Oxidative Decarboxylation of pyruvate and TCA cycle

Oxidative decarboxylation of pyruvic acid Definition:- It is conversion of pyruvic acid (the end product of glycolysis) to acetyl-CoA. It is known as the link reaction because it forms an important link between the metabolic pathways of glycolysis and the citric acid cycle. This reaction is usually catalyzed by the pyruvate dehydrogenase complex.

Site: In eukaryotes, pyruvate decarboxylation takes place exclusively inside the mitochondrial matrix of all the tissues except RBCs. In prokaryotes similar reactions take place in the cytoplasm and at the plasma membrane.

Pyruvate is the end product of glycolysis, which located in the cytosolic fraction of the cell. Pyruvate transported into the mitochondria via pyruvate transporter Within the mitochondrion, pyruvate is oxidatively decaroboxylated to acetyl-CoA, by pyruvate dehydrogenase complex. This is the link between the citric acid cycle and glycolysis.

Pyruvate Dehydrogenase Complex (PDC). The pyruvate dehydrogenase complex is a multienzyme complex located in the mitochondrial matrix; it converts, irreversibly, pyruvate to acetyl-CoA. The irreversibility of this reaction prevents the formation of pyruvate from acetyl-CoA, and explain the why glucose cannot be formed from acetyl-CoA.

PDH is composed of three enzymes Pyruvate decarboxylase Dihydrolipoyl transacetylase Dihydrolipoyl dehydrogenase Also, the pyruvate dehydrogenase complex contains five coenzyme, TPP (thiamine), lipoic acid, FAD, NAD+ and CoASH ( Coenzyme A)

Functions The pyruvate dehydrogenase complex converts pyruvate, the end product of aerobic glycolysis, into acetyl CoA, a major fuel for the citric acid cycle.

Citric Acid Cycle Tricarboxylic acid cycle (Kreb's cycle) The citric acid cycle is known as Kreb’s cycle. It is also called tricarboxylic acid cycle (TCA cycle) because the involvement of tricarboxylate, citrate and isocitrate in the cycle

Definition:- It is the series of reactions in mitochondria which oxidize acetyl CoA to C02, H20 and energy. Site: Mitochondria of all tissue cells except RBCs which not contain mitochondria. The enzymes of the cycle are present in mitochondrial matrix except succinate dehydrogenase which is tightly bound to inner mitochondrial membrane.

Steps: After the glycolysis takes place in the cell's cytoplasm, the pyruvic acid molecules travel into the interior of the mitochondrion. Once the pyruvic acid is inside, carbon dioxide is enzymatically removed from each three-carbon pyruvic acid molecule to form acetic acid. The enzyme then combines the acetic acid with an enzyme, coenzyme A, to produce acetyl coenzyme A, also known as acetyl CoA. Once acetyl CoA is formed, the Krebs cycle begins. The cycle is split into eight steps.

Condensation of acetyl CoA and oxalacetate by citrate synthase to give citric acid

2. Interconversion of citrate, cis aconitate to isocitrate: The reaction is catalyzed by Aconitase enzyme. This reversible reaction represents the removal and addition of H20 to this tricarboxylic acid.

3. Conversion of isocitrate to α-ketogluterate: - Isocitrate dehydrogenase catalyses the irreversible oxidative decarboxylation of isocitrate, yielding a -ketoglutarate, NADH and release CO2.

4. Conversion of α-ketogluterate to succinyl CoA: Oxidative decarboxylation of α-ketogluterate is catalyzed by α- ketoglutarate dehydrogenase, similar to PDH, requiring the same 5 coenzymes (TPP, lipoic acid, FAD, NAD+ and CoASH.). The reaction releases the second CO2 and produces the second NADH of the cycle.

5. Cleavage of succinyl-CoA by succinyl-CoA synthase, which cleaves the high-energy bond of succinyl-CoA, yielding succinate.

6. Dehydrogenation of succinic acid to fumarate by succinate dehydrogenase:  

7. Hydration of fumaric acid Fumarate is hydrated to malate catalyzed by fumarase.

This reaction produces the third and final NADH of the cycle. 8. Oxidation of malate and regeneration of oxaloacetate by malate dehydrogenase. This reaction produces the third and final NADH of the cycle. The oxaloacetate can further condense with another acetyl-CoA molecule and the cycle continues.

Energy production

Functions of Kreb's cycle: 1. It is the final pathway for complete oxidation of all food-stuffs CHO, lipids and protein which are converted to acetyl CoA. 2. It is the major source of energy for cells except cells without mitochondria as RBCs. 3. It is the major source of succinyl CoA which used for porphyrin and Hb synthesis.

4. Synthetic functions of Kreb's cycle: Amphibolic Nature of the cycle Citric acid cycle is amphibolic (both catabolic + anabolic) in nature. The catabolic role: 1. It is the final pathway for the oxidation of CHO, fat, and proteins, because glucose, fatty acids and amino acids are all metabolized to acetyl-CoA. 2. It is the major source of energy for cells containing mitochondria.

The anabolic role: The intermediates in the cycle provide precursors for many anabolic pathways. For example: 1. Synthesis of fatty acids from citrate. 2. Synthesis of Some nonessential amino acids from α-ketoglutarate. 3. Synthesis of glucose by gluconeogenesis via oxaloacetate.

Total ATP produce from complete oxidation of one molecule of glucose during glycolysis, oxidative decarboxylation of pyruvate and CAC: