Probing active site chemistry in SHV beta-lactamase variants at ambler position 244 - Understanding unique properties of inhibitor resistance

Inhibitor-resistant class A beta-lactamases are an emerging threat to the use of beta-lactam/beta-lactamase inhibitor combinations (e.g. amoxicillin/clavulanate) in the treatment of serious bacterial infections. In the TEM family of Class A beta-lactamases, single amino acid substitutions at Arg-244 confer resistance to clavulanate inactivation. To understand the amino acid sequence requirements in class A beta-lactamases that confer resistance to clavulanate, we performed site-saturation mutagenesis of Arg-244 in SHV-1, a related class A beta-lactamase found in Klebsiella pneumoniae. Twelve SHV enzymes with amino acid substitutions at Arg-244 resulted in significant increases in minimal inhibitory concentrations to ampicillin/clavulanate when expressed in Escherichia coli. Kinetic analyses of SHV-1, R244S, R244Q, R244L, and R244E beta-lactamases revealed that the main determinant of clavulanate resistance was reduced inhibitor affinity. In contrast to studies in the highly similar TEM enzyme, we observed increases in clavulanate k(inact) for all mutants. Electrospray ionization mass spectrometry of clavulanate inhibited SHV-1 and R244S showed nearly identical mass adducts, arguing against a difference in the inactivation mechanism. Testing a wide range of substrates with C3-4 carboxylates in different stereochemical orientations, we observed impaired affinity for all substrates among inhibitor resistant variants. Lastly, we synthesized two boronic acid transition state analogs that mimic cephalothin and found substitutions at Arg-244 markedly affect both the affinity and kinetics of binding to the chiral, deacylation transition state inhibitor. These data define a role for Arg-244 in substrate and inhibitor binding in the SHV beta-lactamase.