TCA CYCLE BIOCHEMISTRY - II [PHL -224] Dr. VIDYA.D Asst. Professor, College Of Pharmacy (Women), Prince Sattam Bin Abdul Aziz University, Al -Kharj

Objective Introduction Site Pathway Energetics Regulation Clinical importance

Other names Krebs cycle Citric acid Cycle

Overall goal Makes ATP Makes NADH Makes FADH 2 but requires some carbohydrates to run

REACTIONS OF TCA CYCLE “The wheel is turnin’ and the sugar’s a burnin’”

A.OXIDATIVE DECARBOXYLATION OF PYRUVATE

Reaction of pyruvate dehydrogenase complex (PDC) 2 per glucose (all of Krebs) Oxidative decarboxylation Makes NADH

Pyruvate dehydrogenase Complex (PDC) It is a multi-enzyme complex containing three enzymes : 1: Pyruvate dehydrogenase, uses Thiamine pyrophosphate as cofactor bound to E1 2: Dihydrolipoyl transacetylase,Lipoic acid bound, CoA as substrate 3: Dihydrolipoyl DehydrogenaseFAD bound, NAD + as substrate

REGULATION OF PYRUVATE DEHYDROGENASE COMPLEX

B. SYNTHESIS OF CITRATE

Citrate synthase 1.Binding of Oxaloacetate to the enzyme results in conformational change which facilitates the binding of the next substrate, the acetyl Coenzyme A. There is a further conformational change which leads to formation of products.

Aconitase: This enzyme catalyses the isomerization reaction by removing and then adding back the water ( H and OH ) to cis-aconitate in at different positions. Isocitrate is consumed rapidly by the next step thus deriving the reaction in forward direction. C. ISOMERIZATION OF CITRATE

D. OXIDATION & DECARBOXYLATION OF ISOCITRATE

Isocitrate dehydrogenase: Oxidative decarboxylation Rate limiting step. Enzyme activated by ADP and Ca++ and inhibited by ATP and NADH

E. OXIDATION & DECARBOXYLATION OF α-KETOGLUTARATE

 - Ketoglutarate dehydrogenase complex:This is a complex of different enzymatic activities similar to the pyruvate dehydrogenase complex.It has the same mechanism of reaction with E1, E2 and E3 enzyme units. NAD+ is an electron acceptor.

F. CLEAVAGE OF SUCCINYL CoA

Succinyl CoA synthatse:

G. OXIDATION OF SUCCINATE

Succinate Dehydrogenase: Oxidation of succinate to fumarate.This is the only citric acid cycle enzyme that is tightly bound to the inner mitochondrial membrane.It is an FAD dependent enzyme.

H. HYDRATION OF FUMARATE catalyzed by Fumarase: Hydration of Fumarate to malate.

I. OXIDATION OF MALATE

Malate dehydrogenase: Oxidation of malate to oxaloacetate: It is an NAD + dependent enzyme.Reaction is pulled in forward direction by the next reaction (citrate synthase reaction) as the oxaloacetate is depleted at a very fast rate.

Net From Kreb’s cycle (oxidative Oxidative process 3 NADH FADH 2 ATP X 2 per glucose 6 NADH 2 FADH 2 2 ATP All ultimately turned into ATP phosphorylation…later)

Total Energy/glucose / TCA CYCLE In mitochondrion: Each NADH makes 3 ATP Each FADH 2 makes 2 ATP ATP So… From in mitochondrion 3NADH x 3 ATP/ NADH 2 = 1 FADH 2 X 2 ATP/FADH 2 = 1 ATPX1 ATP = 9 ATP 2ATP TOTAL From 1 pyruvate = 12 ATP so 2 pyruvate will yield 12 x2 = 24 ATP

Total Energy/ glucose FROM Glycolysis 2 NADH2 X 3 ATP/ NADH 2 = 6ATP 4 ATP X 1 ATP = 4 ATP ENERGY PAY OFF PHASE = 10 ATP ENERGY INVESTMENT PHASE= 2 ATP TOTAL ATP FROM GLYCOLYSIS = 8 ATP FROM TCA CYLCE = 24 ATP ATP

ATP generation during oxidation of Glucose Net ATP production depends on shuttle used for the transfer of reducing equivalents from cytosol to mitochondria.

Importance inthemuscleunder preventionoflacticacidosis anaerobic conditions.lacticacidosis important in producing ATP, an energy source, during muscle activity. Cycle is Efficient when muscle activity has ceased. so it allows the oxygen such that the Krebs cycle and electron transport chain can produce energy at peak efficiency. The drug metformin can precipitate lactic acidosis in patients with renal failure because metformin inhibits the cori cycle. Normally, the excess lactate would be cleared by the kidneys, but in patients with renal failure, the kidneys cannot handle the excess lactic acid.metformin

Metabolic defects Extremely rare 1. Defect in PDH - Lactic acidosis - Neurologycal dosorders 2. Defect In Pyruvate carboxylase - Oxaloacetate - Hyperammonemia - Lactic acidosis - Hyperalaninemia

Lactic acid cycle (or) Cori cycle Named after its discoverer Carl Ferdinand CoriCarl Ferdinand Cori Refers to the metabolic pathway in which lactate produced by anaerobic glycolysis.lactate glycolysis In the muscles Lactate moves to the liver and is converted to glucose, which then returns to the muscles and is metabolized back to lactate.

Cori’s cycle

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