ELECTRON-TRANSFER PROPERTIES AND ACTIVE-SITE STRUCTURE OF THE TYPE-1 (BLUE) COPPER PROTEIN UMECYANIN

The electron self-exchange rate constant for the Type 1 blue copper pr otein umecyanin from horseradish roots has been determined as 6.1 x 10 (3) M(-1) S-1 at pH 7.5, I = 0.100 M, 25 degrees C by an NMR line-broa dening method. The value obtained is one of the lower self-exchange ra te constants determined for this class of protein; this is attributed to the presence of positively charged residues near to the electron-tr ansfer site. The self-exchange rate constants calculated by means of a Marcus analysis of data for the cross-reactions (25 degrees C) of ume cyanin with azurin and cytochrome c(551) (both from Pseudomonas aerugi nosa) are substantially less at 8.0 M(-1) S-1 and 13.9 M(-1) S-1, resp ectively, and are independent of pH in the range 7.0-8.0, I = 0.100 M. The discrepancy between the self-exchange rate constants obtained by these two different methods can be rationalised if it is assumed that umecyanin reacts with the two proteins employed in the cross-reaction studies through the same site, but that this site is different from th at used for the self-exchange process. A comparison of the primary str ucture of umecyanin with those of other Type 1 copper proteins has rev ealed that a glutamine rather than a methionine is likely as the fourt h ligand of Cu at the active site. Other comparisons are made with ste llacyanin, and the electron-transfer reactivity of the two proteins is discussed.