Unusual properties of plastocyanin from the fern Dryopteris crassirhizoma.

The effect of pH on the (1)H NMR spectrum, reduction potential, and self-exchange rate constant of the novel plastocyanin (PCu) from the fern plant Dryopteris crassirhizoma has been studied. The results are compared with those for the higher-plant PCu from parsley. In the (1)H NMR spectrum of D. crassirhizoma PCu(I), there is no sign that either of the His ligands is protonated at pH* down to 5.4. The reduction potentials of D. crassirhizoma and parsley PCu are 382 and 379 mV, respectively, at pH 7.4. When the pH value is decreased, the reduction potential of parsley PCu is seen to increase quite dramatically, consistent with protonation at His87 in PCu(I). A pK(a) of 5.8 is obtained from the electrochemistry data, consistent with a value of 5.6 determined by NMR. The reduction potential of D. crassirhizoma PCu exhibits a much less pronounced dependence on pH. The self-exchange rate constant of D. crassirhizomaPCu(I) is 3.4 x 10(3) M(-1) s(-1) at pH* 7.9. This is the smallest self-exchange rate constant reported to date for a PCu and can be rationalized by considering the altered distribution of charged residues on the surface of the D. crassirhizoma protein compared to the charge distributions of other higher-plant PCus. The self-exchange rate constant increases to 9 x 10(3) M(-1) s(-1) at pH* 5.4, consistent with enhanced protein-protein association at lower pH*, and the absence of His87 protonation in D. crassirhizoma PCu(I) in the accessible pH range.