Abstract
Two phylloquinone molecules (A1), one being predominantly coordinated by PsaA subunit residues (A1A) the other by those of PsaB (A1B), act as intermediates in the two parallel electron transfer chains of Photosystem I. The oxidation kinetics of the two phyllosemiquinones by the iron-sulfur cluster FX differ by approximately one order of magnitude, with being oxidized in about 200 ns and in about 20 ns. These differences are generally explained in terms of asymmetries in the driving force for FX reduction on the two electron transfer chains. Site directed mutations of conserved amino acids composing the A1 binding site have been engineered on both reaction center subunits, and proved to affect selectively the oxidation lifetime of either , for PsaA mutants, or , for PsaB mutants. The mutation effects are here critically reviewed, also by novel modeling simulations employing the tunneling formalism to estimate the electron transfer rates. Three main classes of mutation effects are in particular addressed: (i) those leading to an acceleration, (ii) those leading to a moderated slowing (~5-folds), and (iii) those leading to a severe slowing (>20-folds) of the kinetics. The effect of specific amino acid perturbations contributing to the poising of the phylloquinones redox potential and, in turn, to PSI functionality, is discussed.
Highlights
Dutton (1992) and Moser and Dutton (1996)
In the following paragraphs the discussion will be focused on the kinetics of on the driving force associated to these
It is arguable that this is a reasonable approach, because of the uncertainties on some of the parameters already addressed and because, whereas the modeling provides information on the population evolutions, the experimentally measured kinetics depend on other properties of the cofactors
Summary
Photosystem I (PSI) is a key component of both the linear and the cyclic electron transport chains of oxygenic photosynthesis. Electron transfer (ET) reactions take place in a protein-cofactor super-complex known as the core, which is overall well conserved throughout evolution. Most of the cofactors involved both in light harvesting and ET reactions are bound by the heterodimer composed of the PsaA and PsaB protein subunits (Jordan et al, 2001; Qin et al, 2015; Mazor et al, 2017). Phylloquinone Redox Tuning in PSI involved in ET reactions are related to a C2 (mirror) symmetry (Figure 1) with respect to an axis perpendicular to the putative membrane plane (Jordan et al, 2001; Qin et al, 2015; Mazor et al., 2017), which is a common structural motif of photosynthetic reaction centers. FX represents a point of convergence of the two active ET branches, as it is shared by both, and so are the terminal acceptors FA and FB (Figure 1)
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