Structure-function correlations in copper clusters in proteins

Abstract- A coupled binuclear copper active site is present in a wide variety of proteins and enzymes which perform different biological functions utilizing 0,. In the multicopper oxidases, the Type 2 and Type 3 centers comprise a trinuclear Cu cluster which represents the active site for the multielectron reduction of 0,. A coupled binuclear copper active site is present in a wide variety of proteins and enzymes which perform different biological functions utilizing 02. The hemocyanins (Hc) reversibly bind 02, the tyrosinases (Ty) are monoxygenases which hydroxylate monophenols to o-diphenols and oxidize these to o-quinones, and the multicopper oxidases (laccase (Lc), ascorbate oxidase and ceruloplasmin), which contain additional copper centers (Type 1 and Type 2) catalyze the four electron reduction of 02 to water. Our original chemical and spectroscopic studies over a series of protein active site derivatives of Hc and Ty demonstrated (ref. 1,2) that these proteins have very similar active sites. The key features of this spectroscopically effective model for the coupled binuclear copper active site, reproduced in Fig. 1, are that dioxygen binds as peroxide, bridging two tetragonal Cu(1I)'s in a cis p-l,2 fashion, with an additional endogenous bridge between the coppers (RO-). The endogenous bridge is likely hydroxide based on the crystal structure (ref. 3) of deoxy Hc. The major difference between the Hc and Ty sites is the high accessibility of the Ty active site to exogenous ligands (ref. 4). We found that substrate analogues bind directly to the copper in Ty and compete with peroxide for the same binding site (ref, 5). These studies resulted in the proposed structural mechanism for hydroxylation and oxidation catalysis given in Fig. 2. Alternatively, chemical and spectroscopic studies of a series of protein active site derivatives of Lc (which is the simplest multicopper oxidase, containing on