Light-Induced Dynamics in Photosystem I Electron Transfer

Abstract

Protein dynamics are likely to play important, regulatory roles in many aspects of photosynthetic electron transfer, but a detailed description of these coupled protein conformational changes has been unavailable. In oxygenic photosynthesis, photosystem I catalyzes the light-driven oxidation of plastocyanin or cytochrome c and the reduction of ferredoxin. A chlorophyll (chl) a/a′ heterodimer, P700, is the secondary electron donor, and the two P700 chl, are designated PA and PB. We used specific chl isotopic labeling and reaction-induced Fourier-transform infrared spectroscopy to assign chl keto vibrational bands to PA and PB. In the cyanobacterium, Synechocystis sp. PCC 6803, the chl keto carbon was labeled from 13C-labeled glutamate, and the chl keto oxygen was labeled from 18O2. These isotope-based assignments provide new information concerning the structure of PA+, which is found to give rise to two chl keto vibrational bands, with frequencies at 1653 and 1687cm−1. In contrast, PA gives rise to one chl keto band at 1638cm−1. The observation of two PA+ keto frequencies is consistent with a protein relaxation-induced distribution in PA+ hydrogen bonding. These results suggest a light-induced conformational change in photosystem I, which may regulate the oxidation of soluble electron donors and other electron-transfer reactions. This study provides unique information concerning the role of protein dynamics in oxygenic photosynthesis.