Acetyl-CoA-Mediated Post-Biosynthetic Modification of Desferrioxamine B Generates - and N--Acetylated Isomers Controlled by a pH Switch.

Biosynthesis of the hydroxamic acid siderophore desferrioxamine D (DFOD, ), which is the -acetylated analogue of desferrioxamine B (DFOB, ), has been delineated. Enzyme-independent Ac-CoA-mediated -acetylation of produced , in addition to three constitutional isomers containing an N--acetyl group installed at either one of the three hydroxamic acid groups of . The formation of -Ac-DFOB (DFOD, ) and the composite of N--acetylated isomers N--Ac-DFOB[001] (), N--Ac-DFOB[010] (), and N--Ac-DFOB[100] () (defined as the N--Ac motif positioned within the terminal amine, internal, or -acetylated region of , respectively), was pH-dependent, with - dominant at pH < 8.5 and dominant at pH > 8.5. The trend in the pH dependence was consistent with the p values of the NH (p ∼ 10) and N-OH (p ∼ 8.5-9) groups in . The - and N--acetyl motifs can be conceived as a post-biosynthetic modification (PBM) of a nonproteinaceous secondary metabolite, akin to a post-translational modification (PTM) of a protein. The pH-labile N--acetyl group could act as a reversible switch to modulate the properties and functions of secondary metabolites, including hydroxamic acid siderophores. An alternative (most likely minor) biosynthetic pathway for showed that the nonribosomal peptide synthetase-independent siderophore synthetase DesD was competent in condensing -acetyl--succinyl--hydroxy-1,5-diaminopentane (-Ac-SHDP, ) with the dimeric hydroxamic acid precursor (AHDP-SHDP, ) native to biosynthesis to generate . The strategy of diversifying protein structure and function using PTMs could be paralleled in secondary metabolites with the use of PBMs.