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An Axial Met Ligand at a Type 1 Copper Site is Preferable for Fast Electron Transfer

The structure, spectroscopy and electron transfer (ET) reactivity of the type 1 copper sites of cupredoxins have been extensively studied by site-directed mutagenesis. All of these investigations have highlighted the importance of the ligating residues, in particular the equatorial Cys ligand, at the typically distorted tetrahedral metal centre (see Figure 1). The axially coordinating amino acid, which is usually a Met located ~3 from the metal, is the only nonconserved type 1 copper site residue. In the stellacyanin subfamily of the cupredoxins, a Gln residue is found in the axial position and coordinates in a monodentate fashion through its side-chain amide oxygen atom (see Figure 1). The influence of this natural variation at a type 1 copper site on ET reactivity is not well understood and the exact physiological function of the stellacyanins is not known. Replacement of the axial Met ligand with a Gln has been carried out in two cupredoxins and has been found to result in a decrease in ET reactivity. Herein we describe the influence of the axial Gln to Met mutation on the ET reactivity of a stellacyanin. The Gln95Met variant of umecyanin (UMC), the stellacyanin from horseradish roots, has spectroscopic properties for the Cu form that are consistent with the substitution of the axial oxygen ligand with the thioether sulfur of the introduced Met (see Figure 2). This is particularly apparent from the EPR parameters, since, due to coordination by an axial oxygen, the large gz and small Az values for the wild-type Figure 1. The active site structures of the type 1 copper sites of plastocyanin (PCu) and umecyanin (UMC).