The reactivity of plastocyanin from a green algal source, Scenedesmus obliquus (estimated PCuI charge -9 at pH ~7), has been studied for the first time in order to further assess the effect of amino acid sequence variations in naturally occurring plastocyanins. At 25°C, I = 0.10M (NaCl), and pH < 7.5, rate constants with [Fe(CN)6]3- and [Co(phen)3]3+ as oxidants for PCuI and with [Co(phen)3]2+ as reductant for PCuPCuII are similar in magnitude to those obtained previously for the higher plant plastocyanins and show similar dependences on pH. Attention is drawn to the possibility that in this and some other instances there may be a small residual reactivity of PCu1 with [Fe(CN)6]3- at low pH. A novel feature at pH > 7.5 is the increase in rate constants giving protein pXj values of 7.6 ([Fe(CN)6]3-) and 8.2 ([Co(phen)3]3+), confirmed by NMR spectroscopy (p= 7.8) as a deprotonation/ protonation of His59, a residue not present in the higher plant plastocyanins. Also from NMR measurements a sensitivity to active-site protonation at His59 and to His59 protonation at the active site is indicated, although there is no direct link between the two and the imidazole ring of His59is 10-12 Å from the Cu site. There is however no evidence for electron transfer from the 59-61 binding site. At pH 7.56 no saturation kinetic behavior is observed with [Co(phen)3]3+ (K < 25 M-1), which is attributable to the lower (-3 instead of -4) charge at residues 42-45, but at pH 9.1, when His59 is acid dissociated, such behavior is observed (K = 50-100 M-1). From 1H NMR line-broadening experiments at pH 6.3 and 7.0 using Cr(III) analogue complexes, it has been demonstrated that [Cr(CN)6]3- produces almost no specific broadening effects. With [Cr(phen)3]3+ binding to S.obliquus PCuI is observed over a delocalized region of the protein surface including thenegatively charged residues 42-44 and 60-61. Deletions at positions 57 and 58 in the aligned sequences bring these two regions sufficiently close together for reaction at the 42-44 site to be affected by the state of protonation of His59.