PubMed1982
It was found that the rate of proton efflux from the cells, vH+, and the turnover number of bacteriorhodopsin photoconversion cycle per second, vM, are gradually decreased during continuous illumination of Halobacterium halobium with orange light (550-650 nm, 10(3) W/m2). The steady-state value of vH+ after 3 min of illumination is 3 times lower than vH+ at the 1st second of illumination. At the same time the concentration of the photochemical cycle intermediate, M412, the quantum yield of its formation, phi M, and the quantum efficiency of proton efflux, phi H+, are decreased (phi M and phi H+ up to 3-4-fold), whereas the concentration of the initial form of bacteriorhodopsin, BR570, and lifetime of M412, tau, are increased. These light-induced effects are abolished by uncoupler, carbonylcyanide m-chlorophenylhydrazone (CCCP) and are enhanced by the phosphorylation inhibitor, N,N'-dicyclohexylcarbodiimide (DCCD). This suggests that the observed changes of M412 and BR570 concentrations, tau, VH+, VM, phi M and phi H+ are due to the light-induced changes in the transmembrane electric potential, delta psi, and in pH inside and outside the cell, which control the photochemical cycle reactions. The decrease of phi M and phi H+ is discussed on the basis of an earlier proposed scheme of the branched photochemical cycle. It is assumed that the back reactions of the cycle (shunts) not coupled with the transmembrane proton translocation, e.g. conversion of L550 (P550) into BR570 without M412 formation, etc., become more probably at high values of the proton electrochemical potential gradient.