Krebs cycle

- Krebs cycle is a series of 8 reactions that oxidize the acetyl group of acetyl-CoA to two CO2 molecules with the production of NADH and FADH2, thus generating free energy that will be used in the synthesis of ATP - Krebs cycle is the common mode of oxidative degradation in eukaryotes and aerobic prokaryotes - it accounts for the major portion of carbohydrate, fatty acid, and amino acid oxidation and generates numerous biosynthetic precursors - Krebs cycle is therefore amphibolic - it operates both catabolically and anabolically - The major function of the cycle is to extract hydrogen atoms from substrates in the form of NADH and FADH2 which will subsequently be oxidized in the respiratory chain to produce energy (ATP) - Energy production: the krebs cycle in cooperation with the respiratory chain, when oxidizing one acetyl molecule, produces standard 12 ATP - The Krebs cycle can be divided into 4 stages: I - condensation of the acetyl residue with oxaloacetate, with the formation of citrate and its isomerization into isocitrate - reactions 1 (citrate synthase) and 2 (aconitase) II - decarboxylation - reactions 3 (isocitrate dehydrogenase) and 4 (α-ketoglutarate dehydrogenase) III - substrate-level phosphorylation - synthesis of a macroergetic molecule GTP - reaction 5 (Succinyl-CoA synthetase). GTP can be converted to ATP IV - regeneration of oxaloacetate to continue the cycle - reactions 6 (Succinate dehydrogenase), 7 (Fumarase) and 8 (malate dehydrogenase) ! Reactions 1, 3 and 4 are irreversible and can determine the rate of the Krebs cycle, they are regulated