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Interaction of S. aureus penicillinase with penicillin and methicillin. Penicillinase hydrolyzes the amide bond (highlighted in red) of the β-lactam ring of penicillin and ampicillin (A). Methicillin is resistant to cleavage by S. aureus penicillinase (a β-lactamase) due to the presence of an ortho-dimethoxyphenyl group (highlighted in green) that sterically hinders the enzyme from hydrolyzing its target amide bond (B).
