Inactivation of Bacillus cereus beta-lactamase I by 6 beta-bromopencillanic acid: mechanism

The mechanism of the inactivation of Bacillus cereus beta-lactamase I by 6 beta-bromopenicillanic acid, a probable suicide substrate [see Loosemore, M.J., Cohen, S.A., & Pratt, R.F. (1980) Biochemistry (preceding paper in this issue)], is described. Inactivation is accompanied by covalent modification of the protein with the appearance of a characteristic chromophore at 326 nm. Ultraviolet (UV) absorption, nuclear magnetic resonance (NMR), and circular dichroic (CD) spectra of the modified protein, of a modified peptide derived from the protein by enzymatic digestion, and of relevant model compounds suggest that acylation of the enzyme by 6 beta-bromopenicillanic acid is accompanied by rearrangement and cyclization of the inhibitor to a 2,3-dihydro-1,4-thiazine-3,6-dicarboxylic acid derivative, which is the observed chromophore. The acylated residue is shown to be Ser-70. The mechanism of action of beta-lactamase inhibitors is discussed.