Studies on well coupled Photosystem I-enriched subchloroplast vesicles. Content and redox properties of electron-transfer components

Abstract

Stable and well coupled Photosystem (PS) I-enriched vesicles, mainly derived from the chloroplast stroma lamellae, have been obtained by mild digitonin treatment of spinach chloroplasts. Optimal conditions for chloroplast solubilization are established at a digitonin/chlorophyll ratio of 1 ( w w ) and a chlorophyll concentration of 0.2 mM, resulting in little loss of native components. In particular, plastocyanin is easily released at higher digitonin/chlorophyll ratios. On the basis of chlorophyll content, the vesicles show a 2-fold enrichment in ATPase, chlorophyll-protein Complex I, P-700, plastocyanin and ribulose-1,5-bisphosphate carboxylase as compared to chloroplasts, in line with the increased activities of cyclic photophosphorylation and PS I-associated electron transfer as shown previously (Peters, A.L.J., Dokter, P., Kooij, T. and Kraayenhof, R. (1981) in Photosynthesis I (Akoyunoglou, G., ed.), pp. 691–700, Balaban International Science Services, Philadelphia). The vesicles have a low content of the light-harvesting chlorophyll-protein complex and show no PS II-associated electron transfer. Characterization of cytochromes in PS I-enriched vesicles and chloroplasts at 25°C and 77 K is performed using an analytical method combining potentiometric analysis and spectrum deconvolution. In PS I-enriched vesicles three cytochromes are distinguished: c-554 ( E′ 0 = 335 mV), b-559 LP ( E′ 0 = 32 mV) and b-563 ( E′ 0 = − 123 mV); no b-559 HP is present (LP, low-potential; HP, high-potential). Comparative data from PS I vesicles and chloroplasts are consistent with an even distribution of the cytochrome b-563- cytochrome c-554 redox complex in the lateral plane of exposed and appressed thylakoid membranes, an exclusive location of plastocyanin in the exposed membranes and a dominant location of plastoquinone in the appressed membranes. The results are discussed in view of the lateral heterogeneity of redox components in chloroplast membranes.