Extinction coefficients of cytochromes b559 and c550 of Thermosynechococcus elongatus and Cyt b559/PS II stoichiometry of higher plants

Abstract

"Reduced minus oxidized" difference extinction coefficients Deltavarepsilon in the alpha-bands of Cyt b559 and Cyt c550 were determined by using functionally and structurally well-characterized PS II core complexes from the thermophilic cyanobacterium Thermosynechococcus elongatus. Values of 25.1+/-1.0 mM(-1) cm(-1) and 27.0+/-1.0 mM(-1) cm(-1) were obtained for Cyt b559 and Cyt c550, respectively. Anaerobic redox titrations covering the wide range from -250 up to +450 mV revealed that the heme groups of both Cyt b559 and Cyt c550 exhibit homogenous redox properties in the sample preparation used, with E(m) values at pH 6.5 of 244+/-11 mV and -94+/-21 mV, respectively. No HP form of Cyt b559 could be detected. Experiments performed on PS II membrane fragments of higher plants where the content of the high potential form of Cyt b559 was varied by special treatments (pH, heat) have shown that the alpha-band extinction of Cyt b559 does not depend on the redox form of the heme group. Based on the results of this study the Cyt b559/PSII stoichiometry is inferred to be 1:1 not only in thermophilic cyanobacteria as known from the crystal structure but also in PSII of plants. Possible interrelationships between the structure of the Q(B) site and the microenvironment of the heme group of Cyt b559 are discussed.

Quantum efficiency distributions of photo-induced side-pathway donor oxidation at cryogenic temperature in photosystem II

“…absorption were used to quantify Cytb 559 and Car oxidation. We used the extinction coefficient for the reduced form of Cytb 559 at room temperature reported by Kaminskaya et al (2005) of 25,000 M -1 cm -1 , where the FWHM is 10.7 nm. At low temperature the width was FWHM = 6.5 nm, and so the corrected low-temperature extinction coefficient that we used was 41 9 10 3 M -1 cm -1 .…”

A megacomplex composed of both photosystem reaction centres in higher plants

“…For P700, an extinction coefficient of 64 mM À 1 cm À 1 was used 25 . For cytochrome b559, an extinction coefficient of 25 mM À 1 cm À 1 was used 26 . The chlorophyll concentration and a/b ratio were determined as described previously 27 .…”

A Reaction Center-dependent Photoprotection Mechanism in a Highly Robust Photosystem II from an Extremophilic Red Alga, Cyanidioschyzon merolae

“…The cytochrome c 550 concentration was calculated from the maximal absorption at 550 nm and the extinction coefficient of 27.0 mM Ϫ1 cm Ϫ1 (42). For calculation of the cytochrome b 559 content, maximum absorbance at 557 nm and an extinction coefficient of 25.1 mM Ϫ1 cm Ϫ1 were used (42). Each spectrum was recorded three times.…”

“…To ensure complete oxidation or reduction of cytochromes, an additional grain of ferricyanide or dithionite was added before reacquiring the oxidized and reduced spectra, respectively. The cytochrome c 550 concentration was calculated from the maximal absorption at 550 nm and the extinction coefficient of 27.0 mM Ϫ1 cm Ϫ1 (42). For calculation of the cytochrome b 559 content, maximum absorbance at 557 nm and an extinction coefficient of 25.1 mM Ϫ1 cm Ϫ1 were used (42).…”