A comparative flash-photolysis study of electron transfer from pea and spinach plastocyanins to spinach Photosystem 1. A reaction involving a rate-limiting conformational change |
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| Authors: | Kalle Sigfridsson Shiping He Sandeep Modi Derek S Bendall John Gray Örjan Hansson |
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| Institution: | (1) Department of Biochemistry and Biophysics, Lundberg Laboratory, Göteborg University, Medicinaregatan 9C, S-413 90 Göteborg, Sweden;(2) Chalmers University of Technology, Medicinaregatan 9C, S-413 90 Göteborg, Sweden;(3) Department of Plant Sciences, University of Cambridge, Downing Street, CB2 3EA Cambridge, UK;(4) Department of Biochemistry, University of Cambridge, Tennis Court Road, CB2 1QW Cambridge, UK;(5) Cambridge Centre for Molecular Recognition, University of Cambridge, Tennis Court Road, CB2 1QW Cambridge, UK;(6) Present address: Department of Immunology, Leicester Royal Infirmary, LE1 5WW Leicester, UK;(7) Present address: Centre for Mechanisms of Human Toxicity, University of Leicester, Lancaster Road, LE1 9HN Leicester, UK |
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| Abstract: | Two mutants of plastocyanin have been constructed by site-directed mutagenesis in spinach and pea to elucidate the binding and electron transfer properties between plastocyanin and spinach Photosystem 1. The conserved, surface-exposed Tyr-83 has been replaced by phenylalanine and leucine in plastocyanin from both species and the proteins have been expressed in Escherichia coli. The reaction mechanism of electron transfer from plastocyanins to photooxidized P700 in Photosystem 1 has been studied by laser-flash absorption spectroscopy. The experimental data were interpreted with a model involving a rate-limiting conformational change, preceding the intracomplex electron transfer. The pea proteins show an overall facilitated reaction with spinach Photosystem 1, compared to spinach plastocyanins. The changes are small but significant, indicating a more efficient electron transfer within the transient complex. In addition, for the spinach leucine mutant, the equilibrium within the plastocyanin-Photosystem 1 complex is more displaced towards the active conformation than for the corresponding wild-type. Absorption spectra, EPR and reduction potentials for the mutants are similar to those of the corresponding wild-type, although small shifts are observed in the spectra of the Tyr83Leu proteins. Based on these results, it is suggested that Photosystem 1 from spinach is capable of using both pea and spinach plastocyanin as an efficient electron donor and that the former even can stimulate the Photosystem 1 reduction. The origin of the stimulation is discussed in terms of differences in surface-exposed residues. Since the effects of the mutations are small, it can be concluded that electron transfer to Photosystem 1 does not occur via Tyr-83.Abbreviations cyt-
cytochrome
- IPTG-
isopropyl- -d-thiogalactopyranoside
- P,P700-
reaction-center chlorophyll
- Pc-
plastocyanin
- PS 1-
Photosystem 1
- SDS-PAGE-
sodium dodecyl sulfate polyacrylamide gel electrophoresis
- WT-
wild-type |
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| Keywords: | blue copper protein gated electron transfer photosynthesis site-directed mutagenesis |
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