Carbohydrate Metabolism: TCA Citric Acid Cycle PDF

INTRODUCTION  The citric acid cycle (also called the krebs cycle or the tricarboxylic acid (TCA) cycle is a series of enzymatically catalysed reactions that form a common pathway for the final oxidation of all metabolic fuels (carbohydrates, free fatty acids, ketone bodies and amino acids)...

INTRODUCTION  The citric acid cycle (also called the krebs cycle or the tricarboxylic acid (TCA) cycle is a series of enzymatically catalysed reactions that form a common pathway for the final oxidation of all metabolic fuels (carbohydrates, free fatty acids, ketone bodies and amino acids) which are catabolised to the substrate (acetyl CoA) of the citric acid cycle. Its central function is the oxidation of acetyl CoA (ie. acetyl group) to CO2 and H2O.  This oxidation is the major site of oxygen consumption and ATP production in most animals including humans. FUNCTION OF TCA  The citric acid cycle is an amphibolic pathway i.e. it is involved in both anabolic and catabolic processes. 1. Anabolic reactions: The intermediates of citric acid cycle are used as precursors in the biosynthesis of many compounds like synthesis of glucose from carbon skeletons of amino acids, and providing building blocks for heme synthesis. 2. Catabolic reactions: The cycle provides a means for the degradation of two carbon acetyl residues which are derived from carbohydrates, fatty acid and amino acids. CITRIC ACID CYCLE REACTIONS IN THE CITRIC ACID CYCLE Synthesis of citrate from acetyl CoA and oxaloacetate:  Citrate synthase catalyses this aldol condensation reaction with the release of CoA. There are certain inhibitors to this reaction, which include:  Inhibitors: Citrate synthase is inhibited by ATP, NADH, succinyl CoA and acyl CoA derivative of fatty acids (fatty acyl CoA). The rate of the reaction is also determined by the availability of the substrate.  ROLE OF CITRATE: 1. Citrate in addition to being an intermediate of citric acid cycle provides a source of acetyl CoA for the cytosolic synthesis of fatty acids. 2. Citrate inhibits phosphofructokinase-I, the rate limiting enzyme of glycolysis, 3. Citrate activates acetyl CoA carboxylase (the rate limiting enzyme of fatty acid synthesis) ISOMERIZATION OF CITRATE  In this step, citrate is isomerized to isocitrate by aconitase which has iron-sulphur centre as its prosthetic group. OXYDATION AND DECARBOXYLATION OF ISOCITRATE TO α-KETOGLUTARATE  Isocitrate dehydrogenase catalyses this reaction, yielding the first three NADH molecules produced by the cycle, and the first release of CO2. Note, the aim of the citric acid cycle is to oxidize acetyl units to 2 molecules of CO2. In fact, this is one of the rate limiting steps of the citric acid cycle. The enzyme isocitrate dehydrogenase is activated by ADP and inhibited by ATP and NADH. Oxidative decarboxylation of a- ketoglutarate to succinyl CoA  This conversion of a- ketoglutarate to succinyl CoA is catalyzed by the a- ketoglutarate dehydrogenase complex.  This reaction releases the 2nd CO2 and produces the 2nd NADH of the cycle. The equilibrium of the reaction is far in the direction of succinyl CoA a high energy thioester similar to acetyl CoA. CLEAVAGE OF SUCCINYL CoA TO SUCCINATE  Succinate thiokinase (succinyl CoA synthetase) cleaves the high energy thioester linkage in succinyl CoA to release succinate and CoA along with the substrate level phosphorylation of GDP (Guanosine Diphosphate) to GTP (GTP and ATP are inter-convertible by nucleoside diphosphate kinase reaction). OXIDATION OF SUCCINATE TO FUMARATE  This reaction is catalyzed by succinate dehydrogenase and FAD (Flavin Adenine Dinucleotide) is needed as a cofactor. Malonate, a structural analogue of succinate, competitively inhibits succinate dehydrogenase. It is also competitively inhibited by oxaloacetate. HYDRATION OF FUMARATE TO L- MALATE  Fumarase catalyses this reversible reaction. OXIDATION OF MALATE TO OXALOACETATE  Malate is oxidized to oxaloacetate by malate dehydrogenase and NAD+ is required as coenzyme. This is the third step of NADH production in the citric acid cycle by the electron transport chain along with the generation-of ATP molecules.