Streptogramin B is a subgroup of the streptogramin antibiotics family. These natural products are cyclic hexa- or hepta depsipeptides produced by various members of the genus of bacteria Streptomyces. Many of the members of the streptogramins reported in the literature have the same structure and different names; for example, pristinamycin IA = vernamycin Bα = mikamycin B = osteogrycin B.[1]
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The biosynthesis of streptogramin B is carried out by large multifunctional enzymes called non-ribosomal peptide synthetases (NRPS). In the NRPS system, each amino acid is activated as an aminoacyladenylate and is linked to the enzyme as a thioester with a phosphopantetheinyl group. An elongation reaction then occurs by transferring the activated carboxyl to the amino group in the next amino acid, thus executing the N-to-C stepwise condensation.
NRPSs contain several modules on a single polypeptide. Each of these modules can catalyze activation, condensation and a modification reaction specific to one kind of amino acid.[2] A typical elongation module consists of an adenylation domain (A), a peptidyl carrier protein domain (PCP) and a condensation domain (C). Some other domains may be present that are responsible for modifications to the residues, such as epimerization domain (E) and N-methyltransferase domain (MT). The domain responsible for the termination is the thioesterase domain (TE) located in the final module.[3]
The general amino acid composition of streptogramin B consists of: 3-hydroxypicolinic acid, L-threonine, D-aminobutyric acid, L-proline, 4-N,N-(dimethylamino)-L-phenylalanine, 4-oxo-L-pipecolic acid and phenylglycine.
Streptogramins A and B synergically inhibit bacterial cell growth by inhibiting protein synthesis, but separately they are bacteriostatic. The molecular target of streptogramins is the 23S rRNA. Both streptogramin A and B bind to the P binding site of the 50S ribosome subunit. The type A streptogramin binding causes a conformational change to the 50S subunit, which increases the activity of the type B by a 100-fold. Streptogramin B prevents the elongation of protein chains and causes the release of incomplete peptides.[1]
The streptogramin antibiotics were identified almost 50 years ago, but have only recently found clinical use as a consequence of the increase in multidrug-resistant bacteria. They present poor solubility in aqueous solution, and this has limited their clinical use; however, the natural products still find use as feed additives in agriculture.[4]
Medicinal chemists at Rhône-Poulenc worked in the preparation of semi-synthetic, water-soluble, derivatives of pristinamycin IA (B type streptogramin) and pristinamycin IIA (A type streptogramin) giving rise to quinupristin and dalfopristin, respectively, which, when administered in a 3:7 ratio, comprise the 1999 FDA approved drug Synercid.[1]