Krebs Cycle Tricarboxylic Acid Cycle Citric Acid Cycle Krebs Cycle Tricarboxylic Acid Cycle

Citric Acid Cycle Reduces Acetyl CoA to CO2, with production of NADH,FADH2 and GTP Acetyl CoA comes from: Lipids Pyruvate from Glycolysis Amino Acid breakdown Acetoacetate or Acetyl-CoA pyruvate Produces oxaloacetate from various compounds for glucose synthesis

Oxidative Decarboxylation of Pyruvate The first step of Citric Acid Cycle oxidation of carbohydrates is decarboxylation of pyruvate. The Acetyl group is attached to CoA. The process oxidation results in reduction of NAD+ to NADH The process happens on a large complex in 3 stages.

Pyruvate Dehydrogenase Complex Enzyme Abbreviation Number of chains Prosthetic group Reaction catalyzed Pyruvate dehydrogenase component E1 24 TPP Oxidative decarboxylation of pyruvate Dihydrolipoyl transacetylase E2 Lipoamide Transfer of the acetyl group to CoA Dihydrolipoyl dehydrogenase E3 12 FAD Regeneration of the oxidized form of lipoamide

Pyruvate Dehydrogenase Complex

Mechanism of Pyruvate Decarboxylation TPP is first deprotonated. It can then attack pyruvate to displace CO2.

Transfer of Acetyl to CoA Transfer to Lipoamide Transfer to CoA Oxidation of Lipoamide

Flipping of Lipamide between subunits

Acetyl CoA breakdown Acetyl CoA is broken down to produce 1 GTP or ATP 3 NADH 1 FADH2

Citrate Synthase Citrate Synthase adds acetyl group to oxaloacetate.

Citrate Synthase conformation change ensure correct reaction

Citryl CoA Synthesis Mechanism Citrate synthase goes through a second conformation change after this reaction to allow citryl-CoA hydrolysis.

Aconitase converts Citrate to Isocitrate The reaction is stereospecific to one side of citrate. The enzyme uses an iron-sulfur complex for binding citrate.

Isocitrate is then oxidized & decarboxylated Oxalosuccinate is an unstable intermediate, which is decarboxylated on the enzyme. This is an important step in regulating the cycle.

a-Ketoglutarate dehydrogenase catalyzes oxidative decarboxylation The reaction is by the same mechanism as pyruvate dehydrogenase. The same kind of large complex with E1, E2 and E3 is used. E3 (dihyrolipoyl dehydrogenase is identical in the two complexes.

Succinyl CoA synthase runs in reverse to generate GTP Some organisms use ATP instead of GTP. These NTPs are converted by dinucleotide kinase

Oxidation of Succinate to Oxaloacetate Oxaloacetate is regenerated by oxidation of succinate to the keto acid in 3 steps. Succinate Dehydrogenase oxidizes succinate to fumarate with reduction of an enzyme-bound FAD to FADH2. Succinate dehydrogenase in membrane bound & directly transfers e- to e- transport chain. Fumarate is hydrated to malate. Hydration is stereospecific. Malate is oxidized to oxaloacetate with production of NADH.

Citric Acid Cycle Summary

Regulation of Citric Acid Cycle Decarboxylation of pyruvate is committed step, since in animals acetyl-CoA cannot be converted back to pyruvate & glucose. This step is highly regulated. Other regulated steps include the two decarboxylation steps.

Pyruvate Dehydrogenase Regulation

Citric Acid Cycle Generation of Biosynthetic Precursors Anaplerotic reactions can replenish the reaction intermediates. i.e. generation of oxaloacetate by pyruvate carboxylation. No net conversion of Acetyl CoA to intermediates or pyruvate.

The Glyoxylate Cycle Plants and many bacteria can convert acetyl CoA to TCA cycle intermediates by the Glyoxylate cycle. Allows these organisms to grow on acetate.

Conclusions The Citric Acid Cycle allows organisms to extract electrons from pyruvate and other Acetyl-CoA precursors for transport to the mitochondria electron transport chain. One NADH is made converting pyruvate to Acetyl-CoA. Three NADH, one FADH2 & 1 GTP/ATP is made in the citric acid cycle. The citric acid cycle can be used to make precursors for other molecules. Citric acid cycle intermediates can be made from pyruvate and amino acids to increase flow through or replace those used in biosynthesis. Also from acetyl CoA in plants & some bacteria, but not animals. The citric acid cycle is regulated by NADH, and ultimately by oxygen availability.