RESUMO
A model of heme-quinone redox interaction has been developed for cytochrome b559 in photosystem II. The quinone QC in the singly protonated form may function as an interacting quinone. The electrostatic effect between the charges on the heme iron of the cytochrome and QCH leads to appearance of three forms of the cytochrome with different redox potentials. A simple and effective mechanism of redox regulation of the electron transfer pathways in photosystem II is proposed.
Assuntos
Grupo dos Citocromos b/metabolismo , Complexo de Proteína do Fotossistema II/metabolismo , Beta vulgaris/enzimologia , Beta vulgaris/metabolismo , Cianobactérias/enzimologia , Cianobactérias/metabolismo , Grupo dos Citocromos b/química , Cinética , Oxirredução , Complexo de Proteína do Fotossistema II/química , TermodinâmicaAssuntos
Membrana Celular/metabolismo , Grupo dos Citocromos b/metabolismo , Complexo de Proteína do Fotossistema II/metabolismo , Plastoquinona/análogos & derivados , Membrana Celular/química , Sistema Livre de Células , Grupo dos Citocromos b/química , Ativação Enzimática/efeitos da radiação , Luz , Oxirredução/efeitos da radiação , Complexo de Proteína do Fotossistema II/química , Complexo de Proteína do Fotossistema II/efeitos da radiação , Plastoquinona/química , Plastoquinona/metabolismoAssuntos
Grupo dos Citocromos b/metabolismo , Grupo dos Citocromos b/efeitos da radiação , Complexo de Proteína do Fotossistema II/metabolismo , Complexo de Proteína do Fotossistema II/efeitos da radiação , Beta vulgaris/metabolismo , Beta vulgaris/efeitos da radiação , Transporte de Elétrons , Cinética , Oxirredução , Processos Fototróficos , EspectrofotometriaRESUMO
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.
Assuntos
Grupo dos Citocromos b/química , Grupo dos Citocromos b/metabolismo , Complexo de Proteínas do Centro de Reação Fotossintética/química , Complexo de Proteínas do Centro de Reação Fotossintética/metabolismo , Chenopodiaceae , Concentração de Íons de Hidrogênio , Membranas Intracelulares/química , Membranas Intracelulares/metabolismo , Oxirredução , Fragmentos de Peptídeos/química , Fragmentos de Peptídeos/metabolismo , Complexo de Proteína do Fotossistema II , Potenciometria , Espectrofotometria , Spinacia oleraceaRESUMO
Cytochrome b559 in D1-D2-Cyt b-559 complexes from spinach can be photoreduced in the presence of DBMIB at a temperature of 180-240 K upon continuous illumination. The reduction of Cyt b-559 is accompanied by oxidation of P680. At 240 K recombination of P680+ and reduced Cyt b-559 is complete in several seconds. At 220 K and below, the state P680+Cyt b-559red can be trapped for a long time. This indicates that the photoreduced heme is incapable of electron transfer to P680+ at 220 K and below. On the other hand, the chemically reduced heme of Cyt b-559 is oxidized by P680+ at 77 K. These results are consistent with the presence of two kinds of Cyt b-559 hemes in D1-D2-Cyt b-559 complexes which participate in different ways in the photochemical reactions.
