The citric acid cycle —also known as the Krebs cycle, Szent-Györgyi-Krebs cycle or the TCA cycle (tricarboxylic acid cycle)—is a series of chemical reactions to release stored energy through the oxidation of acetyl-CoA derived from carbohydrates, fats, and proteins. The Krebs cycle is used by organisms that respire (as opposed to organisms that ferment) to generate energy, either by anaerobic respiration or aerobic respiration. In addition, the cycle provides precursors of certain amino acids, as well as the reducing agent NADH, that are used in numerous other reactions. Its central importance to many biochemical pathways suggests that it was one of the earliest components of metabolism. Even though it is branded as a 'cycle', it is not necessary for metabolites to follow only one specific route; at least three alternative segments of the citric acid cycle have been recognized.
Acetyl-CoA is a molecule that participates in many biochemical reactions in protein, carbohydrate and lipid metabolism. Its main function is to deliver the acetyl group to the citric acid cycle to be oxidized for energy production. Coenzyme A consists of a β-mercaptoethylamine group linked to the vitamin pantothenic acid (B5) through an amide linkage and 3'-phosphorylated ADP. The acetyl group of acetyl-CoA is linked to the sulfhydryl substituent of the β-mercaptoethylamine group. This thioester linkage is a "high energy" bond, which is particularly reactive. Hydrolysis of the thioester bond is exergonic (−31.5 kJ/mol).
Biological carbon fixation or сarbon assimilation is the process by which inorganic carbon is converted to organic compounds by living organisms. The compounds are then used to store energy and as structure for other biomolecules. Carbon is primarily fixed through photosynthesis, but some organisms use a process called chemosynthesis in the absence of sunlight.
The glyoxylate cycle, a variation of the tricarboxylic acid cycle, is an anabolic pathway occurring in plants, bacteria, protists, and fungi. The glyoxylate cycle centers on the conversion of acetyl-CoA to succinate for the synthesis of carbohydrates. In microorganisms, the glyoxylate cycle allows cells to use two carbons, such as acetate, to satisfy cellular carbon requirements when simple sugars such as glucose or fructose are not available. The cycle is generally assumed to be absent in animals, with the exception of nematodes at the early stages of embryogenesis. In recent years, however, the detection of malate synthase (MS) and isocitrate lyase (ICL), key enzymes involved in the glyoxylate cycle, in some animal tissue has raised questions regarding the evolutionary relationship of enzymes in bacteria and animals and suggests that animals encode alternative enzymes of the cycle that differ in function from known MS and ICL in non-metazoan species.
In enzymology, a citramalate CoA-transferase is an enzyme that catalyzes the chemical reaction
In enzymology, a citrate CoA-transferase is an enzyme that catalyzes the following chemical reaction:
The enzyme 2-dehydro-3-deoxy-phosphogluconate aldolase, commonly known as KDPG aldolase, catalyzes the chemical reaction
The enzyme citramalate lyase catalyzes the chemical reaction
The enzyme citramalyl-CoA lyase catalyzes the chemical reaction
The enzyme citryl-CoA lyase catalyzes the chemical reaction
The enzyme methylisocitrate lyase catalyzes the chemical reaction
In enzymology, a citrate—CoA ligase is an enzyme that catalyzes the chemical reaction
In enzymology, a [citrate (pro-3S)-lyase] ligase is an enzyme that catalyzes the chemical reaction
The enzyme citrate lyase deacetylase (EC 3.1.2.16) catalyzes the reaction
In enzymology, a 2-methylcitrate synthase (EC 2.3.3.5) is an enzyme that catalyzes the chemical reaction
ATP citrate synthase (also ATP citrate lyase (ACLY)) is an enzyme that in animals represents an important step in fatty acid biosynthesis. By converting citrate to acetyl-CoA, the enzyme links carbohydrate metabolism, which yields citrate as an intermediate, with fatty acid biosynthesis, which consumes acetyl-CoA. In plants, ATP citrate lyase generates cytosolic acetyl-CoA precursors of thousands of specialized metabolites, including waxes, sterols, and polyketides.
In enzymology, a citrate (Re)-synthase (EC 2.3.3.3) is an enzyme that catalyzes the chemical reaction
In enzymology, a deacetyl-[citrate-(pro-3S)-lyase] S-acetyltransferase (EC 2.3.1.49) is an enzyme that catalyzes the chemical reaction
Decylcitrate synthase (EC 2.3.3.2) is an enzyme that catalyzes the chemical reaction in enzymology.
In molecular biology, the citrate synthase family of proteins includes the enzymes citrate synthase EC 2.3.3.1, and the related enzymes 2-methylcitrate synthase EC 2.3.3.5 and ATP citrate lyase EC 2.3.3.8.
