The light reactions

It is now generally believed that there are two photochemical reactions, occurring in series in photosynthesis. The subject is comprehensively reviewed in the multi-author work, edited by Staehelin and Arntzen

(1986): see also Mathis and Paillotin (1981), Parson (1991), Chitnis (2001), Nobel (2005) and Nelson and Yocum (2006). Light reaction 1 is associated with the reduction of NADP; light reaction 2 brings about the liberation of oxygen from water. Each of the photoreactions takes place in a reaction centre in association with specialized light-harvesting pigment-proteins and electron transfer agents. The set of specific functional components associated with light reaction 1 is referred to as photosystem I; the set associated with light reaction 2 is referred to as photosystem II. The functional unit consisting of a single photosystem I and a single photosystem II working together, plus a set of light-harvesting pigmentproteins, is commonly referred to as a photosynthetic unit.

Each photosystem has a chlorophyll a/carotenoid-protein, which both harvests light and is intimately associated with the reaction centre. This pigment-protein represents a funnel through which all the excitation energy collected by other pigment-proteins must pass before it is delivered to the reaction centre. Core complex I, and core complex II chlorophyll a/b-carotene-proteins present in all plants (see §8.3) perform this role for photosystems I and II, respectively.

The extent to which each of the other light-harvesting pigment-proteins of algae and higher plants, described in §8.3, transfers energy specifically to one photosystem or the other, or to both, is uncertain. There is some evidence that the major light-harvesting pigment-proteins, such as the various chlorophyll/carotenoid-containing LHC Ils of higher plants and green algae, and the biliproteins of red algae, transfer most, but not all, of their energy initially to photosystem II. Transfer of some of this energy from photosystem II to photosystem I (referred to as 'spillover', or 'state transition') then takes place, thus making it possible for the two photo-reactions to continue at the same rate. The review by Butler (1978) may be consulted for a detailed account of this topic. There is now evidence (discussed in Allen, 2003; Falkowski and Raven, 2007) that spillover involves a phosphorylation of the the major photosystem II light-harvesting protein (e.g. the chl a/b LHC II of green algae and higher plants), and that as a consequence of the increase in negative charge, some of the light-harvesting complex becomes detached from photosystem II and migrates to photosystem I.

The marine cyanobacterium, Synechocystis, may be an exception to the rule that biliproteins belong to photosystem II, in that both photosystems appear to have their own light-harvesting biliproteins. Kondo et al. (2007) present evidence that this alga has two different kinds of phycobilisome. In addition to a 'normal' phycobilisome, containing phycocyanin and

680* Phaeo.

Qa h3

plastoquinone pool cyt.f , * . plastocyanin

P700 Photosystem I

Fe-S proteins *

ferredoxin FD. NADP reductase NADP

Photosystem II

Fig. 8.17 The light phase of photosynthesis. The details are discussed in the text.

allophycocyanin, transferring energy primarily to photosystem II, and loosely attached to the thylakoid membrane, there is another type of phycobilisome, lacking allophycocyanin and firmly attached to the membrane, which transfers energy mainly, but not solely, to photosystem I.

Turning now to what actually happens in the light phase of photosynthesis, the current view of the sequence of events is summarized in Fig. 8.17. In photosystem I the reaction centre, P7oo, consists of a chlorophyll dimer: one of the two molecules is chlorophyll a, the other is chlorophyll a', the C132~epimer of chlorophyll a. When P700 acquires excitation energy it loses an electron to a primary acceptor, A0, which is believed to be another chlorophyll a dimer. The reduced primary acceptor is then oxidized by a phylloquinone molecule (vitamin K). From the quinone the electron is transferred to Fx, a 4Fe-4S cluster ligated by protein cysteinyl residues in reaction centre proteins PsaA and PsaB, and then to FA and/or FB, 4Fe-4S clusters ligated by cysteinyl residues in reaction centre protein PsaC. Electrons are then transferred to the iron-sulfur protein, ferredoxin, and from there, via the flavoprotein ferredoxin-NADP reductase, to NADP, giving NADPH2.

Simultaneously with photosystem I, activation of photosystem II takes place. When the reaction centre chlorophyll in photosystem II, P680 -believed to be a chlorophyll a dimer - acquires excitation energy it loses an electron first to an associated phaeophytin molecule, which then rapidly reduces a molecule of plastoquinone (QA), bound to protein. Electrons are then transferred to another specialized plastoquinone molecule (Qb), which, when fully reduced (two electrons), is displaced from its binding pocket in the core complex by another - oxidized - molecule of plastoquinone and diffuses within the thylakoid membrane until it reaches the cytochrome b6/f complex where it reduces cytochrome b6. Cytochrome b6 reduces cytochrome f, which in turn reduces the copper-containing protein, plastocyanin, from which an electron is then transferred to the oxidized photosystem I reaction centre chlorophyll, P^00, restoring it to its original (reduced) state.

The oxidized reaction centre chlorophyll, P(|80, is reduced by transfer of an electron from a tyrosyl residue in a reaction centre protein. Within the complex there is a cluster of four Mn ions together with one Ca2+ and a Cl_ ion. It is believed that this inorganic ion cluster reduces the tyrosine radical, and the oxidized form of the cluster can then remove electrons from water, leading to the liberation of oxygen. Putting all these processes together, the transfer of hydrogen from water to NADP, giving rise to oxygen and NADPH2, is now complete.

Oxidation of a reduced plastoquinone molecule by the cytochrome b6/f complex in the electron transfer chain between the two photosystems requires removal of two hydrogen atoms. The electrons are transferred to the cytochromes and two protons are released into the intrathylakoid space (lumen). Also, the removal of electrons from water by photosystem II is accompanied by the transfer of H+ to the intrathylakoid space. For every oxygen molecule liberated there is a movement of not less than eight protons from outside the thylakoid to the thylakoid lumen. It is believed that the pH gradient and electric potential set up in this way operate, by means of a reversible ATPase in the membrane, working backwards, to bring about ATP synthesis, i.e. photophosphorylation.

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