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TCA cycle

General Background The TCA tricarboxylic acid cycle is a pathway of aerobic respiration that generates both energy and reducing power. It can also generate precursors for various biosynthetic pathways. The pathway is common, and a variation of it exists in all aerobic organisms . The input to the cycle is : ACETYL-COA, an activated form of : ACET that is generated by the degradation of carbohydrates, fats and proteins. A common source of acetyl-coA is : PYRUVATE, which is generated by : PWY66-400 and converted to acetyl-CoA by the : PYRUVDEHYD-PWY "pyruvate dehydrogenase complex". The TCA cycle is so named because the first step in the pathway is the condensation of : ACETYL-COA with : OXALACETIC_ACID to form : CIT, an acid with three carboxylate groups. The cycle is also known as the citric acid cycle, the Szent-Gyorgyi-Krebs cycle and the Krebs cycle, named after the scientists who first described it. One turn of the cycle in eukaryotes produces 3 molecules of : NADH, one molecule of : Reduced-Quinones quinol and 2 molecules of : CARBON-DIOXIDE. The reduced molecules of NADH and quinol serve as electron donors for oxidative phosphorylation. As electrons flow throught the electron transport chain to a terminal acceptor, protons are pumped across the inner mitochondrial membrane generating a proton motive force (PMF). When the protons return to the mitochondrial matrix, they power ATP synthase which phosphorylates ADP to ATP. The total energy gained from the catabolism of one molecule of glucose by glycolysis, the TCA cycle, and oxidative phosphorylation is about 30 ATP molecules in eukaryotes. About This Pathway In eukaryotic cells, the TCA cycle occurs in the mitochondrial matrix, while in prokaryotes the pathway occurs in the cytosol. There are several differences between the mammalian TCA cycle, which is described here, and the TCA cycles that occur in prokaryotes, the most common of which is described in :TCA. While the prokaryotic pathway utilizes only one, ATP-dependent enzyme to catalyze the interconversion of : SUC-COA with : SUC, the mammalian TCA cycle utilizes two isoforms, : CPLX66-14 and : CPLX-7862. The level of utilization of each isoform is tissue dependent . Another difference concerns isocitrate dehydrogenase. The mammalian : CPLX66-11 "isocitrate dehydrogenase" (: EC- "EC") uses NAD+ as a cofactor, as opposed to the prokaryotic NADP+-dependent enzyme (: EC- "EC"). Mammals do possess an NADP+-dependent isozyme, but it is used in glutamate biosynthesis and does not participate in the TCA cycle . The conversion of : MAL to : OXALACETIC_ACID is catalyzed in mammals by an NAD+-dependent : HS07366-MONOMER "malate dehydrogenase" (: EC- "EC"). In some prokaryotic organisms this step of the cycle is catalyzed by an NADP+-dependent : EG12069-MONOMER (: EC- "EC"). Flux through the cycle is regulated on many different levels. Although the : CPLX66-272 is not a component of the cycle, it plays a pivotal role in determining TCA cycle flux . : CPLX-7863 "Citrate synthase", : CPLX66-11 and : CPLX66-42 are other major regulators of the cycle. At the protein modification level, lysine acetylation of enzymes controls cycle activity .

from BIOCYC source record: HUMAN_PWY66-398
Type: pathway
Taxonomic scope
organism-specific biosystem
Homo sapiens

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