1. Excitation energy transfer kinetics of trimeric, monomeric and subunit-depleted Photosystem I from Synechocystis PCC 6803
- Author
-
Avratanu Biswas, László Kovács, Petar H. Lambrev, Parveen Akhtar, and Nathan Nelson
- Subjects
Chlorophyll ,0106 biological sciences ,Circular dichroism ,Kinetics ,Quantum yield ,Photosynthesis ,Photochemistry ,Photosystem I ,Thylakoids ,01 natural sciences ,Biochemistry ,03 medical and health sciences ,Molecular Biology ,030304 developmental biology ,Photosystem ,0303 health sciences ,Photosystem I Protein Complex ,biology ,Chemistry ,Synechocystis ,Cell Biology ,biology.organism_classification ,Förster resonance energy transfer ,Energy Transfer ,Protein Multimerization ,010606 plant biology & botany - Abstract
Photosystem I is the most efficient photosynthetic enzyme with structure and composition highly conserved among all oxygenic phototrophs. Cyanobacterial Photosystem I is typically associated into trimers for reasons that are still debated. Almost universally, Photosystem I contains a number of long-wavelength-absorbing ‘red’ chlorophylls (Chls), that have a sizeable effect on the excitation energy transfer and trapping. Here we present spectroscopic comparison of trimeric Photosystem I from Synechocystis PCC 6803 with a monomeric complex from the ΔpsaL mutant and a ‘minimal’ monomeric complex ΔFIJL, containing only subunits A, B, C, D, E, K and M. The quantum yield of photochemistry at room temperature was the same in all complexes, demonstrating the functional robustness of this photosystem. The monomeric complexes had a reduced far-red absorption and emission equivalent to the loss of 1.5–2 red Chls emitting at 710–715 nm, whereas the longest-wavelength emission at 722 nm was not affected. The picosecond fluorescence kinetics at 77 K showed spectrally and kinetically distinct red Chls in all complexes and equilibration times of up to 50 ps. We found that the red Chls are not irreversible traps at 77 K but can still transfer excitations to the reaction centre, especially in the trimeric complexes. Structure-based Förster energy transfer calculations support the assignment of the lowest-energy state to the Chl pair B37/B38 and the trimer-specific red Chl emission to Chls A32/B7 located at the monomer–monomer interface. These intermediate-energy red Chls facilitate energy migration from the lowest-energy states to the reaction centre.
- Published
- 2021