Steady-state kinetic analysis of pterin-dependent phenylalanine hydroxylase from Chromobacterium violaceum indicated that the enzyme follows a partially ordered reaction mechanism. The data suggested that oxygen is the first substrate to bind to the enzyme. This result was further supported by rapid-quench experiments in which the enzyme–oxygen complex was trapped to yield product. Additional support for the presence of an enzyme–oxygen complex was derived from magnetic susceptibility measurements of molecular oxygen in the presence and absence of cuprous phenylalanine hydroxylase. The magnetic susceptibility of dissolved oxygen decreased in the presence of the enzyme, supporting a direct oxygen–metal interaction.
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