Coupling of light-induced electron transfer to proton uptake in photosynthesis

Light energy is transformed into chemical energy in photosynthesis by coupling a light-induced electron transfer to proton uptake. The resulting proton gradient drives ATP synthesis. In this study, we monitored the light-induced reactions in a 100-kDa photosynthetic protein from 30 ns to 35 s by FTIR difference spectroscopy. The results provide detailed mechanistic insights into the electron and proton transfer reactions of the Q(A) to Q(B) transition: reduction of Q(A) in picoseconds induces protonation of histidines, probably of His126 and His128 in the H subunit at the entrance of the proton uptake channel, and of Asp210 in the L subunit inside the channel at 12 s and 150 s. This seems to be a prerequisite for the reduction of Q(B), mainly at 150 mus. Q(A)(-) is reoxidized at 1.1 ms, and a proton is transferred from Asp210 to Glu212 in the L subunit, the proton donor to Q(B)(-). Notably, our data indicate that Q(B) is not reduced directly by Q(A)(-) but presumably through an intermediary electron donor.