Redox properties and acid-base equilibria of zucchini mavicyanin

The reduction potential of mavicyanin isolated from zucchini peelings, whic h is a blue copper protein belonging to the subclass of the phytocyanins, h as been determined through direct electrochemistry as a function of tempera ture and pH. The enthalpy and entropy changes accompanying protein reductio n were found to be very similar with those determined previously for other phytocyanins and to differ remarkably from those of azurins and plastocyani ns. This finding contributes to further characterize phytocyanins as a dist inct cupredoxins family also on thermodynamic grounds and improves our unde rstanding of how the reduction potential of these metal centers in proteins is modulated by coordinative and solvation properties. The E-o' of mavicya nin is found to be sensitive to two acid-base equilibria at the extremes of pH. One occurs below pH 4, and is related to the protonation and detachmen t from the Cu(I) center of a histidine ligand. The other, observed above pH 8, causes a remarkable change in the electrostatic potential and/or the fi eld strength around the copper. (C) 2001 Elsevier Science B.V. All rights r eserved.