Three Forms of Cytochrome b ₅₅₉ and Their Relation to the Photosynthetic Activity of Chloroplasts

Abstract

Spinach chloroplasts were found to contain three forms of cytochrome b559 that have the same a-peak at 559 nm, but are distinguished from one another by their oxidation-reduction potentials. It is now widely held that the main feat of photosynthesis, the conversion of light energy into chemical energy, is accomplished by a light-induced, thermodynamically "uphill," electron flow (1, 2) to which are coupled reactions that trap chemical energy in such biochemically useful forms as ATP. The light-induced electron flow concept, as applied to both cyclic and noncyclic photophosphorylation in chloroplasts, has nowv been further refined (3, 4) to accommodate new findings about the properties of cytochromes and the nature of other electron carriers in the photosynthetic apparatus (5-8).With regard to chloroplast cytochromes, special interest has recently been focused on cytochrome b559 (see earlier literature cited in ref. 5). Knaff and Arnon (5) observed that cytochrome b559 is photooxidized by short-wavelength (System II) monochromatic light at the temperature of liquid nitrogen, i.e., a temperature low enough to minimize, and perhaps exclude, thermochemical reactions but one that would not impede the primary photon-activated electron transfer step. The lowtemperature photo-oxidation of cytochrome b559 was confirmed by Erixon and Butler (9, 10), Floyd, Chance, and DeVault (11), Boardman, Anderson, and Hiller (12), and Bendall and Sofrova (13). Moreover, some earlier reports (14-16) that cytochrome f undergoes photo-oxidation at the temperature of liquid nitrogen proved, on recent reexamination (11-13), to have been due to the photo-oxidation of cytochrome b559.Despite the present wide agreement about the capacity of cytochrome b559 to undergo low-temperature photo-oxidation, there is a considerable divergence of views (3,4,10,12,13) concerning its role in the light-induced noncyclic electron flow in chloroplasts. An essential condition for assessment of its role as an electron carrier is a knowledge of its oxidation-reduction potential, but here the literature includes contradictory reports. Knaff and Arnon (3, 8) obtained a midpoint oxidationreduction potential of 330 mV (at pH 8.2)-a value similar to those reported by Bendall (17), Ikegami, Katoh, and Takamiya (18), and Boardman et al. (12). By contrast, Fan and Cramer (19) reported for cytochrome b559 in spinach chloroplasts a midpoint potential of 80 mV (pH 7.0), and Hind and Nakatani (20) reported 55 mV (pH 7) for this cytochrome in subchloroplast fragments prepared from spinach chloroplasts by incubation with Triton X-100.More recently, Cramer et al. (21) concluded that cytochrome b559 in chloroplasts exists in two forms, one (reducible by hydroquinone) with a high, and one (reducible by ascorbate) with a low, oxidation-reduction potential, the two forms being interconvertible. Earlier, Bendall (17) mentioned that chloroplasts contain a second b-type cytochrome with an aband at 559 nm that is not reducible by ascorbate, but is reducible by dithionite.The...

Differential scanning calorimetry of chloroplast membranes: identification of an endothermic transition associated with the water-splitting complex of photosystem II

“…ferrocyanide or hydroquinone. Different chemical and physical treatments inhibiting 02 evolution, including heat (Wada & Arnon, 1971;Horton et al, 1978), trypsin (Cox & Bendall, 1972), and incubation in hydroxylamine (Horton & Croze, 1977), are known to cause a decrease in the level of highpotential cytochrome 6-559. In the present case, there is a good correlation between the loss of 02 evolution and the decrease in content of hydroquinone-reducible cytochrome 6-559.…”

An open reading frame encoding a putative haem-binding polypeptide is contranscribed with the pea chloroplast gene for apocytochrome f

“…The low-potential form of cytochrome b-559 may be detected in freshly isolated chloroplast membranes [3,4,33], but it has been claimed to be an altered form of the high-potential form of cytochrome b-559 associated with photosystem II [9,10]. A variety of treatments, including heat, solvents and sonication, is reported to lower the redox potential of the photosystem II-associated cytochrome b-559 [3,42], but not to the redox potential of the low-potential form of cytochrome b-559 in freshly isolated chloroplasts [ 33 ]. The low-potential form of cytochrome b-559 is solubilised by digitonin treatment of thylakoid membranes, unlike the photosystem II-associated cytochrome b-559 [ 1 ].…”

Protein identification of purified particles isolated from spinach thylakoids by deoxycholate electrophoresis

“…Oxidized minus reduced cytochrome spectra were performed as in [32] with a Aminco Chance DW 2a spectrophotometer. The same molar extinction coefficient (20cm 2 #mole -t ) was used for the c~ peak of the cytochromes, f, b6 and b559.…”