In an effort to probe the structure of the reaction intermediate of metallo-beta-lactamase L1 when reacted with nitrocefin and other beta-lactams, time-dependent absorption and rapid-freeze-quench (RFQ) EPR spectra were obtained using the Co(II)-substituted form of the enzyme. When using nitrocefin as the substrate, time-dependent absorption spectra demonstrate that Co(II)-substituted L1 utilizes a reaction mechanism, similar to that of the native Zn(II) enzyme, in which a short-lived intermediate forms. RFQ-EPR spectra of this intermediate demonstrate that the binding of substrate results in a change in the electronic properties of one or both of the Co(II)'s in the enzyme that is consistent with a change in the coordination sphere of this metal ion. This observation provides evidence that the reaction intermediate is a metal-bound species. RFQ-EPR studies also demonstrate that other beta-lactams, such as cephalothin, meropenem, and penicillin G, proceed through an electronically similar complex and that the role of metal is similar in all cases. EPR spectroscopy has also identified distinct product-bound species of L1, indicating that reversible product binding must be considered in all future kinetic mechanisms. Consideration of the time-dependent optical and EPR studies in light of available crystallographic information indicates the intimate involvement of the metal ion in the Zn(2)-binding site of L1 in the hydrolytic reaction.

Metallo-␤-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 ␤-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 presteady-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.

A structural feature shared by the metallo-␤-lactamases is a flexible loop of amino acids that extends over their active sites and that has been proposed to move during the catalytic cycle of the enzymes, clamping down on substrate. To probe the movement of this loop (residues 152-164), a site-directed mutant of metallo-␤-lactamase L1 was engineered that contained a Trp residue on the loop to serve as a fluorescent probe. It was necessary first, however, to evaluate the contribution of each native Trp residue to the fluorescence changes observed during the catalytic cycle of wild-type L1. Five site-directed mutants of L1 (W39F, W53F, W204F, W206F, and W269F) were prepared and characterized using metal analyses, CD spectroscopy, steady-state kinetics, stopped-flow fluorescence, and fluorescence titrations. All mutants retained the wild-type tertiary structure and bound Zn(II) at levels comparable with wild type and exhibited only slight (<10-fold) decreases in k cat values as compared with wild-type L1 for all substrates tested. Fluorescence studies revealed a single mutant, W39F, to be void of the fluorescence changes observed with wild-type L1 during substrate binding and catalysis. Using W39F as a template, a Trp residue was added to the flexile loop over the active site of L1, to generate the double mutant, W39F/D160W. This double mutant retained all the structural and kinetic characteristics of wild-type L1. Stopped-flow fluorescence and rapid-scanning UV-visible studies revealed the motion of the loop (k obs ‫؍‬ 27 ؎ 2 s ؊1 ) to be similar to the formation rate of a reaction intermediate (k obs ‫؍‬ 25 ؎ 2 s ؊1 ).

Metallo-beta-lactamases (mbetals) are zinc-dependent enzymes that hydrolyze a wide range of beta-lactam antibiotics. The mbetal active site features an invariant Asp-120 that ligates one of the two metal ions (Zn2) and a metal-bridging water/hydroxide (Wat1). Previous studies show that substitutions at Asp-120 dramatically affect mbetal activity, but no consensus exists as to its role in beta-lactam turnover. Here we present crystal structures of the Asn and Cys mutants of Asp-120 of the L1 mbetal from Stenotrophomonas maltophilia. Both mutants retain a dinuclear zinc center with Wat1 present. In the essentially inactive Cys enzyme Zn2 is displaced to a more buried position relative to that in the wild-type enzyme. In the catalytically impaired Asn enzyme the coordination of Zn2 is altered, neither it nor Wat1 is coordinated by Asn-120, and the N-terminal 19 amino acids, important to cooperative interactions between subunits in the wild-type enzyme, are disordered. Comparison with the structure of L1 complexed with the hydrolyzed oxacephem moxalactam suggests that in the Cys mutant Zn2 can no longer make stabilizing interactions with anionic nitrogen species formed in the hydrolytic reaction. The diminished activity of the Asn mutant arises from a combination of loss of intersubunit interactions and impaired proton transfer to, and reduced interaction of Zn2 with, the substrate amide nitrogen. We conclude that, while interactions of Asp-120 with active site water molecules are important to proton transfer and possibly nucleophilic attack by Wat1, its primary role is to optimally position Zn2 for catalytically important interactions with the charged amide nitrogen of substrate.