Spectral Analysis on the Structure and Functional Role of β-Carotene in Cytochrome b6f Complex,Spectral Analysis on the Structure and Functional Role of β-Carotene in Cytochrome b6f Complex

Abstract

The cytochrome b6/f complex (Cyt b6/f) was isolated and purified from spinach chloroplasts by an improved method. The pure Cyt b6/f preparation was composed of four major subunits, Cyt f (33KD), Cytb6 (23KD), Rieske Fe-S protein (19KD) and subunit IV (17KD). Cytb6 to Cyt f ratio of the preparation equals 2. After the Cyt b6/f preparation was reconstituted with lipids isolated from chloroplast membrane of spinach, its electron transfer activity (PQ2H2 ? PC) was approximately 80 electrons per Cyt f per second. In this work, the molecular configuration and function of b -carotene (b -Car) in the Cyt b6/f preparation were investigated by HPLC, absorption, CD, resonance Raman and fluorescence excitation spectra. The experimental results as follows were obtained: 1.The purified Cyt b6/f preparation contains about 1.5 chlorophyll a (Chl a) and 1 carotene molecule per Cyt f. 2.The b -Car in the Cyt b6/f preparation takes a 9-cis configuration and remain in an asymmetric protein environment. 3.The light energy absorbed by the b -Car can not efficiently transfer to Chl a in the Cyt b6/f while the protective function of the b -Car to photodamage of Chl a induced by the strong light is more pronounced.