Reactions of anthranilate hydroxylase with salicylate, a nonhydroxylated substrate analogue. Steady state and rapid reaction kinetics.

Steady state and rapid reaction kinetics of the flavoprotein anthranilate hydroxylase (EC 1.14.12.2) have been examined with the nonhydroxylated substrate analogue, salicylate. Since the reaction with salicylate does not involve events in which aromatic substrate is oxygenated, it provides a simpler model for studying the hysteresis exhibited by this enzyme. It is shown that the first turnover of the enzyme is slower than subsequent turnovers owing in part to slow initial binding reactions of salicylate with the enzyme. The reductive half-reaction of the first turnover is also slow since rapid reduction of the enzyme flavin requires bound aromatic substrate. The oxidative half-reaction involves reaction of the reduced enzyme-salicylate complex with oxygen to form a flavin C4a-hydroperoxide, which then decays to oxidized flavoenzyme and H2O2. Several lines of evidence indicate that salicylate remains bound to the enzyme at the end of the catalytic cycle so that in turnovers subsequent to the first, the slow steps involving salicylate binding are avoided.