RECONSTITUTION AND EXCHANGE OF QUINONES IN THE A1 SITE OF PHOTOSYSTEM-I - AN ELECTRON-SPIN POLARIZATION ELECTRON-PARAMAGNETIC RESONANCE STUDY

The electron spin polarized (ESP) electron paramagnetic resonance (EPR) signal observed in spinach Photosystem I (PS I) particles was examined in preparations depleted of vitamin K1 by solvent extraction, followed by reconstitution with a series of quinones and quinone analogues. The ESP EPR signal was previously attributed to a radical pair that included vitamin K1- (Rustandi, R.R., et al. (1990) Biochemistry 29, 8030-8032) and, in addition, vitamin K1 was assigned as the secondary acceptor A1 in PS I (Snyder, S.W., et al. (1991) Proc. Natl. Acad. Sci. USA, 88, 9895-98%). The ESP EPR signal was observed in untreated PS I preparations, was not detected in the solvent-extracted PS I samples and was restored upon reconstitution using certain quinones. The ability to restore the ESP EPR signal was dependent upon two properties of the reconstituted acceptor. First, the solution reduction potential of the reconstituted acceptor must be more positive than about -750 mV where the solution reduction potential of vitamin K1 is -710 mV. Second, the structure of the reconstituted acceptor requires either a minimum of two aromatic rings (i.e., naphthoquinone) or a benzoquinone (or larger) derivative substituted with an alkyl tail. A model was developed to describe both the requirements for electron transfer to A1 and also previous results for electron transfer from A1- to the iron-sulfur centers (Biggins, J. (1990) Biochemistry 29, 7259-7264). When untreated PS I preparations were incubated with perdeuterated vitamin K1 (DK1) the endogenous K1 was observed to exchange with DK1. The replacement rate was strongly dependent upon temperature (h-days at 4-degrees-C, min at 37-degrees-C) and upon illumination (min). Naphthoquinones lacking a long alkyl tail were unable to exchange with endogenous vitamin K1. Although no known physiological role exists for vitamin K1 ejection from its A1 binding site, the quinone appears to be somewhat labile. Direct exchange of vitamin K1 with exogenously supplied quinones indicates that the PS I A1 site might be a target for the design of new herbicides in green plants.