Redox and spectral properties of cytochrome b559 in different preparations of photosystem II

A detailed analysis of the properties of cytochrome bus (Cyt b(559)) in photosystem II (PS II) preparations with different degrees of structural complexity is presented. It reveals that (i) D1-D2-Cyt b(559) complexes either in solubilized form or incorporated into liposomes contain only one type of Cyt b559 with E-m values of 60 +/- 5 and 100 +/- 10 mV, respectively, at pH 6.8; (ii) in oxygen-evolving solubilized PS II core complexes Cyt b559 exists predominantly (>85%) as an LP form with an E-m,E-7 Of 125 +/- 10 mV and a minor fraction with an E-m,E-7 Of -150 +/- 15 mV; (iii) in Oxygen-evolving PS II membrane fragments three different redox forms are discernible with E-m values of 390 +/- 15 mV (HP form), 230 +/- 20 mV (IP form), and 105 +/- 25 mV (LP form) and relative amplitudes of 58, 24, and 18%, respectively, at pH 7.3; (iv) the E-m values are almost pH-independent between pH 6 and 9.5 in all sample types except D1-D2-Cyt b559 complexes incorporated into liposomes with a slope of -29 mV/pH unit, when the pH increases from 6 to 9.5 (IP and LP form in PS II membrane fragments possibly within a restricted range from pH 6.5 to 8); (v) at pH > 8 the HP Cyt b559 progressively converts to the IP form with increasing pH; (vi) the reduced-minus-oxidized optical difference spectra of Cyt b(559) are very similar in the A range of 360-700 nm for all types except for the HP form which exhibits pronounced differences in the Sorer band; and (vii) PS II membrane fragments and core complexes are inferred to contain about two Cyt b559 hemes per PS ii. Possible implications of conformational changes near the heme group and spin state transitions of the iron are discussed.