that all enzymes, like acetate kinase, are double­ displacement enzymes, and do their work by covalent catalysis. For one thing, I could not think of a single instance of an enzyme for which single­ displacement catalysis had been proved, and inquiries on this point among knowledgeable friends elicited the same negative response. Moreover, it was long known that the two other kinds of chemical catalysis~homo­ geneous and heterogeneous~occur through the intermediary formation of a covalent bond between catalyst and reactant. I began to feel confident that chemical catalysis by enzymes must happen the same way.