Excitation energy transfer kinetics of trimeric, monomeric and subunit-depleted Photosystem I from Synechocystis PCC 6803. Issue 7 (16th April 2021)

• Record Type: Journal Article
• Authors: Akhtar, Parveen; Biswas, Avratanu; Kovács, László; Nelson, Nathan; Lambrev, Petar H.
• Is Part Of: Biochemical journal. Volume 478:Issue 7(2021)
• Page Start: 1333
• Page End: 1346
• Publication Date: 2021-04-16
• Subjects: circular dichroism -- cyanobacteria -- fluorescence resonance energy transfer -- photosynthesis -- time-resolved spectroscopy; Biochemistry -- Periodicals; 572
• DOI: 10.1042/BCJ20210021
• Languages: English
• ISSNs: 0264-6021

Abstract

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 Δ psaL 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.