This Article Full Text Full Text (PDF) All Versions of this Article: biophysj.105.059824v1 90/2/552    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Holzwarth, A. R. Articles by Lubitz, W. Search for Related Content PubMed PubMed Citation Articles by Holzwarth, A. R. Articles by Lubitz, W.

Ultrafast Transient Absorption Studies on Photosystem I Reaction Centers from Chlamydomonas reinhardtii. 2: Mutations near the P700 Reaction Center Chlorophylls Provide New Insight into the Nature of the Primary Electron Donor

Max-Planck-Institut für Bioanorganische Chemie, D-45470 Mülheim an der Ruhr, Germany

Correspondence: Address reprint requests to Prof. A. R. Holzwarth, Max-Planck-Institut für Bioanorganische Chemie. Tel.: 49-208-306-3571; Fax: 49-208-306-3951; E-mail: holzwarth{at}mpi-muelheim.mpg.de.

The energy transfer and charge separation kinetics in several core Photosystem I particles of Chlamydomonas reinhardtii with point mutations around the P_A and P_B reaction center chlorophylls (Chls) have been studied using ultrafast transient absorption spectroscopy in the femtosecond to nanosecond time range to characterize the influence on the early electron transfer processes. The data have been analyzed in terms of kinetic compartment models. The adequate description of the transient absorption kinetics requires three different radical pairs in the time range up to 100 ps. Also a charge recombination process from the first radical pair back to the excited state is present in all the mutants, as already shown previously for the wild-type (Müller, M. G., J. Niklas, W. Lubitz, and A. R. Holzwarth. 2003. Biophys. J. 85:3899–3922; and Holzwarth, A. R., M. G. Müller, J. Niklas, and W. Lubitz. 2005. J. Phys. Chem. B. 109:5903–59115). In all mutants, the primary charge separation occurs with the same effective rate constant within the error limits as in the wild-type (»350 ns^–1), which implies an intrinsic rate constant of charge separation of <1 ps^–1. The rate constant of the secondary electron transfer process is slowed down by a factor of 2 in the mutant B-H656C, which lacks the ligand to the central metal of Chl P_B. For the mutant A-T739V, which breaks the hydrogen bond to the keto carbonyl of Chl P_A, only a slight slowing down of the secondary electron transfer is observed. Finally for mutant A-W679A, which has the Trp near the P_A Chl replaced, either no pronounced effect or, at best, a slight increase on the secondary electron transfer rate constants is observed. The effective charge recombination rate constant is modified in all mutants to some extent, with the strongest effect observed in mutant B-H656C. Our data strongly suggest that the Chls of the P_A and P_B pair, constituting what is traditionally called the "primary electron donor P700", are not oxidized in the first electron transfer process, but rather only in the secondary electron transfer step. We thus propose a new electron transfer mechanism for Photosystem I where the accessory Chl(s) function as the primary electron donor(s) and the A₀ Chl(s) are the primary electron acceptor(s). This new mechanism also resolves in a straightforward manner the difficulty with the previous mechanism, where an electron would have to overcome a distance of 14 Å in <1 ps in a single step. If interpreted within a scheme of single-sided electron transfer, our data suggest that the B-branch is the active branch, although parallel A-branch activity cannot be excluded. All the mutations do affect to a varying extent the energy difference between the reaction center excited state RC* and the first radical pair and thus affect the rate constant of charge recombination. It is interesting to note that the new mechanism proposed is in fact analogous to the electron transfer mechanism in Photosystem II, where the accessory Chl also plays the role of the primary electron donor, rather than the special Chl pair P680 (Prokhorenko, V. and A. R. Holzwarth. 2000. J. Phys. Chem. B. 104:11563–11578).

This article has been cited by other articles:

				T. Tomo, Y. Kato, T. Suzuki, S. Akimoto, T. Okubo, T. Noguchi, K. Hasegawa, T. Tsuchiya, K. Tanaka, M. Fukuya, et al. Characterization of Highly Purified Photosystem I Complexes from the Chlorophyll d-dominated Cyanobacterium Acaryochloris marina MBIC 11017 J. Biol. Chem., June 27, 2008; 283(26): 18198 - 18209. [Abstract] [Full Text] [PDF]

				C. Slavov, M. Ballottari, T. Morosinotto, R. Bassi, and A. R. Holzwarth Trap-Limited Charge Separation Kinetics in Higher Plant Photosystem I Complexes Biophys. J., May 1, 2008; 94(9): 3601 - 3612. [Abstract] [Full Text] [PDF]

				A. R. Holzwarth, M. G. Muller, M. Reus, M. Nowaczyk, J. Sander, and M. Rogner Kinetics and mechanism of electron transfer in intact photosystem II and in the isolated reaction center: Pheophytin is the primary electron acceptor PNAS, May 2, 2006; 103(18): 6895 - 6900. [Abstract] [Full Text] [PDF]