Microbial physiology of sidechain degradation of sterols

A large number of valuable starting materials for steroids synthesis (e.g. 4-androstene-3,17-dione, 1,4-androstadiene-3,17-dione, 9α-hydroxy-4-androsten-17-one) have been produced by microbial transformation methods. This review helps to evaluate the microbial physiological interest of the widely used sterol sidechain degradation processes. Four inducible groups of the catabolic enzymes are involved in the sterol sidechain degradation pathway; the fatty acid β-oxidation system, the ω-oxidase reaction, a methyl-crotonyl-CoA carboxylation system and the propionyl-CoA carboylase system. Altogether nine catabolic enzymes are involved in the β-sitosterol sidechain degradation pathway. They work in 14 consecutive enzymatic steps. Summing up: three molecules of FADH2, three molecules of propionyl-SCoA, three of NADH and one molecule of acetic acid are formed, while the sidechain of one mole of sitosterol is removed selectively. The metabolism of the propionates and the acetate yield 18 molecules of NADH and 7 molecules of FADH2. Taking into consideration the whole process more than 80 molecules of ATP could be formed during the sitosterol sidechain degradation process.