Functional integration of beta-carotenes and xanthophylls in photoprotection of PS II of thylakoid membrane: a theoretical approach

Abstract

The existing data do not suggest any mechanism of interaction between b-carotene in reaction centre (RC) and xanthophylls in light harvesting Chl-binding proteins (LHC) in photoprotecting chloroplasts. Out of two b -cars in RCII, one has been modeled after the position of b-carcis of bacterial reaction centre and the second b-car in D1 protein with all-trans form (b -cartrans). (1) The existence of a fast futile cycle around RCII with turnover time less than 50m s has tempted us to suggest that b-cartrans along with the accessory Chl a can initiate such a cycle and annul the effect of long lived P680+ during photoinhibition. (2) Formation of a molecular quenching complex consisting of zeaxanthin, Chl a and LHC is worked out. The possible excitonic coupling between b-carcis at RCII and the quenching complex is suggested. The excitation energy from 3b -carcis may be chanelled to the quenching complex at interface of RCII and LHCII. The b -carstrans present in core antenna (CP43, CP47) may mediate this triplet energy transfer from RCII to the quenching complex in the following way : RCb -car cis (T1)+ CPb -cartrans (S0) ® RCb-carcis (S0)+CPb -cartrans (T1) CPb -cartrans(T1)+ Zea(S0)® CPb -cartrans (S0)+Zea(T1) Thermal Zea (TI) Zea(S0)+Heat Relaxation The model explains that during photoinhibitory condition, the carotenoid pigments from RC II to the LHC II from a pigment array suggesting a continuum of carotenoids, responsible for photoprotection of PS II. The model also explains that zeaxanthin, in addition to nonphotochemical quenching of Chl a fluorescence, operates in another channel for photoprotection through b-car.