Crotonyl coenzyme

Li, F. et ed (2008) Coupled ferredoxin and crotonyl coenzyme A (CoA) reduction with NADH catalyzed by the butyryl-CoA dehydrogenase/Etf complex from Qostridium kluyveri. J. Bacterial., 190, 843-850. 

The degradation of pimelate is initiated by formation of the coenzyme A ester and is followed by a series of steps with the production of glutaryl-CoA that is decarboxylated to crotonyl-CoA... 

Biotin (vitamin B ) is widespread in foods and is also synthesized by intestinal bacteria. It is a coenzyme for the carboxylation of pyruvate, acetyl-coenzyme-A (CoA), propionyl CoA, and /1-methyl-crotonyl CoA and is involved in fatty acid formation and in energy release from carbohydrates. In humans deficiencies only occur in patients with an abnormal gut flora and manifests itself as exfoliative dermatitis and alopecia. 

The product of acetyl-CoA carboxylase reaction, malonyl-CoA, is reduced via malonate semialdehyde to 3-hydroxypropionate, which is further reductively converted to propionyl-CoA. Propionyl-CoA is carboxylated to (S)-methylmalonyl-CoA by the same carboxylase. (S)-Methylmalonyl-CoA is isomerized to (R)-methylmal-onyl-CoA, followed by carbon rearrangement to succinyl-CoA by coenzyme B 12-dependent methylmalonyl-CoA mutase. Succinyl-CoA is further reduced to succinate semialdehyde and then to 4-hydroxybutyrate. The latter compound is converted into two acetyl-CoA molecules via 4-hydroxybutyryl-CoA dehydratase, a key enzyme of the pathway. 4-Hydroxybutyryl-CoA dehydratase is a [4Fe-4S] cluster and FAD-containing enzyme that catalyzes the elimination of water from 4-hydroxybutyryl-CoA by a ketyl radical mechanism to yield crotonyl-CoA [34]. Conversion of the latter into two molecules of acetyl-CoA proceeds via normal P-oxidation steps. Hence, the 3-hydroxypropionate/4-hydroxybutyrate cycle (as illustrated in Figure 3.5) can be divided into two parts. In the first part, acetyl-CoA and two bicarbonate molecules are transformed to succinyl-CoA, while in the second part succinyl-CoA is converted to two acetyl-CoA molecules. 

Reduction and dehydration. The ketone carbonyl group in acetoacetyl ACP is next reduced to an alcohol by NADPH (nicotinamide adenine dinucleotide phosphate), a reducing coenzyme closely related to NADH. Subsequent dehydration of the resulting /3-hydroxy thiol ester (E2 reaction) in step 7 yields crotonyl ACP, and the carbon-carbon double bond of crotonyl ACP is further reduced by NADPH in step 8 to yield butyryl ACP. 

Eggers J, Steinbiichel A (2013) Poly(3-hydroxybutyrate) degradation in Ralstonia eutropha H16 is mediated stereoselectively to (S)-3-hydroxybutyryl coenzyme A (CoA) via crotonyl-CoA. J Bacteriol 195 3213-3223... 

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