Both chlorophylls a and d are essential for the photochemistry in photosystem II of the cyanobacteria, Acaryochloris marina

Abstract

We have measured the flash-induced absorbance difference spectrum attributed to the formation of the secondary radical pair, P(+)Q(-), between 270 nm and 1000 nm at 77 K in photosystem II of the chlorophyll d containing cyanobacterium, Acaryochloris marina. Despite the high level of chlorophyll d present, the flash-induced absorption difference spectrum of an approximately 2 ms decay component shows a number of features which are typical of the difference spectrum seen in oxygenic photosynthetic organisms containing no chlorophyll d. The spectral shape in the near-UV indicates that a plastoquinone is the secondary acceptor molecule (Q(A)). The strong C-550 change at 543 nm confirms previous reports that pheophytin a is the primary electron acceptor. The bleach at 435 nm and increase in absorption at 820 nm indicates that the positive charge is stabilized on a chlorophyll a molecule. In addition a strong electrochromic band shift, centred at 723 nm, has been observed. It is assigned to a shift of the Qy band of the neighbouring accessory chlorophyll d, Chl(D1). It seems highly likely that it accepts excitation energy from the chlorophyll d containing antenna. We therefore propose that primary charge separation is initiated from this chlorophyll d molecule and functions as the primary electron donor. Despite its lower excited state energy (0.1 V less), as compared to chlorophyll a, this chlorophyll d molecule is capable of driving the plastoquinone oxidoreductase activity of photosystem II. However, chlorophyll a is used to stabilize the positive charge and ultimately to drive water oxidation.

Redox potentials of primary electron acceptor quinone molecule (Q A ) − and conserved energetics of photosystem II in cyanobacteria with chlorophyll a and chlorophyll d

“…In addition, some other studies using partially purified samples support our view (28,29). However, for another study using partially purified samples, a different component (Chl a and Chl d heterodimer) was proposed for the special pair (30). Additionally, in an earlier study, Shevela et al (31) proposed that the redox potentials of the wateroxidizing complex in its different S i states (donor side of PS II) are almost the same for A. marina cells and spinach thylakoids.…”

Primary Photophysical Processes in Photosystem II: Bridging the Gap between Crystal Structure and Optical Spectra

“…Calculations of the TÀS spectra of D1D2cytb559 complexes and comparison with experimental data revealed that the triplet is localized on Chl D1 . [32] Direct evidence was provided by Schlodder et al [60] from TÀS spectra on mutants ( Figure 5) of PS-II core complexes from Synechocystis sp. PCC 6803.…”

“…This time con- Figure 12. The experimental P + Q A À ÀPQ A difference spectrum [60] at 77 K is compared with calculations [70] assuming a Chla homodimer or a Chla-Chld heterodimer in the "special pair" of A. marina. Comparison between experimental fluorescence decay of PS-II core complexes at 300 K, measured after excitation with a 100 fs optical pulse that was centered at 663 nm (*), [74] in comparison to the "exact" calculations (c) and calculations that assume fast exciton equilibration in compartments consisting of CP43-Chlz D1 , CP47-Chlz D2 , and the six core pigments of the RC (a) or fast exciton equilibration in the whole PS-II core complex (g).…”

The Thermodynamics and Kinetics of Electron Transfer between Cytochrome b6f and Photosystem I in the Chlorophyll d-dominated Cyanobacterium, Acaryochloris marina

“…However, in the case of A. marina, the redshifted oxidation spectrum of P 740 (8,38,39) strongly overlaps with the oxidation spectrum of plastocyanin (the redox changes of which are commonly followed in the 810 -870 nm region), making the direct observation of plastocyanin redox changes using optical spectroscopy difficult.…”