5380
J. Phys. Chem. B 1999, 103, 5380-5387
Amino Acid Protonation States Determine Binding Sites of the Secondary Ubiquinone and Its Anion in the Rhodobacter sphaeroides Photosynthetic Reaction Center1 Anthony K. Grafton† and Ralph A. Wheeler* Department of Chemistry and Biochemistry, UniVersity of Oklahoma, Norman, Oklahoma 73019 ReceiVed: January 7, 1999; In Final Form: April 28, 1999
Molecular dynamics simulations of native ubiquinone-10 binding in the photosynthetic reaction center of Rhodobacter sphaeroides are presented that support the theory that the neutral and radical anionic quinones QB and QB•- bind in different locations. The differences in binding are attributed to differing protonation states of the nearby amino acids GLU L212 and ASP L213. QB binding at the “dark-adapted” QB site observed by Stowell et al. is most consistent with protonation of GLU L212. QB•- binding at the experimentally observed “light-adapted” QB•- site is consistent only with protonation of both GLU L212 and ASP L213. The experimentally established pH dependence of electron-transfer rate, combined with our MD results, implies that protonation of ASP L213 must occur before electron transfer. Additionally, the molecular dynamics results suggest that movement of the semiquinone anion QB•- between sites (for different amino acid protonation states) is spontaneous near room temperature and cannot by itself account for the higher of two experimentally observed activation energies for electron transfer from QA to QB.
A fundamental understanding of the step-by-step process of photosynthesis remains elusive in part because the atomic-level three-dimensional structures of photosystems I and II are currently unknown.2 Bacterial photosynthetic reaction centers, such as that from the purple bacterium Rhodobacter sphaeroides, are useful as models of the plant photosystem3,4 in part because numerous atomic-scale X-ray structures of these systems have been described in detail.5,6 Even in these relatively wellcharacterized systems, unanswered questions remain. After electron transfer from the primary donor (the special pair) to the primary quinone (QA), the electron is then transferred to a secondary quinone, QB,7-15 which in the case of Rhodobacter sphaeroides (Rb. sphaeroides) is ubiquinone-10 (see Figure 1).16-18 The anionic form of the secondary ubiquinone-10, QB•-, is subsequently protonated, a second electron and a second proton are transferred to QB, and the doubly reduced QBH2 diffuses out of the reaction center (RC) to be replaced by another QB. The first electron transfer between quinones has been the subject of many experimental studies. Kinetic studies have shown that the rate-limiting step in QA•- f QB•- electron transfer has an activation energy of about 15 kcal/mol.19,20 More recent work shows biphasic electron transfer at temperatures below 288 K, with activation energies of
