Reconstitution of carotenoids into peripheral light harvesting complex 2 of Rhodobacter azotoformans in vitro and its effects on energy transfer

Abstract:

[Objective] To explore the relationships between carotenoid (Car) structure and energy transfer efficiency in peripheral light-harvesting complex 2 (LH2) from anoxygenic phototrophic bacteria (APB). [Methods] Partially carotenoid-deficient LH2 (LC-LH2) was obtained from Rhodobacter azotoformans 134K20 by diphenylamine (DPA) inhibition. Three Cars including spheroidene (SE), rhodopin (RP) and okenone (OK) were obtained from three species of APB by thin layer chromatography and high performance liquid chromatography. The reconstitutions of different Cars into LC-LH2 were performed by using ultrasonic procedures in 10 mmol/L Tris-HCl buffer, pH 8.0, containing 0.1% LDAO. The structure and function of reconstituted LH2 (N-LH2) were studied by using UV-VIS spectra, Raman spectra and fluorescence spectroscopy. [Results] The deficiency of SE in LC-LH2 was about 64.7%. Three Cars with different conjugation length, substituent and polarity could be reconstituted into the partially SE-deficient LC-LH2, and the reconstitution efficiency was in the range from 24.0% to 29.4%, and the reconstitution efficiencies of SE and OK were higher than that of RP. Compared to the intrinsic planner conformation of SE in LC-LH2, the incorporated Cars also adopted a planner conformation in N-LH2. The Car to bacteriochlorophyll (BChl) energy transfer efficiency in different N-LH2 changed in the order of SE-LH2>RP-LH2>OK-LH2. The energy transfer efficiency in N-LH2 had a negative correlation with the conjugated length of Cars, whereas independent of the polarity of Cars. [Conclusion] In N-LH2, Cars bind to the apoproteins with a planner conformation, the conjugated length of Cars still plays a dominant role in controlling the Car to BChl energy transfer efficiency; however, the substituent and polarity of Cars have negligible effects on energy transfer efficiency.