University:
University of Phoenix
Course:
BIO 350 | General Biology I
Academic year:

2023
Views:

499

Pages:

3
Author:

Kasey Mitchell

Kreb's Cycle / TCA Cycle Kreb’s cycle was discovered by Hans Adolf Krebs. The cycle is known as Citric Acid Cycle or TCA (Tricarboxylic Acid) Cycle. The enzymes of Krebs Cycle are located in the mitochondrial matrix in close proximity to ETC. This enables the synthesis of ATP by oxidative phosphorylation. Steps Involving Krebs Cycle: The cycle consists of 10 enzymatic steps which involve 2 carboxylation and 4 dehydrogenation. Condensation: Condensation of Acetyl Coenzyme A and Oxaloacetate catalyzed by enzyme citrate synthase. Dehydration I: Citrate undergoes reorganization in presence of enzyme aconitase forming Cis-aconitate and water. Hydration I: Cis-aconitate is converted into Iso-citrate in presence of water and enzyme aconitase. Dehydration II: Isocitrate is dehydrogenated to oxalosuccinate in presence of enzyme isocitrate dehydrogenase and Mn²⁺ producing NADH. Carboxylation I: Oxalosuccinate is decarboxylated to produce α-ketoglutarate by enzyme decarboxylase releasing one CO₂. Dehydration III and Carboxylation II: α-ketoglutarate is both dehydrogenated with help of NAD⁺ and decarboxylated by enzyme α-ketoglutarate dehydrogenase. The enzyme complex contains TPP and lipoic acid. The product combines with CoA to form Succinyl CoA. Substrate Level Phosphorylation of GDP: Succinyl CoA is converted to Succinate by enzyme succinyl thiokinase. This reaction requires GDP. In presence of inorganic phosphate, high-energy bond of Succinyl CoA is transferred to GDP, converting it to GTP. Dehydrogenation IV: Succinate is metabolized to Fumarate by enzyme succinate dehydrogenase. In this reaction, direct transfer of hydrogen from substrate to FAD. Hydration II: Water molecule added to Fumarate to produce Malate by enzyme fumarase. Dehydrogenation V: Malate is dehydrogenated to Oxaloacetate by enzyme malate dehydrogenase. Oxaloacetate picks off another molecule of acetyl CoA to repeat the cycle, as two molecules of acetyl CoA are produced from 1 molecule of glucose. Net Reaction of Kreb’s Cycle: 2 Acetyl CoA + 6 H₂O + 6 NAD⁺ + 2 FAD + 2 GDP + 2 Pᵢ ⟶ 4 CO₂ + 2 H₂O + 6 NADH + 2 FADH₂ + 2 GTP + 2 CoA