Pigment-protein complexes light-harvesting complex

The antennae complexes (light harvesting complex proteins or LHCs) are a large family of related proteins that bind xanthophylls (typically lutein, violaxanthin and neoxanthin), Chi a and Chi b and collectively are the most abundant pigment protein complexes in the plastid (Peter and Thomber, 1991 Bassi et al., 1993). 

Two experimental systems will be briefly described to illustrate some of the ideas presented in the previous section. The examples span the range of system complexity from a diatomic molecule (I2) [28] to a supramolec-ular pigment-protein complex (the core light-harvesting antenna of photosynthetic bacteria, LH1 [18, 19]). 

But one may go further and say that these are the primary steps, the only ones which are true photochemistry and which are, therefore, unique to photosynthesis. The pigment-protein complexes responsible for light harvesting and for the electron transfer sequences within the reaction centres of Photosy stem 1 (PS1) and Photosy stem 2 (PS2) are the heart of the photosynthetic process. 

Fig. 2. Spectra of the reaction center (A), chromatophores (B) and the light-harvesting complexes (C) of the purple bacterium Chromatium vinosum. From Thornber (1986) Biochemical characterization and structure of pigment-proteins of photosynthetic organism. In LA Staehelin and CJ Arntzen (eds) Encyclopedia of Plant Physiology, New series, Vol 19, p 100. Springer Verlag.

Fig. 1. Models for the light-harvesting complexes surrounding the PS-II reaction center in the thylakoid (A) Simplified model of the thylakoid in cross section, (B) More detailed model of PS II. (A) taken from Fig. 21 (A) in Chapter 1. The light-harvesting complex portion in (B) adapted from H-E Akerlund (1993) Function and organization of photosysfem II. In C Sundqvist and M Ryberg (eds) Pigment-Protein Complexes in Plastids Synthesis and Assembly, p 419. Acad Press.

The primary photosynthetic process is carried out by a pigment protein complex the reaction centre (RC) embedded in a lipid bilayer membrane (Figure 6.19) and surrounded by light-harvesting complexes.1477,1481,1482 Thus energy is transferred from LH1 to a bacteriochlorophyll special pair (P) and then through a bacteriochlorophyll molecule (BC monomer) to bacteriopheophytin (BP a chlorophyll molecule lacking the central Mg2 + ion), followed by electron transfer to a quinone Qa in hundreds of ps. The neutral P is then restored by electron transfer from the nearest intermembrane space protein cytochrome c (Cyt c) in hundreds of ns. The rate constants of the... 

Light-harvesting complex (multiple proteins and pigments), reaction center (multisubunit protein with associated pigments and electron carriers)... 

Cammarata KV, Plumley FG and Schmidt GW (1992) Pigment and protein composition of reconstituted light-harvesting complexes and effects of some protein modifications. Photosynth Res 33 235-250... 

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