Citric Acid Cycle

What is it?

Series of rxns that oxidize acetyl CoA to 2CO2 in a manner thatconserves the liberated free energy for ATP production

Breakdown products of glc, fats and amino acids are all oxidized viathe cycle

Intermediates also a source for many biosynthetic pathways -“amphibolic” which means serves in both catabolic and anabolicprocesses

Also called “TCA cycle” - tricarboxylic acid cycle or “Krebs cycle”

Citric Acid Cycle

Cellular respiration

Citric Acid CyclePyruvate dehydrogenase complex

3 mitochondrial euk enzymes5 coenzymes

Citric Acid CyclePyruvate dehydrogenase complex5 coenzymes - Coenzyme A (pantothenate)Reactive thiol is an acyl carrier

Citric Acid CyclePyruvate dehydrogenase complex5 coenzymes - NAD+ (niacin)Involved in redox, electron carrier

Citric Acid CyclePyruvate dehydrogenase complex5 coenzymes - FAD (riboflavin, vitamin B2)Involved in redox, electron carrier

Citric Acid CyclePyruvate dehydrogenase complex5 coenzymes - TPP (thiamine, vitamin B1)Helps in cleavage of bonds next to carbonyl

Citric Acid CyclePyruvate dehydrogenase complex5 coenzymes - lipoateReversible oxidation at thiols, serves as electron carrier and acylcarrier

Citric Acid CyclePyruvate dehydrogenase complex

Citric Acid CyclePyruvate dehydrogenase complex - regulationEukaryotic complex regulated by phosphorylation

RegulationPyruvate: substrate for complexADP: indicates low energy charge NADH, acetyl CoA: endproducts of complexCa2+: certain hormones (insulin) raise [Ca2+]

Citric Acid CycleEvery turn of cycle:1. Joining an acetyl group to oxaloacetate2. Oxidizing 2 carbons off as CO2, leaving succinate3. Converting succinate back to oxaloacetate4. Producing 1 GTP and reduced cofactors

Citric Acid Cycle

Net reaction:AcetylCoA + 3NAD+ + FAD + GDP + Pi + 2H2O → 2CO2 + CoA + 3NADH + FADH2 +GTP + 2H+

Citric Acid CycleGoal:capture energy from acetate → 2CO21. One GTP made: equivalent to 1 ATP2. Lots of reduced cofactors, reducing power can be converted to ATP

-ΔG irreversible

-ΔG irreversible

α-ketoglutarate dehydrogenase complex-ΔG irreversible

Only enzyme bound to inner mitochondrial membrane

+ΔGmade favorable by fastproduct removal

Citric Acid CycleRegulationRate of cycle is determined by:1. Availability of substrates (acetyl CoA, NAD+, FAD)2. Inhibition by accumulating products3. Allosteric feedback inhibition of enzymes

NAD+ and FAD are regeneratedby oxidative phosphorylation,which needs O2

Citric Acid CycleRegulationThree nonequilibrium rxns:1. Citrate synthase2. Isocitrate dehydrogenase3. α-ketoglutarate dehydrogenase

Citric Acid CycleWhy so complicated?Hub of intermediary metabolismGreatest selective advantage

Citric Acid Cycle

Citric Acid CycleReaction 1 - Citrate synthaseCondensation of acetylCoA with oxaloacetate to form citrateIrreversible

Citric Acid CycleReaction 2 - AconitaseRemoval of H2O, stereospecifically add back H2OReversible, pulled to right because isocitrate rapidly consumed

Prochiral carbon allows for stereospecific rxn

Citric Acid CycleReaction 3 - isocitrate dehydrogenaseOxidative decarboxylation of isocitrateIrreversible

Citric Acid CycleReaction 4 - α-ketoglutarate dehydrogenase complexOxidative decarboxylation of α-ketoglutarateIdentical to pyruvate dehydrogenase reactionThree homologous enzymes (E1, E2, E3)Five coenzymes (TPP, lipoate, FAD, NAD+, CoA)Irreversible

Citric Acid CycleReaction 5 - Succinyl-CoA synthetaseHydrolysis of thioester bond energizes formation of GTPSynthetase indicates participation of nucleoside triphosphateReversible

Citric Acid CycleReaction 6 - Succinate dehydrogenaseOxidation of succinate to fumarateEnzyme bound to inner mito membrane, only membrane-bound enzReversible

Malonate (analog of succinate) is a competitive inhibitor of enz,blocks activity of cycle

Citric Acid CycleReaction 7 - FumaraseHydration of fumarate to L-malateReversible

Substrate

Product

Citric Acid CycleReaction 8 - Malate dehydrogenaseOxidation of malate to oxaloacetateNAD+ linked to enzymeHigh +ΔG˚, pulled to products by continual removal of oxaloacetate