Scientists Unlock Mystery in Important Photosynthesis Step
ScienceDaily (Feb. 20, 2010) — An international team of scientists, including two from Arizona State University, has taken a significant step closer to unlocking the secrets of photosynthesis, and possibly to cleaner fuels.
This is Kevin Redding in his lab at Arizona State University. Together with coworkers from the Max Plank Institute, he has taken a significant step closer to unlocking the secrets of photosynthesis. (Credit: Mary Zhu)
The ASU scientists working with collaborators at the Max Planck Institute at Mülheim a.d. Ruhr in Germany have been investigating the PSI reaction center.
They have made an important observation that is nut-shelled in the title of a paper published in the online Early Edition of theProceedings of the National Academy of Sciences (PNAS).
Kevin Redding, an associate professor in the department of chemistry and biochemistry in the College of Liberal Arts and Sciences, is leading the research at ASU. His lab created mutations in a single-celled green alga (Chlamydomonas reinhardtii or 'Chlamy' for short). Using these mutants, Redding and collaborators have shown that the primary light-triggered electron transfer event in the PSI reaction center can be initiated independently in each of its parallel branches. At the same time, they showed that PSI has two charge separation devices that effectively work in parallel to increase the overall efficiency of electron transfer.
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Independent initiation of primary electron transfer in the two branches of the photosystem I reaction center
Marc G. Müller a, Chavdar Slavov a,1, Rajiv Luthra b,1, Kevin E. Redding b,2, and Alfred R. Holzwarth a,2
-Author Affiliations
aMax-Planck-Institut für Bioanorganische Chemie, D-45470 Mülheim a.d. Ruhr, Germany;
bDepartment of Chemistry and Biochemistry, Arizona State University, 1711 South Rural Road, Tempe, AZ 85287-1604
Edited by A. William Rutherford, CEA Saclay, Gif-sur Yvette, France, and accepted by the Editorial Board January 08, 2010 (received for review May 22, 2009)
↵1C.S. and R.L. contributed equally to this work.
Abstract
Photosystem I (PSI) is a large pigment-protein complex that unites a reaction center (RC) at the core with ∼100 core antenna chlorophylls surrounding it. The RC is composed of two cofactor branches related by a pseudo-C2 symmetry axis. The ultimate electron donor, P700 (a pair of chlorophylls), and the tertiary acceptor, FX (a Fe4S4 cluster), are both located on this axis, while each of the two branches is made up of a pair of chlorophylls (ec2 and ec3) and a phylloquinone (PhQ). Based on the observed biphasic reduction of FX, it has been suggested that both branches in PSI are competent for electron transfer (ET), but the nature and rate of the initial electron transfer steps have not been established. We report an ultrafast transient absorption study of Chlamydomonas reinhardtii mutants in which specific amino acids donating H-bonds to the 131-keto oxygen of either ec3A (PsaA-Tyr696) or ec3B (PsaB-Tyr676) are converted to Phe, thus breaking the H-bond to a specific ec3 cofactor. We find that the rate of primary charge separation (CS) is lowered in both mutants, providing direct evidence that the primary ET event can be initiated independently in each branch. Furthermore, the data provide further support for the previously published model in which the initial CS event occurs within an ec2/ec3 pair, generating a primary ec2+ec3-radical pair, followed by rapid reduction by P700 in the second ET step. A unique kinetic modeling approach allows estimation of the individual ET rates within the two cofactor branches.
Chlamydomonas electron transfer directionality femtosecond absorption photosystem I
ultrafast spectroscopy
Footnotes
2To whom correspondence may be addressed. E-mail: holzwarth@mpi-muelheim.mpg.de or kevin.redding@asu.edu.
Author contributions: K.R. and A.R.H. designed research; M.G.M., C.S., and R.L. performed research; R.L. contributed new reagents/analytic tools; M.G.M. and C.S. analyzed data; and M.G.M., C.S., K.R., and A.R.H. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. A.W.R. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/cgi/content/full/0905407107/DCSupplemental.
†Note that the analysis of the “WT data” is based on our previously published (27) transient absorption data on C. reinhardtii PSI purified from the CC2696 mutant (6, 27), which lacks PSII and most of the cellular LHC complement. The PsaA-Y696F and PsaB-Y676F mutants were introduced into a strain that also lacks PSII but also has a reduced PSI antenna size. This is the reason why the antenna to RC signal strength in Fig. 5 is different between the WT and the mutants. However, since this is only an antenna effect, it has no consequences for the ET issues discussed in this paper.
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