Characterization of the bifunctional aminoglycoside-modifying enzyme ANT(3')-Ii/AAC(6')-IId from Serratia marcescens

A newly discovered bifunctional antibiotic resistance enzyme from Serratia marcescens catalyzes adenylation and acetylation of aminoglycoside antibiotics. The structure assignment of the enzymic products indicated that acetylation takes place on the 6'-amine of kanamycin A and the adenylation on 3'- and 9-hydroxyl groups of streptomycin and spectinomycin, respectively. The adenyltransferase domain appears to be highly specific to spectinomycin and streptomycin, while the acetyltransferase domain shows a broad substrate profile. Initial velocity patterns indicate that both domains follow a sequential kinetic mechanism. The use of dead-end and product inhibition, the solvent isotope effect, and the solvent viscosity effect reveals that the adenyltransferase domain catalyzes the reaction by a Theorell-Chance kinetic mechanism, where ATP binds to the enzyme prior to the aminoglycoside and the modified antibiotic is the last product to be released. The acetyltransferase domain follows an ordered bi-bi kinetic mechanism, in which the antibiotic is the first substrate that binds to the active site and CoASH is released prior to the modified aminoglycoside. The merging of two genes to create bifunctional resistance enzymes with expanded profiles has now been documented in four instances, including the subject of study in this report, which suggests a new trend in the emergence of resistance to aminoglycoside antibiotics among pathogens.