Olga Kaminskaya

Reaction pattern of photosystem II: oxidative water cleavage and protein flexibility.

This short communication addresses three topics of photosynthetic water cleavage in Photosystem II (PS II): (a) effect of protonation in the acidic range on the extent of the ‘fast’ ns kinetics of P680+· reduction by YZ, (b) mechanism of O–O bond formation and (c) role of protein flexibility in the functional integrity of PS II. Based on measurements of light-induced absorption changes and quasielastic neutron scattering in combination with mechanistic considerations, evidence is presented for the protein acting as a functionally active constituent of the water cleavage machinery, in particular, for directed local proton transfer. A specific flexibility emerging above a threshold of about 230 K is an indispensable prerequisite for oxygen evolution and plastoquinol formation.

Biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments Evidence for two types of cytochrome b559/plastoquinone redox equilibria

In photosystem II membrane fragments with oxidized cytochrome (Cyt) b559 reduction of Cyt b559 by plastoquinol formed in the membrane pool under illumination and by exogenous decylplastoquinol added in the dark was studied. Reduction of oxidized Cyt b559 by plastoquinols proceeds biphasically comprising a fast component with a rate constant higher than (10s)(-1), named phase I, followed by a slower dark reaction with a rate constant of (2.7min)(-1) at pH6.5, termed phase II. The extents of both components of Cyt b559 reduction increased with increasing concentrations of the quinols, with that, maximally a half of oxidized Cyt b559 can be photoreduced or chemically reduced in phase I at pH6.5. The photosystem II herbicide dinoseb but not 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) competed with the quinol reductant in phase I. The results reveal that the two components of the Cyt b559 redox reaction reflect two redox equilibria attaining in different time domains. One-electron redox equilibrium between oxidized Cyt b559 and the photosystem II-bound plastoquinol is established in phase I of Cyt b559 reduction. Phase II is attributed to equilibration of Cyt b559 redox forms with the quinone pool. The quinone site involved in phase I of Cyt b559 reduction is considered to be the site regulating the redox potential of Cyt b559 which can accommodate quinone, semiquinone and quinol forms. The properties of this site designated here as QD clearly suggest that it is distinct from the site QC found in the photosystem II crystal structure.

Study of the nature of biphasic reduction of cytochrome b559 by plastoquinol in photosystem II membrane fragments

273 Cytochrome b559 is an obligate component of the PS II complex in green plants and cyanobacteria; however, the function of this integral hemeecontaining protein is not understood completely. It is assumed that cytochrome b559 is involved in cyclic electron transport in PS II, providing a mechanism to protect the photosynthetic apparatus under conditions of donor or acceptor photoinhibition [1, 2]. It is also assumed that cytochrome b559 functions as a quinol oxidase, mediating plastoquinol oxidation by molecuu lar oxygen [3, 4]. Photoreactions of cytochrome b559 have been intensively studied over the past few decades [2, 4]. Reduced cytochrome b559 is the final electron donor for P680 in samples with a nonfunctioning waterroxii dizing complex. Oxidized cytochrome b559 can be photoreduced in a reaction mediated by Q B and the plastoquinone pool in membrane preparations of PS II [5–7] or via a Q B siteeindependent pathway involving pheophytin and Q A in solubilized PS II samples or in membrane preparations with inhibited function of plastoquinone reduction [8, 9]. The PS II complex includes three plastoquinonee binding sites—Q A , Q B , and Q C. The latter has been found only recently in the threeedimensional structure of the PS II complex [10]. The function of the Q C site is unclear. The binding of quinone analogues (herbii cides, ADRY reagents, and tetraphenylboron) at the Q C site leads to a decrease in the redox potential of the highhpotential form of cytochrome b559 [11, 12]. It is assumed that the function of Q C is associated with the regulation of the redox potential of cytochrome b559 as a result of redoxxdependent substitution of the bound quinone cofactor (quinone/quinol) under conn ditions of different degree of reduction of the quinone pool and that this mechanism ensures oxidation of an excess plastoquinol by oxygen [11, 12]. In this study, we have investigated the reaction of cytochrome b559 reduction in PS II membrane fragg ments by plastoquinol, both endogenous (formed in the membrane pool as a result of illumination of samm ples) and exogenous (added to the nonilluminated samples (in the form of decylplastoquinol). Oxygenn evolving PS II membrane fragments were isolated from sugar beet and treated with 1 mM potassium ferricya nide to oxidize highpotential cytochrome b559 as described in [9]. The amplitude of photoreduction of cytochrome b559 and chemical reduction of cytoo chrome was determined by recording the difference absorption spectra at …

Towards an understanding of the nature of the redox forms of cytochrome b559 in photosystem II

151 Photosystem (PS) II provides a photoinduced elecc tron transfer from the primary donor (chlorophyll dimer) through intermediate scavengers (monomeric chlorophyll and pheophytin) to the primary and then secondary quinone acceptors Q A and Q B. The terminal redox reactions accomplished by the PS II complex are the oxidation of water molecules to molecular oxyy gen on the donor side of PS II and the formation of plastoquinol (PQH 2) in the Q B site on the acceptor side [1]. The PS II complex of green plants and cyanobactee ria includes cytochrome b559, a small hemeecontainn ing transmembrane protein. The function of cytoo chrome b559 is not understood completely. Presumm ably, it is involved in the mechanism of protection of PS II against photoinhibition [2]. It was shown that cytochrome b559 may function as a quinol oxidase, an oxygen reductase, and a superoxide oxidase/reducc tase [3]. A characteristic feature of the cytochrome b559, which distinguishes it from other heme proteins, is its redox heterogeneity in native membrane preparations of PS II. In PS II membrane samples, cytochrome b559 is represented by three redox forms with redox potentials (Е m) of 370–400 mV (highhpotential (HP) form), 170–250 mV (form with an intermediate potential (IP)), and 70–100 mV (lowwpotential (LP) form) [4, 5]. The factors that determine the difference between the redox forms of cytochrome b559 are not reliably known. Relative orientation of the planes of the histidine ligands of the heme, the character of diss tribution of protolytic amino groups and hydrogen bonds in the vicinity of the heme group, and the replacement of one of the histidine ligands of the heme were considered as possible variants [2, 3]. The HP form of cytochrome b559, which is predominant in PS II membrane preparations and accounts for about 70% of cytochrome b559, is unstable towards changes in environmental conditions and is readily converted to the forms with a lower potential. In studies of the photoinduced redox reactions of the LP form of cytochrome b559, insensitive to the Q B site inhibitor DCMU, it was assumed that PS II conn tains two additional quinoneebinding sites, besides Q A and Q B [6]. It was assumed that one of these sites mediates the reduction of cytochrome b559 by pheoo phytin, and the other mediates the oxidation of cytoo chrome b559 by the quinone pool. It was further found that the interaction of PS II …

The PS II complex possesses a quinone-binding site that differs from QA and QB and interacts with cytochrome b559

12 Photosystem II (PS II) accomplishes splitting of water and reduction of plastoquinone (PQ) as a result of photoinduced electron transfer from the primary donor, chlorophyll complex P680, to the primary and the secondary quinone acceptors Q A and Q B . The binding site of the secondary quinone Q B is the target site of PS II inhibitors, which displace the secondary quinone and block linear electron transport. The PS II complexes of higher plants and cyanobacteria contain an integral hemoprotein, cytochrome b 559 [1]. The function of cytochrome b 559 is not clear enough; it is believed that this cytochrome is involved in protection of PS II from photoinhibition by formation of a secondary electron transport chain (ETC) around PS II. The cytochrome b 559 in the cyclic ETC mediates oxidation of the pool plastoquinone (PQ) by the PS II donor site.

New interpretation of the redox properties of cytochrome b559 in photosystem II.

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.