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.
A transferase is any one of a class of enzymes that catalyse the transfer of specific functional groups from one molecule to another. They are involved in hundreds of different biochemical pathways throughout biology, and are integral to some of life's most important processes.
Isocitrate dehydrogenase (IDH) (EC 1.1.1.42) and (EC 1.1.1.41) is an enzyme that catalyzes the oxidative decarboxylation of isocitrate, producing alpha-ketoglutarate (α-ketoglutarate) and CO2. This is a two-step process, which involves oxidation of isocitrate (a secondary alcohol) to oxalosuccinate (a ketone), followed by the decarboxylation of the carboxyl group beta to the ketone, forming alpha-ketoglutarate. In humans, IDH exists in three isoforms: IDH3 catalyzes the third step of the citric acid cycle while converting NAD+ to NADH in the mitochondria. The isoforms IDH1 and IDH2 catalyze the same reaction outside the context of the citric acid cycle and use NADP+ as a cofactor instead of NAD+. They localize to the cytosol as well as the mitochondrion and peroxisome.
Aconitase is an enzyme that catalyses the stereo-specific isomerization of citrate to isocitrate via cis-aconitate in the tricarboxylic acid cycle, a non-redox-active process.
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 caffeoyl-CoA O-methyltransferase is an enzyme that catalyzes the chemical reaction
Isocitrate lyase, or ICL, is an enzyme in the glyoxylate cycle that catalyzes the cleavage of isocitrate to succinate and glyoxylate. Together with malate synthase, it bypasses the two decarboxylation steps of the tricarboxylic acid cycle and is used by bacteria, fungi, and plants.
In enzymology, a 1-acylglycerol-3-phosphate O-acyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, an anthocyanin 5-O-glucoside 6'''-O-malonyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a galactarate O-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a glucarate O-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a glucarolactone O-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a malate synthase (EC 2.3.3.9) is an enzyme that catalyzes the chemical reaction
In enzymology, a polysialic-acid O-acetyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a putrescine N-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a quinate O-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a rosmarinate synthase is an enzyme that catalyzes the chemical reaction
In enzymology, a shikimate O-hydroxycinnamoyltransferase is an enzyme that catalyzes the chemical reaction
In enzymology, a [isocitrate dehydrogenase (NADP+)] kinase (EC 2.7.11.5) is an enzyme that catalyzes the chemical reaction:
The biosynthesis of phenylpropanoids involves a number of enzymes.
