Reactivity comparison of iron(II) reductant with type 1 copper(II) in native and type 2 copper-depleted Rhus vernicifera laccase

Rate constants and activation parameters are compared for the reductions of native and type 2 copper-depleted (T2D) Rhus vernicifera laccase type 1 Cu(II) by hydroxyethylferrocene, Fe(CN)₆^4- and Fe(EDTA)^2-. Oxidation of Fe(CN)₆^4- (k(25°C)=1.45 · 10² M^-1 · s^-1, pH 7, I = 0.5 M) by T2D laccase blue copper is an order of magnitude faster than the corresponding native enzyme rate, and a type 2 Cu(II)-Fe(CN)₆^4- interaction is shown to be responsible for complex kinetic behavior in the reduction of native laccase. Activation parameters (ΔH^‡, ΔS^‡) confirm the presence of a large conformational rearrangement barrier in the electron transfer pathway to laccase type 1 Cu(II), as compared with other blue copper proteins. A systematic compensation pattern between ΔH^‡ and ΔS^‡ in laccase reductions by Fe(II) redox agents suggests a common mechanism, with considerable flexibility in activation requirements, dependent upon the hydrophilicity of the electron donor.