Secrets of Carotenoid Binding (original) (raw)

Secrets of Carotenoid Binding the same manner as the naturally occurring spheroidene in wild-type bRCs. Triplet-triplet energy transfer from P to the carotenoid occurs through an accessory bacteriochlorophyll inter-In this issue of Structure, researchers reveal new crysmediate, BChl B. In order for triplet-triplet energy transfer tal structures of the bacterial reaction center of Rb. between two molecules to occur, there must be van sphaeroides R-26.1 containing either no carotenoid or der Waals contact and they must have appropriately natural or synthetic carotenoid (Roszak et al., 2004). matched excited state energy levels. In this study by These structures give insight in to the mechanism of Roszak et al. (2004), in each of the two carotenoid reconcarotenoid binding. stituted bRC X-ray crystal structures, the carotenoid is in van der Waals contact with BChl B. In fact, the position Here, Frank and colleagues (Roszak et al., 2004) report of the 15-15Ј-cis bond of the natural and reconstituted three new X-ray crystallography structures of the bactecarotenoid relative to the BChl B molecule are nearly rial reaction center (bRC) from the carotenoid-less muidentical. In addition to the structural requirement, the tant of Rhodobacter sphaeroides R-26.1 containing eicarotenoid triplet state must be lower in energy than ther no carotenoid or reconstituted with the natural the triplet state of BChl B in order for energy transfer to carotenoid spheroidene or a synthetic carotenoid, 3,4occur. In another study from the Frank laboratory, it was dihydrospheroidene. The structures suggest that acshown that the triplet state on P is not quenched in bRCs cess to the binding site is constrained by the rotational reconstituted with 3,4-dihydrospheroidene (Farhoosh et motion of a phenylalanine residue. Based on this obseral., 1997). Because energy in the excited state flows vation, the authors postulate a mechanism for carotfrom the highest to the lowest energy level, the authors enoid binding which selects for the correct orientation had hypothesized the excited state of 3,4-dihydrospheof the molecule in the bRC. roidene must be higher than the excited state of BChl B. bRCs normally bind a single carotenoid molecule, However, a possibility remained that 3,4-dihydrosphespheroidene. One of the important roles of the carotroidene is not bound to the bRC in the same manner enoid in the bacterial reaction center is to quench triplet as spheroidene. This possibility can now be excluded states that can form on the primary electron donor P because it can be seen in the structure of the bRC as a result of charge recombination reactions (Krinsky, reconstituted with 3,4-dihydrospheroidene that this ca-1971). These bacteriochlorophyll triplet states react with rotenoid does indeed bind in the same position as sphemolecular oxygen to form excited singlet state oxygen, roidene (Roszak et al., 2004). Therefore, this result supwhich can cause oxidative damage to the cell. The carotports the idea that the relatively higher excited state enoid protects the reaction center from oxidative damenergy level of 3,4-dihydrospheroidene compared to age by preventing singlet oxygen formation.