citric acid cycle

citric acid cycle. Also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, it is a central metabolic hub within the Mitochondrial matrix of eukaryotic cells and the Cytoplasm of prokaryotes. This series of chemical reactions is fundamental to cellular respiration, oxidizing Acetyl-CoA derived from carbohydrates, fats, and proteins to generate high-energy electron carriers and GTP. The cycle is a critical source of precursors for various biosynthetic pathways and is tightly regulated in response to cellular energy demands.

Overview

The citric acid cycle serves as the final common pathway for the oxidation of fuel molecules, funneling carbon atoms from Pyruvate, fatty acids, and amino acids into a cyclic sequence of reactions. It occurs under aerobic conditions and is directly linked to the Electron transport chain, which uses the cycle's reduced products, NADH and FADH2, to drive ATP synthesis. The cycle was largely elucidated through the pioneering work of Hans Adolf Krebs, building upon earlier discoveries by Albert Szent-Györgyi and Carl Martius. Key intermediates of the cycle, such as Oxaloacetate and Alpha-ketoglutarate, are also crucial anaplerotic substrates for Gluconeogenesis and Amino acid synthesis.

Steps of the cycle

The cycle begins with the condensation of Acetyl-CoA and Oxaloacetate, catalyzed by the enzyme Citrate synthase to form Citrate. Aconitase then isomerizes citrate to Isocitrate, which is subsequently oxidized and decarboxylated by Isocitrate dehydrogenase to produce Alpha-ketoglutarate, Carbon dioxide, and NADH. The next step involves another oxidative decarboxylation by the Alpha-ketoglutarate dehydrogenase complex, yielding Succinyl-CoA and a second molecule of NADH. Succinyl-CoA synthetase then catalyzes the substrate-level phosphorylation of GDP to GTP, producing Succinate. Succinate dehydrogenase, an integral protein of the Inner mitochondrial membrane, oxidizes succinate to Fumarate, reducing FAD to FADH2. Fumarase hydrates fumarate to Malate, which is finally oxidized back to oxaloacetate by Malate dehydrogenase, generating the third molecule of NADH and completing the cycle.

Regulation

The flux through the citric acid cycle is precisely controlled at several irreversible steps to match cellular ATP requirements. Key regulatory enzymes include Citrate synthase, Isocitrate dehydrogenase, and the Alpha-ketoglutarate dehydrogenase complex. These enzymes are allosterically inhibited by high ratios of ATP/ADP and NADH/NAD+, signaling an abundant energy supply. Conversely, they are stimulated by Calcium ions, which increase during muscle contraction and hormonal signaling. Acetyl-CoA availability, largely determined by the activity of the Pyruvate dehydrogenase complex, also exerts significant control over cycle entry. Furthermore, Succinyl-CoA acts as a feedback inhibitor of both Alpha-ketoglutarate dehydrogenase and Citrate synthase.

Energetic yield

For each acetyl-CoA molecule oxidized, the citric acid cycle directly produces one molecule of GTP (readily converted to ATP) via substrate-level phosphorylation. More significantly, it generates three molecules of NADH and one of FADH2. These reduced coenzymes donate electrons to the Electron transport chain, driving Oxidative phosphorylation. The complete oxidation of one acetyl-CoA unit, via the combined actions of the cycle and oxidative phosphorylation, typically yields approximately 10 molecules of ATP in many eukaryotic cells. When accounting for the prior conversion of Pyruvate to acetyl-CoA, the full oxidation of one glucose molecule via Glycolysis, the citric acid cycle, and oxidative phosphorylation can produce up to 30-32 ATP.

Evolution and variants

The citric acid cycle is considered an ancient metabolic pathway, with components found in the last universal common ancestor (LUCA). It likely evolved from simpler, non-cyclic pathways for Carbon fixation in anaerobic and autotrophic organisms. Variants of the cycle, such as the reductive or reverse citric acid cycle, operate in certain Archaea and Bacteria, including Chlorobium and Aquificae, functioning in anabolic carbon assimilation. The Glyoxylate cycle, a modified bypass present in plants, Escherichia coli, and some fungi, allows net conversion of Acetyl-CoA into Carbohydrates, enabling growth on two-carbon compounds like Acetate.

Clinical significance

Dysfunction in the citric acid cycle is associated with severe metabolic disorders and contributes to the pathology of numerous diseases. Mutations in genes encoding cycle enzymes, such as Fumarase hydratase and Succinate dehydrogenase, are linked to hereditary cancers like Hereditary leiomyomatosis and renal cell cancer and Paraganglioma. Impaired cycle activity is also observed in Hypoxia and Ischemia, leading to Lactic acidosis. Furthermore, several toxins, such as Arsenic and Fluoroacetate, inhibit key enzymes like the Alpha-ketoglutarate dehydrogenase complex and Aconitase, disrupting cellular energy production. The cycle is a target in cancer metabolism, as many tumors exhibit altered Glutamine metabolism to replenish cycle intermediates. Category:Metabolic pathways Category:Cellular respiration Category:Biochemistry