Three-dimensional model of stellacyanin and its implications for electron transfer reactivity

Experimental data were combined with computational methods in constructing a hypothetical three-dimensional model for the blue single copper protein Rhus stellacyanin (St). The known sequence of stellacyanin and its homology with plastocyanin (Pc) were used together with the results of spectroscopic studies of the protein that yielded the current assignment of two histidines, one cysteine and a disulfide sulfur as copper ligands in stellacyanin. By computer graphics and energy minimization the folding of the protein was predicted. The model structure is somewhat less regular than Pc as judged by surface area and energy comparisons, but it is a stable structure. Besides rotation of one imidazole ring the copper site undergoes no change even in the absence of the copper ion and the model shows that the site can be constructed with the four assumed copper ligands without forming a strained system. The structure also indicates that a carbonyl oxygen atom is near the copper, thus the site may have analogy to the Alcaligenes denitrificans azurin (Az) site, although the amino acid sequence is more homologous to that of Pc. The model indicates that aspartate 49, reductively labeled by Cr(III), is near the copper center and homologous to the site labeled by Cr(III) on Pc. Also homologous to Pc is a tyrosine residue adjacent to the aspartate. This tyrosine has been implicated in Pc electron transfer and thus is probably involved in electron transfer reactivity of St as well. The higher reactivity of St with small-molecule redox reagents compared to Az and Pc, may be due to the proximity of the above-mentioned aspartate 49 to the Cu, or the greater exposure of one of the Cu cysteine ligands, in the predicted structure as compared to that in the known Pc and Az structures.