Alkaline Transition of Rhus vernicifera Stellacyanin, an Unusual Blue Copper Protein

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Abstract
Stellacyanin from Rhus vernificera is a blue copper protein in which the metal is coordinated to a Cys, two His, and a Gln residue. It displays a low redox potential, a fast electron exchange rate, and a reversible alkaline transition. We have studied this transition in Cu(II)- and Co(II)-stellacyanin by means of electronic and NMR spectroscopy. The data indicate that a conformational rearrangement of the metal site occurs at high pH. A drastic alteration in the Gln coordination mode, as initially proposed, is discarded. These results show that the metal site in stellacyanin is more flexible than the sites of other blue copper proteins. The present study demonstrates that the paramagnetic shifts of the bound Cys in the Co(II) derivative are sensitive indicators of the electron delocalization and conformational changes experienced by this residue.