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abstracts
Macrolides are antibiotics that have been in use since the late 1950s to treat a wide range of bacterial infections (e.g. upper respiratory infections, skin and soft-tissue infections, stomach ulcers and some venereal diseases). The structure of these antibiotics contains a lactone ring of either 14, 15, or 16 members, with a variety of sugar moieties attached. Resistance to this class of antibiotics may result from the reaction carried out by macrolide phosphotransferases [MPHs]. MPHs belong to the family of antibiotic kinases which catalyzes the transfer of a phosphate group from a nucleoside triphosphate to a specific hydroxyl on the antibiotic. However, unlike most antibiotic kinases, MPHs utilize GTP as the phosphate donor. Specifically, 2'-macrolide phosphotransferase type I [MPH(2')-I] transfers the gamma-phosphate from GTP to the 2'-hydroxyl of 14- and 15-membered ring macrolides. Crystal structure of the ternary complexes of MPH(2')-I with both 14- and 15-membered lactone macrolides have been determined. To study the basis of substrate selectivity, we have generated mutations of several amino acid residues in the macrolide-binding pocket and examined the catalytic activities of these mutants on the different classes of macrolides, including those containing a 16-membered lactone. Furthermore, we will present kinetic studies of MPH(2')-I containing mutations in the nucleoside-binding pocket in order to study the mechanism for the enzyme's preference for GTP.
abstracts
2'-macrolide phosphotransferase type I [MPH(2')-I] is an antibiotic kinase that renders many macrolides, such as erythromycin, inactive by catalyzing the transfer of a phosphate group from a nucleoside triphosphate to the hydroxyl at the 2'-position of the antibiotic. MPH(2')-I is functionally and structurally analogous to the aminoglycoside kinases (APHs). However, it is distinct from most APHs in that it utilizes GTP exclusively as its phosphate donor. We will present the crystal structure of MPH(2')-I in its apo and ternary complex forms with guanosine nucleotide and different macrolide substrates. We will compare its nucleoside-binding pocket to that of the 2''-aminoglycoside phosphotransferases [APH(2'')], a subclass of aminoglycoside kinases that are capable of utilizing GTP as a phosphate donor. To further decipher the structural basis of the nucleoside specificity of MPH(2')-I, mutations of amino acid resides in the nucleoside-binding pocket have been carried out and their effects on the binding affinity of purine nucleotides were examined by isothermal titration calorimetry. Our preliminary results show that the "gatekeeper" residue plays a role in governing the nucleoside selectivity.
