Abstract:The electron self-exchange rate constant for the Type 1 blue copper protein umecyanin from horseradish roots has been determined as 6.1 x lo3 M-' S -' at pH 7.5, I = 0 . 1 0 0~~ 25°C by an NMR line-broadening method. The value obtained is one of the lower self-exchange rate constants determined for this class of protein; this is attributed to the presence of positively charged residues near to the electron-transfer site. The self-exchange rate constants calculated by means of a Marcus analysis of data for the cross-reactions (25 "C) of umecyanin with azurin and cytochrome cssl (both from Pseudomonas aeruginosa) are substantially less at 8.0 M-' s -' and 1 3.9 M -s -', respectively, and are independent of pH in the range 7.0-8.0, Z = 0 . 1 0 0~. 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 that used for the self-exchange process. A comparison of the primary structure of umecyanin with those of other Type 1 copper proteins has revealed that a glutamine rather than a methionine is likely as the fourth ligand of Cu at the active site. Other comparisons are made with stellacyanin, and the electron-transfer reactivity of the two proteins is discussed.