Light-activated reactions reaction centers

Luciferase turnover. The luciferase-catalyzed light-emitting reaction that forms oxyluciferin is fast, but the hydrolysis reaction of oxyluciferin into etioluciferin by luciferase is slow. The turnover rate (catalytic center activity) of luciferase was reported to be about 30/s for the luminescence reaction, and 0.03/s for the hydrolysis of oxyluciferin (Shimomura et al., 1969). 

Figure 12.1. Schematic diagram of a photosynthesis reaction center. Light is absorbed by pigments in the light-gathering antenna and absorbed energy is transferred to a photochemically active site P, where it is utilized to initiate photosynthetic reactions.

The participation of the phycobiliproteins in the absorption ofphotokinetically active light has been demonstrated above. Peaks of around 565 and 615 nm in the action spectra indicate the involvement of C-phycoerythrin andC-phycocanin. These pigments transfer energy to the reaction center of PS II and suggest the participation of the non-cyclic electron transport and coupled phosphorylation. 

The process by which a stereochemically inactive center is converted to a specific stereoisomeric form. In most cases, the reacting center is prochiral. Such processes can occur with reactions involving an optically active reagent, solvent, or catalyst (eg., an enzyme). The reaction produced by such a process is referred to as an enantioselective reaction. In principle, use of circularly polarized light in photochemical reactions of achiral reactants might also exhibit asymmetric induction. However, reported enantioselectivities in these cases have been very small. 

Photosynthetic reaction centers offer an inherent advantage in the fact that the electron-transfer reactions are activated by light. Thus, very fast laser-based kinetic methods, called laser flash photolysis, can be applied to follow the time course of the reactions. The initial electron transfer step in these systems occurs in approximately 2ps and is followed by several subsequent steps that are progressively slower. Numerous investigators have focused their attention on these reactions, which are described elsewhere see Photosynthesis). 

The pathway and kinetics of electron transfer in photochemically activated reaction centers of chloroplasts and photosynthetic microorganisms have been largely solved thanks to ultrafast lasers. The initial steps of light-activated electron transfer do not involve the breaking and making of chemical bonds unlike the great majority of chemical and biochemical reactions. To study the latter type of reactions. 

Figure 1. The major transmembrane photosynthetic reaction centers (RC) (top) and respiratory complexes (bottom) are composed of light (zigzag) activated chains (dark gray) of redox centers (open polygons) that create a transmembrane electric field and move protons (double arrows) to create a transmembrane proton gradient, fulfilling the requirements of Mitchell s chemiosmotic hypothesis. Diffusing substrates include ubiquinone (hexagon) and other sources of oxidants and reductants. PSI and PSII, photosystems I and II, respectively.

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