Oxidation of oxymyoglobin by poplar plastocyanins a and b.

Oxidation of oxymyoglobin [MbO2 (Fe2+)] by isoplastocyanins a (PCa) and b (PCb) was experimentally investigated and the corresponding redox reaction was modeled using the physicochemical parameters of the isoforms to study the effect of the dimorphism. The kinetic curve of oxidation of MbO2 (Fe2+) by oxidized PCa [PCa(Cu2+)] and PCb [PCb(Cu2+)] and the pH-dependence of the rate constant k1 were determined. In the range of pH 4.8-9.0, PCb reacts with higher k1, compared with PCa. For example, at pH 7.0, k1(PCb) = 4 × 102 M-1s-1, whereas k1(PCa) = 2 × 102 M-1s-1. The observed values of ΔE0 for the reaction pairs Mb-PCa and Mb-PCb were -304 mV and -319 mV, respectively. The effect of the ionic strength (µ) on the rate of the electron transfer was also studied. It was found that: (i) the net charge Z1 of PCa and PCb fully corresponds to that calculated by their primary structures and Z2 of Mb corresponds to that calculated by its titration curve; (ii) the ln k as function of √¯µ was similar for both PCa and PCb; (iii) the curve of the reaction PCb Mb (pH 7.0) was shifted towards higher values of k, in agreement with the larger net negative charge of PCb; and (iv) the character of the electrostatic interactions remained unchanged by a replacement of PCa by PCb and by the change of pH from 7.0 to 4.8.

Studies on electron donation to photosystem I sites by exogenous donors in Spirulina thylakoids.

The kinetic parameters of different sites of electron donation to photosystem I (PS I) were evaluated in Spirulina platensis thylakoids. Reduced 2,6-dichlorophenolindophenol (DCIPH2) exhibited two sites of electron donation, with apparent K(m) values of 8 and 40 microM each. The corresponding value for reduced N-tetramethyl-p-phenylenediamine (TMPDH2) and diaminodurene (DADH2) which donate electrons at a single site to PS I were 103 and 48 microM, respectively. The electron donation by these three exogenous donors were differentially inhibited by KCN (70 mM) affecting the apparent K(m) and Rmax values to varying extent. This cyanide inhibition of PS I catalyzed electron transport suggests the presence of plastocyanin in the photosynthetic electron transport chain of Spirulina platensis.