Role of Aspartate 94 in the Decay of the Peroxide Intermediate in the Multicopper Oxidase Fet3p^†

Fet3p is a multicopper oxidase that contains four Cu ions:  one type 1, one type 2, and a coupled binuclear type 3 site. The type 2 and type 3 centers form a trinuclear cluster that is the active site for O₂ reduction to H₂O. When the type 1 Cu is depleted (C484S mutation), the reaction of the reduced trinuclear cluster with O₂ generates a peroxide intermediate. Kinetic studies of the decay of the peroxide intermediate suggest that a carboxyl residue (D94 in Fet3p) assists the reductive cleavage of the O−O bond at low pH. Mutations at the D94 residue (D94A, D94N, and D94E) have been studied to evaluate its role in the decay of the peroxide intermediate. Spectroscopic studies show that the D94 mutations affect the geometric and electronic structure of the trinuclear cluster in a way that is consistent with the hydrogen bond connectivity of D94. While the D94E mutation does not affect the initial reaction of the cluster with O₂, the D94A mutation causes larger structural changes that render the trinuclear cluster unreactive toward O₂, demonstrating a structural role for the D94 residue. The decay of the peroxide intermediate is markedly affected by the D94E mutation, confirming the involvement of D94 in this reaction. The D94 residue appears to activate a proton of the type 2 Cu⁺-bound water for participation in the transition state. These studies provide new insight into the role of D94 and proton involvement in the reductive cleavage of the O−O bond.