The Central Photochemical Event: Light-Driven Electron Flow:- Spatial Separation of Photosystems I and II Prevents Exciton Larceny
The energy required to excite PSI (P700) is less than that needed to excite PSII (P680) (shorter wavelength corresponds to higher energy). If PSI and PSII were physically contiguous, excitons originating in the antenna system of PSII would migrate to the reaction center of PSI, leaving PSII chronically underexcited and interfering with the operation of the two-center system. This “exciton larceny” is prevented by separation of PSI and PSII in the thylakoid membrane (Fig. 19–52). PSII is located almost exclusively in the tightly appressed membrane stacks of thylakoid grana (granal lamellae); its associated light-harvesting complex (LHCII) mediates the tight association of adjacent membranes in the grana. PSI and the ATP synthase complex are located almost exclusively in the thylakoid membranes that are not appressed (the stromal lamellae), where both have access to the contents of the stroma, including ADP and NADP+. The cytochrome b6f complex is present throughout the thylakoid membrane.
The association of LHCII with PSII is regulated by light intensity and wavelength. In bright sunlight (with a large component of blue light), PSII absorbs more light than PSI and produces reduced plastoquinone (plasto quinol, PQH2) faster than PSI can oxidize it. The resulting accumulation of PQH2 activates a protein kinase that phosphorylates a Thr residue on LHCII (Fig. 19–53). Phosphorylation weakens the interaction of LHCII with PSII, and some LHCII dissociates and moves to the stromal lamellae; here it captures photons for PSI, speeding the oxidation of PQH2 and reversing the imbalance between electron flow in PSI and PSII. In less intense light (in the shade, with more red light), PSI oxidizes PQH2 faster than PSII can make it, and the resulting increase in PQ concentration triggers dephosphorylation of LHCII, reversing the effect of phosphorylation.
FIGURE 19–52 Localization of PSI and PSII in thylakoid membranes. Light harvesting complex LHCII and ATP synthase are located in regions of the thylakoid membrane that are appressed (granal lamellae, in which several membranes are in contact) and in regions that are not appressed (stromal lamellae) and have ready access to ADP and NADP+ in the stroma. Photosystem II is present almost exclusively in the appressed regions, and photosystem I almost exclusively in nonappressed regions exposed to the stroma. LHCII is the “adhesive” that holds appressed lamellae together (see Fig. 19–53).
FIGURE 19–53 Equalization of electron flow in PSI and PSII by modulation of granal stacking. A hydrophobic domain of LHCII in one thylakoid lamella inserts into the neighboring lamella and closely ap presses the two membranes. Accumulation of plastoquinol (not shown) stimulates a protein kinase that phosphorylates a Thr residue in the hydrophobic domain of LHCII, which reduces its affinity for the neigh boring thylakoid membrane and converts appressed regions (granal lamellae) to nonappressed regions (stromal lamellae). A specific protein phosphatase reverses this regulatory phosphorylation when the [PQ]/[PQH2] ratio increases.
