Thylakoid light harvesting complex

EFFECTS OF CATALYTICAL HYDROGENATION IN SITU OF PHOS-PHATIDYLGLYCEROL-ASSOCIATED TRANS HEXADECENOATE ON THE STABILITY OF THE THYLAKOID LIGHT HARVESTING COMPLEX... 

The physical separation of PS II and PS I permits the chloroplasts to respond to changes in illumination. The relative amount of light absorbed by these two systems varies with the distribution of light harvesting complexes (LHCs) between the stacked and unstacked portion of the thylakoid membrane. 

Garab, G., Faludidaniel, A., Sutherland, J. C., and Hind, G. 1988. Macroorganization of chlorophyll a/b light-harvesting complex in thylakoids and aggregates Information from circular differential scattering. Biochemistry 27 2425-30. 

Della Mea, M., Di Sandro, A., Dondini, L., Del Duca, S., Vantini, F., Bergamini, C., Bassi, R., and Serafini-Fracassini, D., A /ea mays 39-kDa thylakoid transglutaminase catalyzes the modification by polyamines of light-harvesting complex II in a light-dependent way, Planta, 219, 754—764, 2004. 

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.

The polypeptides of the light-harvesting complex are most easily resolved by polyacrylamide gel electrophoresis of thylakoid membranes solubilized with sodium or lithium dodecyl sulphate. This produces the green chlorophyll-protein band, CP II, which on staining for protein reveals up to four polypeptides of 24-27 kDa [2], each of which is believed to bind Chi a and Chi b. In most plants two polypeptides predominate, but other minor polypeptides may be resolved [2]. 

Note that LHCll is a separate light-harvesting complex, which supplements the inner antennae (CP29, CP26, CP24 and CP22) associated with the PS-II complex. The presence of this separate LHC II complex has been confirmed by the results of freeze-fracture experiments obtained with thylakoid membranes of both wild-type and mutant plants. The PS-11 inner antenna complex and the LHC-II complex appear as distinctly different classes of membrane particles. On the other hand, the LHC-I proteins associated with the PS-1 complex appear to be complexed to the PS-1 reaction center, much as the inner antennae of PS II are complexed to the PS-II reaction center. 

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.

III. In V7Vo Organization of Light-Harvesting Complexes in the PS-I Thylakoid Membrane... 

Reversible phosphorylation and dephosphorylation of the PSII light-harvesting complex control the functional organization of the photosynthetic apparatus in thylakoid membranes. State I favors linear electron flow, whereas state II favors cyclic electron flow (see Figure 8-40). 

---