Possessing both peroxidase and peroxygenase activities with a broad substrate profile that includes phenols, indoles, and pyrroles, the enzyme dehaloperoxidase (DHP) from Amphitrite ornata is a multifunctional catalytic hemoglobin that challenges many of the assumptions behind the well-established structure-function paradigm in hemoproteins. While previous studies have demonstrated that the F21W variant leads to attenuated peroxidase activity in DHP, here we have studied the impact of this mutation on peroxygenase activity to determine if it is possible to selectively tune DHP to favor one function over another. Biochemical assays with DHP B (F21W) revealed minimal decreases in peroxygenase activity of 1.2-2.1-fold as measured by 4-nitrophenol or 5-Br-indole substrate conversion, whereas the peroxidase activity catalytic efficiency for 2,4,6-trichlorophenol (TCP) was more than sevenfold decreased. Binding studies showed a 20-fold weaker affinity for 5-bromoindole (K d = 2960 ± 940 μM) in DHP B (F21W) compared to WT DHP B. Stopped-flow UV/visible studies and isotope labeling experiments together suggest that the F21W mutation neither significantly changes the nature of the catalytic intermediates, nor alters the mechanisms that have been established for peroxidase and peroxygenase activities in DHP. The X-ray crystal structure (1.96 Å; PDB 5VLX) of DHP B (F21W) revealed that the tryptophan blocks one of the two identified TCP binding sites, specifically TCPinterior, suggesting that the other site, TCPexterior, remains viable for binding peroxygenase substrates. Taken together, these studies demonstrate that blocking the TCPinterior binding site in DHP selectively favors peroxygenase activity at the expense of its peroxidase activity.
Keywords: Globin; Peroxidase; Peroxygenase; Structure–function relationship.