Biochemistry. Photosynthesis from the protein's perspective

Abstract

Slow motions of proteins modulate electron-transfer rates during the early stages of photosynthesis.

The initial photosynthetic electron-transfer reaction

(Left) In the photosynthetic reaction center, an electron is transferred from a special pair of chlorophylls via a bridging chlorophyll to the pheophytin acceptor. A nearby tryptophan—1 of 39 in the protein—is also shown. Wang et al. use the time evolution of the protein's tryptophan absorbance to track the protein's response (in the Sumi-Marcus model, motion along the X coordinate) following photoexcitation of the special pair. (Right) Application of the Sumi-Marcus model by Wang et al. (2) to the electron-transfer step shown in the left panel. The parabolas describe the energy of the electron-transfer system as a function of the motions q for the initial state of the system before photoexcitation (black), the photoexcited state before electron transfer (blue), and the final charge-separated state (red).The slow motion X shifts the photoexcited energy surface, thus modulating the speed of the electron-transfer step (horizontal arrows). The colors for the photoexcited and charge-separated energy surfaces correspond to the colors for the electron donor and acceptor moieties in the left panel.