CRYSTAL STRUCTURE OF CUCUMBER STELLACYANIN AT 1.7 Å RESOLUTION. P. John Hart¹, Aram N. Nersissian², Joan Selverstone Valentine², and David Eisenberg¹. ¹UCLA-DOE Laboratory of Structural Biology and Molecular Medicine, Box 951570, UCLA, Los Angeles, CA 90095. ²Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90024.

Stellacyanins are blue (type I) copper glycoproteins that differ from other cupredoxins (such as plastocyanin and azurin) in many of their properties. They have an unusual copper ligand (Gln instead of Met found in other mononuclear blue copper proteins), they perform more rapid long-range electron transfer, and they exhibit pH-dependent, reversible EPR and electronic absorption spectra. Until now, stellacyanins have eluded structure determination. Here we report the refined three-dimensional crystal structure at 1.7 Å resolution of stellacyanin from cucumber peelings.

The overall fold of the cucumber stellacyanin copper-binding domain is organized in two (-sheets, one of three (-strands and one of four. Two (-helices are found in loop regions between (-strands. One side of the molecule is predominantly negatively-charged, and provides a possible interaction site for redox partners. The characteristic spectroscopic properties and electron transfer reactivity of stellacyanin, relative to other well characterized blue copper proteins, may be explained by a copper binding site that is solvent exposed, and the fact that the copper is held in a nearly tetrahedral geometry by a strong interaction with the Gln ligand.