Structure/Function Relationships in Ceruloplasmin
Ceruloplasmin belongs to the family of multinuclear copper oxidases, which also includes ascorbate oxidase and laccase (Solomon et al., 1996). These proteins are trinuclear copper cluster enzymes, as documented by spectroscopic and functional studies in solution for laccase (Aliendorf et al., 1985; Spira-Solomon et al., 1986) and ceruloplasmin (Calabrese et al., 1988, 1989), and recently confirmed by X-ray crystallography for ascorbate oxidase (Messerschmidt et al., 1992) and ceruloplasmin (Zaitseva et al., 1996). Multiple copper sites are present, with a minimal functional unit constituted by a blue, or type 1, copper site, and three copper atoms clustered in a trinuclear arrangement. The blue copper site takes up an electron from a reducing substrate and transfers it, through long-range intramolecular electron transfer mediated by a Cys-His pathway, to the trinuclear cluster (Fig. 1), which provides four electrons to reduce oxygen to water. Among multicopper oxidases, ceruloplasmin is presently unique in that it possesses additional copper sites beside the four ions of the minimal unit. The three dimensional structure of ceruloplasmin solved at 3 A resolution has shown that the enzyme is made up of six α-barrel domains arranged in a ternary simmetry (Zaitseva et al., 1996). Consistent with spectroscopic data (Wever et al., 1973, Musci et al., 1993), three of these domains (domains 2, 4 and 6) bind type 1 copper ions coordinated by nitrogen and sulphur ligands. Type 1 copper ions bound to domain 4 and 6 (hereafter referred to as Cu4 and Cu6) are coordinated by 2 histidines, a cysteine and a methionine. Type 1 copper bound to domain 2 (Cu2) should be preferably reduced (Musci et al., 1995; Solomon et al., 1996) since the axial methionine is replaced by leucine, in a geometry typical of a high redox potential copper site. The three copper ions of the trinuclear cluster lie at the interface between domain 1 and 6 and are coordinated by 8 histidine ligands and an oxygen ligand (OH or H2O) bridging two of them. Labile metal binding sites have also been observed on domains 4 and 6, close to the respective prosthetic blue sites. Domain 2 appears different also in this respect, since it lacks the proper ligands for the corresponding labile site.
KeywordsAscorbate Oxidase Copper Site Multicopper Oxidase Ferroxidase Activity Labile Metal
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