Recent advances in the biosynthesis of lincosamide antibiotics

Abstract

Lincosamides characterized by an unusual C1 mercapto substituted 8-carbon sugar joined with a proline moiety by an amide linkage, is a small family of antibiotics. The vital role in clinic and the unique structures have attracted much attention for many years. Natural lincosamides contain only four compounds to date: lincomycin A and B, celesticetin and Bu-2545, among which only lincomycin A possesses good in vitro and in vivo potency against a variety of Gram-positive microorganisms, such as Bacillus anthracis , Staphylococcus aureus , Staphylococcus epidermidis , Streptococcus agalactiae , Streptococcus pneumoniae , Streptococcus pyogenes and Streptococcus viridans . Because of the excellent antibiotic activity of lincomycin A, hundreds of synthetic, semisynthetic and biosynthetic fermentation derivatives have been prepared. However, most of them are not suitable for clinical use due to the low biological activity or high toxicity and so on. Only clindamycin, a semisynthetic analog of lincomycin A, is a clinical acceptance and shows better antibiotic activities than lincomycin A. Natural lincosamides have typically been isolated from actinomycetes and are generated from large and complex gene clusters. Despite found and clinical used for nearly half a century, their biosynthetic pathways have not been elucidated completely. During the past few years, great progress was achieved on lincosamides biosynthetic pathway. It is found that the C1 mercapto substituted 8-carbon sugar derived from the condensation of d-ribose 5-phosphate and d-fructose 6-phosphate or d-sedoheptulose 7-phosphate. During the biosynthesis of the octose, two unique low molecular weight thiols: ergothioneine and mycothiol, function through two unusual glycosylations that program lincosamide transfer, activation and modification, providing the first paradigm for ergothioneine associated biochemical processes and for the poorly understood mycothiol dependent biotransformations. The biosynthesis of lincomycin and celesticestin is branched from a pair of homologous proteins, both of which are pyridoxal-5′-phosphate dependent proteins and are highly related in phylogenesis, but cysteine S -conjugated intermediates are processed in different ways and associated with individual downstream enzyme(s) toward distinct S -functionalization of lincomycin and celesticetin. During the formation of the trans 4-propyl- L -proline moiety in lincomycin, two γ-glutamyltranspeptidase homolog proteins (the specific LmbA and a non-specific LmbA2991) catalyze an unusual C−C bond cleavage reaction. Both of them belong to N -terminal nucleophile (Ntn) hydrolase superfamily, and exhibit the activity for post-translational autocatalytic cleavage of a peptide precursor to form a functional heterodimer in which the newly released N -terminal threonine residue acts as a nucleophile to attack a carbonyl group and catalyze the C−C bond cleavage. For the linkage of the octose and the pyrrole rings, the latter are activated by standalone adenylation domains and uploaded to peptidyl carrier proteins, followed by condensation reaction with ergothioneine S -conjugate lincosamine to afford amide bonds. It cannot imagine that such small molecules are biosynthesized via inconceivable process. Although there are still some questions on lincosamides biosynthesis, chances are that more surprise will be unveiled in the near future. Furthermore, on the basis of the biosynthetic mechanism of lincosamides, their biosynthetic pathway could be rationally reconstructed to obtain diverse lincosamide analogues which have distinct physiological activities and are difficult to synthesize, including several unnatural chimeras with antibacterial properties better than lincomycin A, opening the door for combinatorial biosynthesis of hybrid lincosamide antibiotics. But more than that, due to the excellent antibacterial activity of lincomycin A, the lincomycin-producing strain- Streptomyces lincolnensis has been engineered for quite a long time to improve the production of lincomycin A, which may help cut production costs and simplify downstream separation processes. This review mainly covers the most recent advances in the biosynthesis of lincomycin and celesticetin, and on this basis, in the structural diversity expansion and lincomycin-producing strain engineering.