Functional analysis of photosystem I complex by site-directed mutagenesis

Xu, Wu
Major Professor
Parag R. Chitnis
Committee Member
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Biochemistry, Biophysics and Molecular Biology

Photosystem I, one of the two light driven reaction centers of oxygenic photosynthesis, functions as the light-driven plastocyanin-ferredoxin oxidoreductase in the thylakoid membranes of cyanobacteria and chloroplasts. It is a pigment protein complex with 11--14 proteins and several types of organic (e.g. chlorophyll a) and inorganic energy and electron transfer cofactors (e.g. Fe-S cluster). The objectives of this dissertation are: (i) to demonstrate that the spectroscopic and kinetic properties of photosystem I cofactors are determined by the protein environments, (ii) to determine whether the electron transfer in photosystem I is unidirectional or bidirectional, (iii) to identify which aromatic residues in the helix ten of PsaB are involved in electron transfer from plastocyanin to P700.;P700, accessory chlorophyll and A0 are chemically the same as other chlorophyll a molecules, however they have distinct spectroscopic properties. Similarly, phylloquinones that function as A 1 have far lower redox potentials than the phylloquinones in solution. Site-directed mutagenesis analysis showed that the spectroscopic properties of P700, accessory chlorophyll, A0 and A1 were altered when the proteins were changed by mutations. Therefore, the photosystem I proteins are responsible for the precise arrangement of cofactors and determine their spectroscopic properties. There exist two branches for the electron transfer chains in photosystem I; it is unclear that both function or only one functions. To address this question, equivalent pairs of mutations were generated and characterized. Optical kinetics results indicate that the structural changes in both branches influence electron transfer rates. However, transient electron paramagnetic resonance spectroscopy, which cannot detect kinetic events at less than 50 ns time scale could only detect changes when the PsaA protein was altered. These results revealed the functional importance of two branches of electron transfer. The helix ten of PsaB subunit contains several aromatic residues in the region between P700 and the luminal side of thylakoid membrane. Cysteine-scanning mutagenesis of these residues indicated that many of these residues are critical in structural integrity of photosystem I and a phenylalanyl residue at position 647 could be involved in electron transfer from plastocyanin to P700.