Evidence for two functional phylloquinones in Photosystem I from Chlamydomonas reinhardtii.

Abstract

We have been using site-directed mutagenesis in conjunction with spectroscopic techniques to probe structure-function relationships in the Photosytem I reaction centre of the green alga Chlamydomonas reinhardtii. We recently reported [1] that substitution of the conserved tryptophan W693/H/L on the PsaA polypeptide slowed down or abolished forward electron transfer from the phylloquinone to the FeS centre Fx, as monitored by the decay of the Electron Spin Polarised signal (ESP) arising from the P700+ /A1- radical pair at 260K. We concluded that the photoaccumulated phyllosemiquinone EPR signal, and the ESP signal, monitored the phylloquinone on the PsaA branch of electron transfer. We have recently used Electron Nuclear Double Resonance spectroscopy (ENDOR) to demonstrate that we can photoaccumulate under certain conditions the phyllosemiquinone on the PsaB branch of electron transfer, and that there are differences in the electronic structure of the two phyllosemiquinones. By monitoring the decay of the ESP signal at 100K where it reflects the properties of the P700.+/A1.- radical pair we have obtained evidence for forward electron transfer from P700 to A1 on both the PsaA and PsaB sides of the reaction centre. [1] S. Purton, D. Stevens, I. P. Muhiuddin, M. C. W. Evans, S. Carter, S. E. J. Rigby, and P. Heathcote (2001) Biochemistry 40, 2167-2175.