Metal binding Asp-120 in metallo-β-lactamase L1 from Stenotrophomonas maltophilia plays a crucial role in catalysis

Metallo-beta-lactamase L1 from Stenotrophomonas maltophilia is a dinuclear Zn(II) enzyme that contains a metal-binding aspartic acid in a position to potentially play an important role in catalysis. The presence of this metal-binding aspartic acid appears to be common to most dinuclear, metal-containing, hydrolytic enzymes; particularly those with a beta-lactamase fold. In an effort to probe the catalytic and metal-binding role of Asp-120 in L1, three site-directed mutants (D120C, D120N, and D120S) were prepared and characterized using metal analyses, circular dichroism spectroscopy, and pre-steady-state and steady-state kinetics. The D120C, D120N, and D120S mutants were shown to bind 1.6+/-0.2, 1.8+/-0.2, and 1.1+/-0.2 mol of Zn(II) per monomer, respectively. The mutants exhibited 10- to 1000-fold drops in k(cat) values as compared with wild-type L1, and a general trend of activity, wild-type>D120N>D120C and D120S, was observed for all substrates tested. Solvent isotope and pH dependence studies indicate one or more protons in flight, with pK(a) values outside the range of pH 5-10 (except D120N), during a rate-limiting step for all the enzymes. These data demonstrate that Asp-120 is crucial for L1 to bind its full complement of Zn( II) and subsequently for proper substrate binding to the enzyme. This work also confirms that Asp-120 plays a significant role in catalysis, presumably via hydrogen bonding with water, assisting in formation of the bridging hydroxide/water, and a rate-limiting proton transfer in the hydrolysis reaction.