Plasticity of aminoglycoside binding to antibiotic kinase APH(2")-Ia [PublishAheadOfPrint]

APH(2")-Ia aminoglycoside resistance enzyme forms the C-terminal domain of the bifunctional AAC(6')-Ie/APH(2")-Ia enzyme and confers high-level resistance to natural 4,6-disubstituted aminoglycosides. In addition, reports have suggested that the enzyme can phosphorylate 4,5-disubstituted compounds and aminoglycosides with substitutions at the N1-position. Previously determined structures of the enzyme with aminoglycosides bound have not indicated how these non-canonical substrates could bind and be modified by the enzyme. We carried out crystallographic studies to directly observe the interaction of these compounds with the aminoglycoside-binding site and probe the means by which these non-canonical substrates interact with the enzyme. We find that APH(2")-Ia maintains a preferred mode of binding aminoglycosides using the conserved neamine rings when possible, with flexibility that allows it to accommodate additional rings. However, if this binding mode is made impossible because of additional substitutions to the standard 4,5- or 4,6-disubstituted aminoglycoside architecture as in lividomycin A or the N1-substituted aminoglycosides, it is still possible for these aminoglycosides to bind to the antibiotic-binding site, using alternate binding modes, explaining the low rates of non-canonical phosphorylation activities seen in enzyme assays. Furthermore, structural studies of a clinically-observed arbekacin-resistant mutant of APH(2")-Ia reveals an altered aminoglycoside binding site that can stabilize an alternative binding mode for N1-substituted aminoglycosides. This mutation could alter and expand the aminoglycoside resistance spectrum of the wildtype enzyme in response to newly developed aminoglycosides.

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