Active Sites of $\beta$-Lactamases from Bacillus cereus

There are two extracellular $\beta$-lactamases produced by Bacillus cereus 569. One of these enzymes, $\beta$-lactamase I, is inactivated by 6-$\beta$-bromopenicillanic acid: the site of reaction is serine-44. This is a conserved amino acid residue in the other $\beta$-lactamases whose structures have been determined, and it becomes a good candidate for an active-site group in these enzymes. The inactivation may involve a rearrangement leading to a dihydrothiazine. The other extracellular enzyme produced by B. cereus, $\beta$-lactamase II, is exceptional in requiring metal ions for activity. The Zn II and Co II enzymes (the former is more active) have been studied by nuclear magnetic resonance, and by absorption spectroscopy. The groups that bind the metal ion required for activity are three histidine residues and the enzyme's sole thiol group.