Kinetics of a Two-Component p-Hydroxyphenylacetate Hydroxylase Explain How Reduced Flavin Is Transferred from the Reductase to the Oxygenase

p-Hydroxyphenylacetate hydroxylase (HPAH) from Acinetobacter baumannii catalyzes the hydroxylation of p-hydroxyphenylacetate (HPA) to form 3,4-dihydroxyphenylacetate (DHPA). HPAH is composed of two proteins: a flavin mononucleotide (FMN) reductase (C1) and an oxygenase (C2). C1 catalyzes the reduction of FMN by NADH to generate reduced FMN (FMNH-) for use by C2 in the hydroxylation reaction. C1 is unique among the flavin reductases in that the substrate HPA stimulates the rates of both the reduction of FMN and release of FMNH- from the enzyme. This study quantitatively shows the kinetics of how the C1-bound FMN can be reduced and released to be used efficiently as the substrate for the C2 reaction; additional FMN is not necessary. Reactions in which O2 is rapidly mixed with solutions containing C1-FMNH- and C2 are very similar to those in which solutions containing O2 are mixed with one containing the C2-FMNH- complex. This suggests that in a mixture of the two proteins FMNH- binds more tightly to C2 and has already been completely transferred to C2 before it reacts with oxygen. Rate constants for the transfer of FMNH- from C1 to C2 were found to be 0.35 and >or=74 s-1 in the absence and presence of HPA, respectively. The reduction of cytochrome c by FMNH- was also used to measure the dissociation rate of FMNH- from C1. In the absence of HPA, FMNH- dissociates from C1 at 0.35 s-1, while with HPA present it dissociates at 80 s-1; these are the same rates as those for the transfer from C1 to C2. Therefore, the dissociation of FMNH- from C1 is rate-limiting in the intermolecular transfer of FMNH- from C1 to C2, and this process is regulated by the presence of HPA. This regulation avoids the production of H2O2 in the absence of HPA. Our findings indicate that no protein-protein interactions between C1 and C2 are necessary for efficient transfer of FMNH- between the proteins; transfer can occur by a rapid-diffusion process, with the rate-limiting step being the release of FMNH- from C1.