Sequential Proton Loss Electron Transfer in Deactivation of Iron(IV) Binding Protein by Tyrosine Based Food Components

Abstract

The iron­(IV) binding protein ferrylmyoglobin, MbFe­(IV)O, was found to be reduced by tyrosine based food components in aqueous solution through a sequential proton loss electron transfer reaction mechanism without binding to the protein as confirmed by isothermal titration calorimetry. Dopamine and epinephrine are the most efficient food components reducing ferrylmyoglobin to oxymyoglobin, MbFe­(II)­O₂, and metmyoglobin, MbFe­(III), as revealed by multivariate curve resolution alternating least-squares with second order rate constants of 33.6 ± 2.3 L/mol/s (Δ<i>H</i>^⧧ of 19 ± 5 kJ/mol, Δ<i>S</i>^⧧ of −136 ± 18 J/mol K) and 228.9 ± 13.3 L/mol/s (Δ<i>H</i>^⧧ of 110 ± 7 kJ/mol, Δ<i>S</i>^⧧ of 131 ± 25 J/mol K), respectively, at pH 7.4 and 25 °C. The other tyrosine based food components were found to reduce ferrylmyoglobin to metmyoglobin with similar reduction rates at pH 7.4 and 25 °C. These reduction reactions were enhanced by protonation of ferrylmyoglobin and facilitated proton transfer at acidic conditions. Enthalpy–entropy compensation effects were observed for the activation parameters (Δ<i>H</i>^⧧ and Δ<i>S</i>^⧧), indicating the common reaction mechanism. Moreover, principal component analysis combined with heat map were performed to understand the relationship between density functional theory calculated molecular descriptors and kinetic data, which was further modeled by partial least squares for quantitative structure–activity relationship analysis. In addition, a three tyrosine residue containing protein, lysozyme, was also found to be able to reduce ferrylmyoglobin with a second order rate constant of 66 ± 28 L/mol/s as determined by a competitive kinetic method