Redox and Spectral Properties of Cytochromeb₅₅₉in Different Preparations of Photosystem II

Abstract

A detailed analysis of the properties of cytochrome b(559) (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 b(559) 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 b(559) exists predominantly (>85%) as an LP form with an E(m,7) of 125 +/- 10 mV and a minor fraction with an E(m,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 b(559) 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 b(559) 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 lambda range of 360-700 nm for all types except for the HP form which exhibits pronounced differences in the Soret band; and (vii) PS II membrane fragments and core complexes are inferred to contain about two Cyt b(559) hemes per PS II. Possible implications of conformational changes near the heme group and spin state transitions of the iron are discussed.

Dimeric and Monomeric Organization of Photosystem II

“…Cyt b 559 can exist in several potential forms, and its redox state depends on the functional status of the PSII complex (15,(53)(54)(55). We therefore characterized the redox state of Cyt b 559 in the different fractions of the thylakoid membrane, and the results are presented in Fig.…”

Redox properties of the photosystem II cytochromes b559 and c550 in the cyanobacterium Thermosynechococcus elongatus

“…In D, the solid curve is the best fit of the experimental data to the Nernst equation in accordance with one-electron processes (n=1) for one component with an E 0 m value of -240 mV has a high oxidizing E¢ m , which is not affected by the pH, and (2) it is partially converted into a lower potential form by treatments such as Tris washing, mild heating or incubation at alkaline pH. Nevertheless, it seems that the HP form of Cyt b559 in T. elongatus is less susceptible to these denaturing treatments than that in plant PSII [45,46,47]. Several hypotheses have been proposed to explain the high oxidizing E¢ m of the HP form of Cyt b559, the localization of the heme in an extremely hydrophobic environment being the most common reason argued [13,48,49].…”

Differences in the Interactions between the Subunits of Photosystem II Dependent on D1 Protein Variants in the Thermophilic Cyanobacterium Thermosynechococcus elongatus

“…Refs. [42][43][44][45]. Each of these two redox forms exhibits two different EPR signals corresponding to (i) a non-relaxed state (when the Cyt b 559 is oxidized by illumination at temperatures Յ77 K) and (ii) to a relaxed state when the illuminated sample is warmed up to allow the sample to relax to a less constraint geometry.…”