Spectral and thermodynamic properties of the two hemes of the D1D2cytochrome b-559 complex of spinach.

In agreement with previous work [Shuvalov, Heber and Schreiber (1988) FEBS Lett. 258, 27-31] two hemes (low potential (LP) and extra low potential (XLP)) per two pheophytins were found in isolated D1D2Cyt b-559 complexes. Reductive and oxidative redox titrations demonstrate that the Em of the LP form is at about +150 mV. It is independent of pH between pH 7.2 and 9.4. The XLP heme is autoxidizable at pH 7.2 and displays, at this pH, an Em of -45 mV. Both the LP and XLP hemes show absorption peaks at 559 nm. They are proposed to have bis-histidine ligation of the heme iron. At pH 9.4, the XLP heme splits into two forms. One of them has an Em of +40 mV, and absorption peaks at 559 nm showing the bis-histidine ligation. The other displays an Em of -220 mV and the peak is shifted to 562 nm. This last form is proposed to be due to the incorporation of OH- which occupies the 6th coordination position of the heme Fe(III) at high pH. The pK value for the conversion of the XLP heme is close to 7.7. In a structure simulation of the alpha-helices of alpha- and beta-polypeptide, the beta-polypeptide, but not the alpha-polypeptide, reveals a distance between the histidine N and the heme Fe which permits stable N-Fe coordination. In the alpha-polypeptide, OH- can be incorporated between N and Fe. The functional role of the two hemes of cyt b-559 is briefly discussed with respect to water oxidation and cyclic electron transfer.