Structure of an extended-spectrum class A β-lactamase from Proteus vulgaris K1

The structure of a chromosomal extended-spectrum β-lactamase (ESBL) having the ability to hydrolyze cephalosporins including cefuroxime and ceftazidime has been determined by X-ray crystallography to 1.75 Å resolution. The species-specific class A β-lactamase from Proteus vulgaris K1 was crystallized at pH 6.25 and its structure solved by molecular replacement. Refinement of the model resulted in crystallographic R and Rfree of 16.9% and 19.3%, respectively. The folding of the K1 enzyme is broadly similar to that of non-ESBL TEM-type β-lactamases (2 Å rmsd for Cα) and differs by only 0.35 Å for all atoms of six conserved residues in the catalytic site. Other residues promoting extended-spectrum activity in K1 include the side-chains of atypical residues Ser237 and Lys276. These side-chains are linked by two water molecules, one of which lies in the position normally filled by the guanidinium group of Arg244, present in most non-ESBL enzymes but absent from K1. The ammonium group of Lys276, ca 3.5 Å from the virtual Arg244 guanidinium position, may interact with polar R2 substitutents on the dihydrothiazene ring of cephalosporins.