Tyrosinase/catecholoxidase activity of hemocyanins: structural basis and molecular mechanism

The enzymes tyrosinase, catecholoxidase and hemocyanin all share similar ac tive sites, although their physiological functions differ. Hemocyanins serv e as oxygen carrier proteins, and tyrosinases and catecholoxidases (commonl y referred to as phenoloxidases in arthropods) catalyze the hydroxylation o f monophenols or the oxidation of o-diphenols to o-quinones, or both. Tyros inases are activated in vivo by limited proteolytic cleavage, which might o pen up substrate access to the catalytic site, It has recently been demonst rated that if hemocyanins are subjected to similar proteolytic treatments t in vitro) they also exhibit at least catecholoxidase reactivity. On the bas is of their molecular structures, hemocyanins are used as model systems to understand the substrate-active-site interaction between catecholoxidases a nd tyrosinases.