The effect of pH on rate constants (25-degrees-C) for the [Co(dipic)2]- and [Co(phen)3]3+ oxidations of pseudoazurin pACu(I) have been studied in the pH range 3.5-8.7. From the trends observed (decrease in rate constants with decreasing pH) two pK(a)’s are obtained, and from H-1 NMR these are confirmed as being associated with an active site His protonation/deprotonation, pK(a) = 4.84 (average), and a protonation/deprotonation of the uncoordinated His6, pK(a) = 7.21 (average). Alongside plastocyanin and amicyanin, pseudoazurin therefore provides a third example of a type 1 protein the Cu(I) state of which exhibits an active site protonation in the accessible pH range. The spacings of the active site His, Cys, and Met coordinated residues are noted and appear to relate to the magnitude of the pK(a) values observed. The rate constant for the oxidation with [Fe(CN)6]3- is >2.6 X 10(6) M-1 s-1 at pH 5.5, yielding an exceptionally high k(Fe)/k(Co) ratio of >7.8 x 10(3) at this pH, where k(Co) is for the [Co(phen)3]3+ oxidation. The effect of a number of conserved basic residues Lys38, Lys46, Lys57, and Lys77 (the last adjacent to the active site Cys78) on reactivity is discussed. The presence of these basic residues explains the relatively small value of 2.9 x 10(3) M-1 s-1 for the electron self-exchange rate constant at 25-degrees-C, I = 0.100 M.