1:4. Efficient excitation transfer between carotenoids and chlorophylls, based on the structure of the aggregate, i.e., close contacts between peridinins and chlorophylls, has been confirmed by quantum chemical calculations (T.R., A.D., and K.S., unpublished work).
The most abundant light-harvesting complex located in chloroplasts of green plants is LHCII. shown in Fig. 8d. The structure of LHCII, resolved at 3.4 Å (6), features two carotenoids, seven Chls a, and five Chls b as light-absorbing agents. LHCII is located within the thylakoid membrane in the vicinity of PS-II. It has been suggested that LHCII can, according to light conditions, physically move toward PS-I, regulating thereby the relative flow of energy into PS-II and PS-I.
The multiprotein photosynthetic apparatus as shown in Fig. 1 poses the challenge for eventually modeling the conversion of light into ATP in its entirety. Few would have predicted that the protein constituents of the photosynthetic apparatus would be structurally known in principle already today, but many expect that biologists will see more and more often entire protein systems engaged in complex overall functions resolved at atomic resolution. The questions posed by the photosynthetic apparatus will then be typical for biology of the 21st century: how are multiprotein systems genetically controlled, how do they physically aggregate, how did they evolve, and how do they compare between species? The PSU constitutes an ideal subsystem of the photosynthetic apparatus that, because of its smaller size, is more amenable to study while posing the same principal challenges: how do LH-I and LH-II form from their many independent components, what determines the ring size and stability, and how do the completed LH-IIs aggregate around the LH-I-RC complex? The function of the PSU emerges as a true system properly, all components being designed to cooperate in absorbing light effectively and channel its energy to the RC. The common origin of photosynthetic, respiratory, and other organisms makes the PSU and the photosynthetic apparatus a valuable model for understanding, at the level of multiprotein systems, not only photosynthesis but also life in general.
We acknowledge financial support from the National Institutes of Health [Grant P41RR05969], the National Science Foundation [Grants NSF BIR 9318159 and NSF BIR-94–23827(EQ)]. and the Carver Charitable Trust.
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