Effects of selective destruction of iron-sulfur center B on electron transfer and charge recombination in Photosystem I

Abstract

Incubation of spinach thylakoids with HgCl2 selectively destroys Fe-S center B (FB). The function of electron acceptors in FB-less PS I particles was studied by following the decay kinetics of P700(+) at room temperature after multiple flash excitation in the absence of a terminal electron acceptor. In untreated particles, the decay kinetics of the signal after the first and the second flashes were very similar (t 1/2∼2.5 ms), and were principally determined by the concentration of the artificial electron donor added. The decay after the third flash was fast (t 1/2∼0.25 ms). In FB-less particles, although the decay after the first flash was slow, fast decay was observed already after the second flash. We conclude that in FB-less particles, electron transfer can proceed normally at room temperature from FX to FA and that the charge recombination between P700(+) and FX (-)/A1 (-) predominated after the second excitation. The rate of this recombination process is not significantly affected by the destruction of FB. Even in the presence of 60% glycerol, FB-less particles can transfer electrons to FA at room temperature as efficiently as untreated particles.

Electrogenic light reactions in photosystem I: resolution of electron-transfer rates between the iron-sulfur centers.

“…The dominating phase in the optical signals has a half-life that varies from 20 to 70 ms in different reports, and it has been assigned to a charge recombination between P700+ and FA (17,18). A component with a >200-ms halftime (amplitude, 27%) has been described for a PSI preparation similar to the one used by us (17). It was ascribed to the reaction between reduced Fe-S centers and a diffusable electron acceptor-i.e., one that is not part of PSI.…”

Photosystem I of Synechococcus elongatus at 4 Å resolution: comprehensive structure analysis

“…Selective destruction of F B by urea-ferricyanide (Golbeck & Warden, 1982) and mercurials (Kojima et al, 1987;Sakurai et al, 1991;He & Malkin, 1994), signi®cantly reduces the rate of NADP reduction while reconstitution of F B largely restores the rate of NADP reduction (Jung et al, 1995). Thus F B appears more accessible to denaturing chemicals and, in contradiction to Mannan et al (1996), F B would after all be essential for reduction of NADP .…”

Modified Ligands to FA and FB in Photosystem I

“…While serine, an amino acid with a potential oxygen ligand, might be expected to support a cubane cluster, this result was quite unexpected for alanine, an amino acid that contains a methyl side group. Further complicating the issue was that the C14A PsaC and C51A PsaC as well as C14S PsaC (17)(18)(19)) is a notable exception in which the heavy metal may cross-link the cysteine thiolates in the F B site thereby retaining three-dimensional structure.) Significantly, S ϭ 3/2, [4Fe-4S] clusters were tentatively identified in unbound C14S PsaC , C51S PsaC , C14A PsaC , and C51A PsaC proteins at the modified site.…”