ON THE MOLECULAR MECHANISMS OF THE RAPID AND SLOW SOLAR-TO-ELECTRIC ENERGY STORAGE PROCESSES BY THE OTHER NATURAL PHOTOSYNTHETIC SYSTEM, BACTERIORHODOPSIN

摘要

Upon the absorption of solar energy by retinal in bacteriorhodopsin (bR), a very rapid and highly specific photoisomerization of the retinal around the C-13-C-14 bond takes place. This is followed by the formation of a number of intermediates resulting from conformational changes of the protein around the retinal which leads to the deprotonation of the protonated Schiff base of the retinylidene system. This is the switch of the proton pump which leads to the last step in the storage of solar energy in the form of electric energy by this photosynthetic system. The removal of metal cations from bR is found to inhibit the deprotonation process. In the present paper we summarize the results of our studies and the others regarding two important questions in the conversion process: 1) what is(are) the molecular mechanism(s) of the protein catalysis of the photoisomerization process and 2) what is the role of metal cations in the deprotonation process of the protonated Schiff base (the switch of the proton pump)? In order to answer the first question, the results of the subpicosecond photoisomerization rate of retinal in bR and in a number of its relevant mutants are discussed in terms of the steric and electronic factors. In an effort to answer the second question, we discussed the results of the binding studies of Ca2+ to bR, to its mutants and to bR after its C-terminus is cleaved. From these results and the results of Roux et al. on the P-31 NMR of Nd3+ regenerated bR, we concluded that one or two metal cations strongly bound to the protein but not on the surface, are functionally important. The model in which these metal cation(s) control the pK values of Aspartic acids in the 85 and 212 positions and that of the protonated Schiff base (PSB) during the photocycle is discussed. [References: 68]