Some physicochemical peculiarities of poplar plastocyanins a and b

The redox potentials of poplar plastocyanins a and b (PCa, PCb) were determined by spectrophotometric titrations of their reduced forms with [Fe(CN)6]3-. It was found that the two isoforms have the following millimolar extinction coefficients epsilon597 equilibrium constants Keq of one-electron exchange with [Fe(CN)6]4-/[Fe(CN)6]3-, and standard electron potentials E0: PCa: epsilon597 = (4.72 +/- 0.08) mM(-1) cm(-1), Keq = 0.133 +/- 0.009, E0' = (354 +/- 11) mV; PCb: epsilon597 = (5.23 +/- 0.16) mM(-1) cm(-1), Keq = 0.175 +/- 0.010, E0' = (363 +/- 12) mV. The pH dependence of the redox potential of PCb was studied too. It was found, that the value of E0' for PCb is constant in the pH range 6.5-9.5, but decreases in the range 4.8-6.5. On the whole, the dependence resembles that of PC from some well-known plant species, including poplar PCa. The changes of E0' in the pH-dependent region for poplar PCb, however, are smaller and are 13 mV per pH unit, whereas in the other well-known plant species the changes are about 50-60 mV per pH unit. It has been assumed that the weaker pH dependence of EO' of PCb accounts for some structural differences between PCa and PCb.