Two substrate binding sites in ascorbate peroxidase: the role of arginine 172.

Site-directed mutagenesis has been used to probe the role of Arg172 in ascorbate utilization by ascorbate peroxidase. Arg172 was changed to lysine, glutamine, and asparagine. Although each of these variants retains the ability to utilize guaiacol as a reductant, they exhibit large decreases in their steady-state rates of ascorbate utilization. Spectroscopic, steady-state, and transient-state experiments indicate that these variant proteins are capable of reacting with hydrogen peroxide to form Compound I, but their ability to oxidize ascorbate to form Compound II, and subsequently the resting state, is severely impeded. Results are presented which highlight the importance of Arg172, and a model is proposed to explain its role in ascorbate utilization.