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.
Malate dehydrogenase (EC 1.1.1.37) (MDH) is an enzyme that reversibly catalyzes the oxidation of malate to oxaloacetate using the reduction of NAD+ to NADH. This reaction is part of many metabolic pathways, including the citric acid cycle. Other malate dehydrogenases, which have other EC numbers and catalyze other reactions oxidizing malate, have qualified names like malate dehydrogenase (NADP+).
Aromatic-ring-hydroxylating dioxygenases (ARHD) incorporate two atoms of dioxygen (O2) into their substrates in the dihydroxylation reaction. The product is (substituted) cis-1,2-dihydroxycyclohexadiene, which is subsequently converted to (substituted) benzene glycol by a cis-diol dehydrogenase.
Carboxy-lyases, also known as decarboxylases, are carbon–carbon lyases that add or remove a carboxyl group from organic compounds. These enzymes catalyze the decarboxylation of amino acids, beta-keto acids and alpha-keto acids.
In enzymology, a GDP-4-dehydro-D-rhamnose reductase (EC 1.1.1.187) is an enzyme that catalyzes the chemical reaction
In enzymology, a gluconate 5-dehydrogenase (EC 1.1.1.69) is an enzyme that catalyzes the chemical reaction
In enzymology, a glycerate dehydrogenase (EC 1.1.1.29) is an enzyme that catalyzes the chemical reaction
In enzymology, a hydroxyphenylpyruvate reductase (EC 1.1.1.237) is an enzyme that catalyzes the chemical reaction
In enzymology, an indolelactate dehydrogenase (EC 1.1.1.110) is an enzyme that catalyzes the chemical reaction
In enzymology, a malate dehydrogenase (NADP+) (EC 1.1.1.82) is an enzyme that catalyzes the chemical reaction
In enzymology, a 3-hydroxyacyl-CoA dehydrogenase (EC 1.1.1.35) is an enzyme that catalyzes the chemical reaction
(R)-4-hydroxyphenyllactate dehydrogenase (EC 1.1.1.222) is an enzyme that catalyzes a chemical reaction
In enzymology, a 3-(imidazol-5-yl)lactate dehydrogenase (EC 1.1.1.111) is an enzyme that catalyzes the chemical reaction
In enzymology, a 1,6-dihydroxycyclohexa-2,4-diene-1-carboxylate dehydrogenase (EC 1.3.1.25) is an enzyme that catalyzes the chemical reaction
In enzymology, a 2-coumarate reductase or melilotate dehydrogenase (EC 1.3.1.11) is an enzyme that catalyzes the chemical reaction
In enzymology, a cis-1,2-dihydroxy-4-methylcyclohexa-3,5-diene-1-carboxylate dehydrogenase (EC 1.3.1.67) is an enzyme that catalyzes the chemical reaction
In enzymology, a 2-oxoaldehyde dehydrogenase (NAD+) (EC 1.2.1.23) is an enzyme that catalyzes the chemical reaction
In enzymology, a cystine reductase (EC 1.8.1.6) is an enzyme that catalyzes the chemical reaction
Octopine dehydrogenase (N2-(D-1-carboxyethyl)-L-arginine:NAD+ oxidoreductase, OcDH, ODH) is a dehydrogenase enzyme in the opine dehydrogenase family that helps maintain redox balance under anaerobic conditions. It is found largely in aquatic invertebrates, especially mollusks, sipunculids, and coelenterates, and plays a role analogous to lactate dehydrogenase (found largely in vertebrates) . In the presence of NADH, OcDH catalyzes the reductive condensation of an α-keto acid with an amino acid to form N-carboxyalkyl-amino acids (opines). The purpose of this reaction is to reoxidize glycolytically formed NADH to NAD+, replenishing this important reductant used in glycolysis and allowing for the continued production of ATP in the absence of oxygen.
Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvate to lactate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.
