposal of hydrogen © Michael Palmer 2019 9

Alternate metabolic destinations of pyruvate 1. Conversion to acetyl-Co. A by PDH for complete degradation or for synthesis of fatty acids and cholesterol 2. Carboxylation to oxaloacetate, for use in gluconeogenesis or in the citric acid cycle 3. Synthesis of amino acids, e. g. transamination to alanine 4. Reduction to lactate © Michael Palmer 2019 10

Regulation of PDH by allosteric effectors and by phosphorylation © Michael Palmer 2019 11

The overall reaction of the TCA cycle: does it add up? © Michael Palmer 2019 12

The citrate synthase reaction © Michael Palmer 2019 13

Reactions in the TCA cycle: from citrate to succinyl-Co. A © Michael Palmer 2019 14

Reactions in the TCA: from succinyl-Co. A to oxaloacetate © Michael Palmer 2019 15

α-Ketoglutarate dehydrogenase resembles PDH © Michael Palmer 2019 16

Regulation of the citric acid cycle • ATP and NADH inhibit isocitrate dehydrogenase • NADH inhibits α-ketoglutarate dehydrogenase • High levels of NADH will lower the oxaloacetate concentration, which limits citrate synthase activity © Michael Palmer 2019 17