Abstract
The X-ray structure analysis of Photosystem I (PS I) crystals at 4 A resolution [1] revealed the distances between the three iron-sulfur clusters FX, F1 and F2 functioning on the acceptor side of PS I. There is a general consensus on the assignment of the FX cluster that is bound to the PsaA and PsaB polypeptides forming the PS I core heterodimer. However, the correspondence of the clusters F1 and F2 determined from X-ray data to FA and FB clusters defined by their EPR spectra and cysteine ligands on PsaC, remained controversial. Several recent studies [2,3,4] provide evidence that FA is the cluster proximal to FX, and that FB is the distal cluster that donates electrons to ferredoxin. In this work we provide another argument for this assignment based on analysis of the rates of electron transfer between the iron-sulfur clusters obtained from experimental kinetics of P700+ reduction in isolated PS I preparations.
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