Abstract
5-Methyl-3-phenyl-4-isoxazolyl penicillin is a competitive inhibitor of penicillinase. Interaction with this compound causes marked conformational changes in the enzyme. The changes are reflected in increased susceptibility to iodination, urea and heat. Present observations are related to previously recorded evidence, and the conclusion is reached that the side-chain of a penicillin may confer resistance by distorting the catalytic orientation of penicillinase.