Antenna complexes of photosystem

In PS I, as in PS II, there are a number of Chi alb protein complexes having lightharvesting and energy-transfer functions. Such complexes most probably exist in direct contact with the RC (part of the core complex), and certainly exist as peripheral LHC I antenna complexes further removed from the RC. A native PS I complex (80-180 Chi per RC, 100 kDa) with at least 6 polypeptides was isolated by solubilization of the thylakoid membrane with nonionic detergents (for example, Triton X-100) [177,178]. With further detergent treatment, the PS I complex dissociated into the core complex (CC I with the RC) and the peripheral antenna complex (LHC I) (spinach, barley, pea, Chlamydomonas reinhardtii [179-183]. The peripheral antenna complex (pea, spinach Chi alb ratio 4.0 1, typical fluorescence at 730 nm) contains 3-4 antenna polypeptides (19-25 kDa) [181,184,185]. This complex was also dissociated into two different antenna complexes - LHC la (2 polypeptides of 22 and 23 kDa) and LHC Ib (1 polypeptide, 20 kDa) - which differ in their fluorescence characteristics (680 nm and 730 nm) [184]. No structural data on these polypeptides are available at present. It was postulated that in C. reinhardtii, in addition to the peripheral antenna complex, an antenna system (with 4 polypeptides) exists, which connects the peripheral antenna energetically with the core complex CC 1 [183]. 

The core complex (CC 1, CP 1) with RC P700 (10-65 BChl a, 1-2 )3-carotenes found in various preparations of pea, spinach, barley and C. reinhardtii) contains two main types of polypeptides (60-70 kDa) and small amounts of three to five polypeptides of unknown function (some may also have an antenna function) [165,186]. The relatively large number of bound Chi a molecules in the core com- 

---

In contrast to LHCI, the light-harvesting chlorophyll a/b-antennae complex of photosystem II (LHCII) is the major component of the particles on the complementary protoplasmic fracture face of appressed membranes (PFs) (Simpson, 1979, Olive et al., 1979), and does not appear to be a significant component of the reaction centre EFs particles, although this is disputed. The LHCII in PFs particles is, nevertheless, in contact with the reaction centre particles and may provide a pathway for excitation energy transfer between several photosystem II reaction centres. 

Groot M-L, Peterman EJG, Van Stokkum IHM, Dekker JP and Van Grondelle R (1995) Triplet and fluorescing states ofthe CP47 antenna complex of Photosystem II studied as a function of temperature. Biophys J 68 281-290... 

The Biogenesis and Assembly of the Major Antenna Complex of Photosystem II (LHCIIb) in Barley 661... 

As seen earlier in Chapter 2 on bacterial reaction centers, crystallization of the reaction-center protein of the photosynthetic h iCttn xm Rhodopseudomonas viridis by Michel in 1982 and subsequent determination ofthe three-dimensional structure ofthe reaction center by Deisenhofer, Epp, Miki, Huber and Michel in 1984 led to tremendous advances in the understanding ofthe structure-function relationship in bacterial photosynthesis. Furthermore, because of certain similarities between the photochemical behavior of the components of some photosynthetic bacteria and that of photosystem II, research in photosystem-II was greatly stimulated to its benefit by these advances. In this way, it became obvious that the ability to prepare crystals from the reaction-center complexes of photosystems I and II would be of great importance. However, it was also recognized that, compared with the bacterial reaction center, the PS-I reaction center is more complex, consisting of many more protein subunits and electron carriers, not to mention the greater number of core-antenna chlorophyll molecules. 

W Ortiz, E Lam, M Ghirardi and R Malkin (1984) Antenna function of a chlorophyll alb protein complex of photosystem I. Biochim Biophys Acta 766 505-509... 

Jurinovich S, Viani L, Prandi IG, Renger T, Mennucci B. Towards an ab initio description of the optical spectra of light-harvesting antennae application to the CP29 complex of photosystem II. Phys Chem Chem Phys. 2015 17(22) 14405—14416. http // dx.doi.org/10.1039/c4cp05647g. 

Antenna size reduction unfortunately is not trivial and current antenna size mutants of Chlamydomonas reinhardtii, for example, were not able to outperform the wild type (De Mooij et al, 2014). It is important to stress that only light absorption should be reduced and that the capacity to process photons and perform photosynthesis should remain unaltered. In other words, the number of photosystems must remain maximal and only the size of the antenna complex of the photosystems should be reduced (Formighieri et al, 2012 Wobbe and Remade, 2014). 

Ruban, A.V., Pascal, A.A., Lee, P.J., Robert, B., and Horton, P. 2002a. Molecular configuration of xanthophyll cycle carotenoids in photosystem II antenna complexes. J. Biol. Chem 111 42937-42942. 

FIGURE 19-45 Organization of photosystems in the thylakoid membrane. Photosystems are tightly packed in the thylakoid membrane, with several hundred antenna chlorophylls and accessory pigments surrounding a photoreaction center. Absorption of a photon by any of the antenna chlorophylls leads to excitation of the reaction center by exciton transfer (black arrow). Also embedded in the thylakoid membrane are the cytochrome bkf complex and ATP synthase (see Fig. 19-52). 

The capture of solar energy occurs in photosystems. Each photosystem consists of an antenna complex and a photosynthetic reaction center. The... 

Figure 5-19. Schematic representation of reactions occurring at the photosystems and certain electron transfer components, emphasizing the vectorial or unidirectional flows developed in the thylakoids of a chloroplast. Outwardly directed election movements occur in the two photosystems (PS I and PS II), where the election donors are on the inner side of the membrane and the election acceptors are on the outer side. Light-harvesting complexes (LHC) act as antennae for these photosystems. The plastoquinone pool (PQ) and the Cyt b(f complex occur in the membrane, whereas plastocyanin (PC) occurs on the lumen side and ferredoxin-NADP+ oxidoreductase (FNR), which catalyzes electron flow from ferredoxin (FD) to NADP+, occurs on the stromal side of the thylakoids. Protons (H+) are produced in the lumen by the oxidation of water and also are transported into the lumen accompanying electron (e ) movement along the electron transfer chain.

Figure 7 Rogue s gallery of structures of peripheral anteima complexes. As labelled these include Chlorosomes from green sulfur bacteria, fused antenna domains of the Photosystem I core, the CP43 and CP47 proteins of Photosystem II, the Fenna-Matthew-Olson (FMO) protein associated with chlorosomes, LHI proteins surrounding a purple bacterial photo synthetic core, the peridinin-chlorophyll a protein of dinoflagellate algae, the LHCI and LHCII proteins found in plants and many algae, and the LHII protein complex that is associated with LHI in purple bacteria...

---