A cyclic antibiotic synthesized by Bacillus brevis. The molecule has the structure [d-phe–pro–val–orn–leu leu–orn–val–pro–d–phe] and it contains amino acids not usually found in proteins—namely, ornithine (orn) and d-phenylalanine (rather than the usual l-isomer). The synthesis of gramicidin S constitutes one of the best understood examples of a polypeptide that is not synthesized on a ribosome. Two enzymes are required (E1 and E2), which are bound together, forming a unit called gramicidin synthetase. One molecule each of proline, valine, ornithine, and leucine bind in that sequence to sulfhydryl groups of E1. E2 functions to isomerize l- to d-phenylalanine and to transfer it to the proline attached to E1. The two identical polypeptides are then joined head to tail to form a decapeptide. This rare type of polypeptide synthesis is very uneconomical and cannot generate molecules greater than 20 amino acids long.
Subjects: Genetics and Genomics — Chemistry.