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Evidence for two active branches for electron transfer in photosystem I
M Guergova-Kuras1, B Boudreaux, A Joliot
1Institut de Biologie Physico-Chimique, Centre National de la Recherche Scientifique, UPR 1261, 13 Rue Pierre et Marie Curie, 75005 Paris, France.
Summary
Photosystem I (PS I) reaction centers have two active electron transfer branches. Mutations reveal distinct kinetic effects, proving both pathways are functional in photosynthesis.
Area of Science:
- Photosynthesis research
- Molecular biology
- Biochemistry
Background:
- Photosynthetic reaction centers share common structural features with two branches.
- Type II centers direct electron transfer down one branch, using quinone as a terminal acceptor.
- Photosystem I (PS I) was thought to have unidirectional electron transfer, similar to type II centers.
Purpose of the Study:
- To investigate the electron transfer kinetics in mutant PS I reaction centers.
- To determine if both electron transfer branches in PS I are active.
- To challenge the prevailing assumption of unidirectional electron flow in PS I.
Main Methods:
- In vivo kinetic analysis of electron transfer from quinone in mutant PS I.
- Site-directed mutagenesis of residues near quinone binding sites in PS I branches.
- Comparison of kinetic components affected by mutations in each branch.
Main Results:
- Electron transfer from quinone in PS I exhibited two kinetic components.
- Mutation in one branch specifically altered the faster kinetic component.
- Mutation in the homologous site of the other branch specifically affected the slower kinetic component.
Conclusions:
- Both electron transfer branches in Photosystem I are active.
- PS I does not share the strict unidirectionality of electron flow seen in type II centers.
- The study provides crucial insights into the functional mechanisms of PS I.