The reaction of CN- with the binuclear copper site of Neurospora tyrosinase: its relevance for a comparison between tyrosinase and hemocyanin active sites.

The equilibrium and the kinetics of the reaction between Neurospora crassa tyrosinase and cyanide have been studied. Cyanide reacts with the binuclear copper active site of the protein competitively with respect to dioxygen and displaces the metal ions. This process occurs stepwise and involves transient intermediates containing mononuclear Cu(I) sites. The reaction mechanism proved to be the same as described earlier for molluscan and arthropodan hemocyanins, which share with tyrosinase the same copper active site organization, but perform different physiological functions. A comparison of the kinetic parameters between the different proteins shows that the tyrosinase copper active site has a greater accessibility than that of hemocyanin. The relevance of these data in terms of structure-function relationship and evolution of the binuclear copper proteins is discussed.