What is it? Series of rxns that oxidize acetyl CoA to 2CO 2 in a manner that conserves the liberated free energy for ATP production Breakdown products of glc, fats and amino acids are all oxidized via the cycle Intermediates also a source for many biosynthetic pathways - amphibolic which means serves in both catabolic and anabolic processes Also called TCA cycle - tricarboxylic acid cycle or Krebs cycle
Cellular respiration Citric Acid Cycle
Pyruvate dehydrogenase complex 3 mitochondrial euk enzymes 5 coenzymes Citric Acid Cycle
Pyruvate dehydrogenase complex 5 coenzymes - Coenzyme A (pantothenate) Reactive thiol is an acyl carrier
Pyruvate dehydrogenase complex 5 coenzymes - NAD + (niacin) Involved in redox, electron carrier Citric Acid Cycle
Pyruvate dehydrogenase complex 5 coenzymes - FAD (riboflavin, vitamin B2) Involved in redox, electron carrier
Pyruvate dehydrogenase complex 5 coenzymes - TPP (thiamine, vitamin B1) Helps in cleavage of bonds next to carbonyl
Pyruvate dehydrogenase complex 5 coenzymes - lipoate Reversible oxidation at thiols, serves as electron carrier and acyl carrier
Pyruvate dehydrogenase complex Citric Acid Cycle
Pyruvate dehydrogenase complex - regulation Eukaryotic complex regulated by phosphorylation Regulation Pyruvate: substrate for complex ADP: indicates low energy charge NADH, acetyl CoA: endproducts of complex Ca 2+ : certain hormones (insulin) raise [Ca 2+ ]
Every turn of cycle: 1. Joining an acetyl group to oxaloacetate 2. Oxidizing 2 carbons off as CO 2, leaving succinate 3. Converting succinate back to oxaloacetate 4. Producing 1 GTP and reduced cofactors
Net reaction: AcetylCoA + 3NAD + + FAD + GDP + P i + 2H 2 O 2CO 2 + CoA + 3NADH + FADH 2 + GTP + 2H +
Goal: capture energy from acetate 2CO 2 1. One GTP made: equivalent to 1 ATP 2. Lots of reduced cofactors, reducing power can be converted to ATP -ΔG irreversible +ΔG made favorable by fast product removal -ΔG irreversible α-ketoglutarate dehydrogenase complex -ΔG irreversible Only enzyme bound to inner mitochondrial membrane
Regulation Rate of cycle is determined by: 1. Availability of substrates (acetyl CoA, NAD +, FAD) 2. Inhibition by accumulating products 3. Allosteric feedback inhibition of enzymes NAD + and FAD are regenerated by oxidative phosphorylation, which needs O 2
Regulation Three nonequilibrium rxns: 1. Citrate synthase 2. Isocitrate dehydrogenase 3. α-ketoglutarate dehydrogenase Citric Acid Cycle
Why so complicated? Hub of intermediary metabolism Greatest selective advantage Citric Acid Cycle
Reaction 1 - Citrate synthase Condensation of acetylcoa with oxaloacetate to form citrate Irreversible
Reaction 2 - Aconitase Removal of H 2 O, stereospecifically add back H 2 O Reversible, pulled to right because isocitrate rapidly consumed Prochiral carbon allows for stereospecific rxn
Reaction 3 - isocitrate dehydrogenase Oxidative decarboxylation of isocitrate Irreversible
Reaction 4 - α-ketoglutarate dehydrogenase complex Oxidative decarboxylation of α-ketoglutarate Identical to pyruvate dehydrogenase reaction Three homologous enzymes (E1, E2, E3) Five coenzymes (TPP, lipoate, FAD, NAD +, CoA) Irreversible
Reaction 5 - Succinyl-CoA synthetase Hydrolysis of thioester bond energizes formation of GTP Synthetase indicates participation of nucleoside triphosphate Reversible
Reaction 6 - Succinate dehydrogenase Oxidation of succinate to fumarate Enzyme bound to inner mito membrane, only membrane-bound enz Reversible Malonate (analog of succinate) is a competitive inhibitor of enz, blocks activity of cycle
Reaction 7 - Fumarase Hydration of fumarate to L-malate Reversible Citric Acid Cycle Substrate Product
Reaction 8 - Malate dehydrogenase Oxidation of malate to oxaloacetate NAD + linked to enzyme High +ΔG, pulled to products by continual removal of oxaloacetate