Reaction centres

All types of photosynthetic systems are constructed around an exquisitely designed basic blueprint. All contain a reaction centre (RC) protein complex in which the conversion of light energy to electrochemical potential occurs. This energy conversion process involves the movement of electrical charge in the form of an electron across a membrane, generating an electrical gradient as well as a chemical potential gradient in the form of 'redox' energy, as indicated in Fig. 1.6. As stated earlier, the primary electron donor-the pigment-P is always a chlorophyll or bacteriochlorophyll, while the primary electron acceptor A can be either (bacterio)chlorophyll (as in Type 1 RCs) or (bacteri0)pheophytin (as in Type I1 RCs). Typically the generation of the primary radical pair" P'A- occurs within a few picoseconds at very high quantum efficiencies.
 

The subsequent reduction of P'and oxidation of A- occur on a slower time scale. In both Type I and Type I1 RCs, A- is initially re-oxidised by a quinone molecule (Q). In Type I reaction centres, this quinone is phylloquinone, which typically has a low midpoint redox potential" of about -0.6 V and is strongly bound to the RC proteins. In Type I1 RCs, the quinone electron acceptor (called QA) is also tightly bound but has a higher midpoint potential of about -0.1 V. The identity of this QA quinone depends on the organism: it is plastoquinone in plants and algae, and ubiquinone or menaquinone in purple photosynthetic bacteria. The transfer of electrons from A- to Q occurs on a timescale of -200 ps, resulting in the charge transfer state P+AQ-.
 

It is the next step in the reductive electron flow that clearly distinguishes Type I and Type I1 reaction centres. In Type I centres, the electron is passed to an ironsulphur centre (given the symbol Fx) which is contained within the reaction centre protein, as shown in Fig. 3.8. From Fx, the electron proceeds to two further ironsulphur centres ( F A and FB) and ultimately to ferredoxin, which as a water-soluble protein, transfers the reducing equivalent away from the membrane. In contrast, Type11 reaction centres transfer the electron on QA- to a second quinone, QB (as shown in Fig. 3.1). When QB receives a second electron from the next photochemical turnover, it is protonated to form a quinol, which diffuses away from the reactioncentre protein into the lipid matrix of the membrane. In plants and algae, QB is a plastoquinone, while in purple photosynthetic bacteria it is a ubiquinone. These secondary electron transfer events leading to the ejection of reducing equivalents from the reaction centre occur on a timescale stretching from microseconds to milliseconds.

Meanwhile, the reduction of Pi by the electron donor D occurs on the nanosecond to millisecond time scale, depending on conditions. The nature of the electron donor also depends on the particular system. Cytochromes are usually the donors in both Type I and Type I1 RCs of photosynthetic bacteria, while in plants and algae. water is the electron donor to PSII and plastocyanin or cytochrome c6 is the electron donor to PSI. In thosc photosynthetic organisms that evolve O? (plants, algae and cyanobacteria), the Type I (PSI) and Type I1 (PSII) reaction centres are coupled as shown schematically in Fig. 1.5b and in more detail in Fig. 1.1, so as to use two photons to drive each electron through the system, providing sufficient energy to oxidise water and reduce COz. In all cases, the fundamental principle is that energy storage is accomplished by rapidly separating the initial oxidants and reductants of the primary charge separation so as to avoid wasteful recombination reactions.