Enzymology of Gentamicin Biosynthesis
Gentamicin C complex is a mixture of five structurally similar aminoglycoside antibiotics, gentamicins C1, C1a, C2, C2a, and C2b, produced by the actinomycete bacterium Micromonospora echinospora. It is established in clinical use and despite significant toxicity remains valuable to treat severe Gram-negative bacterial infections. There is a pressing need to develop novel versions of such antibiotics to combat the rise of resistance among pathogens. Engineering of the pathway requires a detailed knowledge of the genes, enzymes, and intermediates involved. The final steps of gentamicin biosynthesis begin at gentamicin X2, the last common intermediate of the C complex. 6'-C-Methylation generates two branches, with analogous reactions happening in both. Candidate genes and enzymes for the steps from the first 6'-C-methylated intermediate, G418, to an aminated metabolite JI-20B have already been described, but none for the subsequent loss of two hydroxyl groups from Ring II, or the N-methylation that then occurs. A novel separation method using dynamic countercurrent chromatography was successfully applied to the difficult purification of gentamicin metabolites. The results described here allowed a detailed mechanism to be proposed for almost the entire pathway from G418 to the C complex, and by analogy for the unbranched pathway, too. The last step of the pathway is 6'-N-methylation of gentamicins C1a and C2. Genome mining and cell-free assays were used by the group of Professor Yuhui Sun (Wuhan University) to identify genL, a methyltransferase gene encoded elsewhere on the M. echinospora genome and capable of this catalysis. Here, in vitro reactions with recombinant GenL confirmed its function, and its kinetic parameters were measured with its substrates. The full mechanistic pathway for the late stages of gentamicin C complex biosynthesis has therefore now been elucidated.