Copper sites ascorbate oxidase, geometries

The structure of the ascorbate oxidase tricopper site is illustrated in Fig. 45. The three copper atoms form an almost equilateral triangle of sides ca. 3.7 A. The Cul and Cu2 atoms are bridged by OH" or O " and make up the EPR-silent type 3 pair each copper atom is coordinated to three histidine residues and the Cu—N(His) distances are all comparable and unexceptional. In contrast to the hemocyanin active site (Section IVD), the copper ions have approximately tetrahedral coordination geometry and are not in identical environments. The third copper ion is coordinated to two histidine residues and to either hydroxide or water. There is no evidence for a fJ-s-OH or fiyO donor at the center of the cluster (and the Cu—Cu distances are too long to support such a bridge). 

The mononuclear copper site is located in domain 3 and has the four canonical type-1 copper ligands (His, Cys, His, and Met) also found in plastocyanin and azurin. It is coordinated to the NDl atoms of His 445 and 512, the SG atom of the Cys 507, and the SD atom of Met 517 in a distorted trigonal pyramidal geometry. The SD atom is at the long apex (see Fig. 4). Bond lengths of the type-1 copper for both subunits are displayed in Table III. They are compared with oxidized poplar plastocyanin (95) and azurin from Pseudomonas aeruginosa (96). Figure 5 shows an overlay of the type-1 copper site in azurin, plastocyanin, and ascorbate oxidase. 

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