Streptogramin A

Streptogramin A is group of antibiotics within the larger family of antibiotics known as streptogramins. They are synthesized by the bacteria Streptomyces virginiae. The streptogramin family of antibiotics consists of two distinct groups: group A antibiotics contain a 23-membered unsaturated ring with lactone and peptide bonds while group B antibiotics are depsipeptides (lactone-cyclized peptides). While structurally different, these two groups of antibiotics act synergistically, providing greater antibiotic activity than the combined activity of the separate components. These antibiotics have until recently been commercially manufactured as feed additives in agriculture, although today there is increased interest in their ability to combat antibiotic-resistant bacteria, particularly vancomycin-resistant bacteria.

Streptogramin A is a polyketide in nature, but contains some amino acid components as well. Its gene cluster codes for a hybrid PKS-NRPS protein that consists of eight PKS modules and two NRPS modules. Other enzymes are required for tailoring of streptogramin A, particularly for the unusual methylation reaction. The figure below shows the origins of the synthetic components of streptogramin A.

The streptogramin A PKS-NRPS is composed of 6 proteins: VirA contains modules 1 though 6; VirF, VirG, and VirH contain modules 6 through 10; VirI is the AT domain that acts for every PKS module; and VirJ contains the TE domain. The starter unit for the biosynthesis of streptogramin A is isobutyryl-CoA, which is given by the amino acid valine after it has undergone transamination and branched-chain keto acid dehydrogenation. Two rounds of chain extension with malonate follow. An NRPS module introduces a glycine residue into the growing polyketide chain, followed by two more rounds of chain extension with malonate. At this point, four enzymes use acetyl-CoA to add a methyl group to position 12 on the macromolecule. The mechanism of the reaction is proposed below. VirC and HMG-CoA synthase bear striking structural similarities and while the mechanism for VirC is not known, it can be proposed to be similar to that of HMG-CoA synthase.

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