TAILIEUCHUNG - Báo cáo khóa học: Chromophore selectivity in bacterial phytochromes Dissecting the process of chromophore attachment

Bacterial phytochromes (Bphs) are ancestors of the well characterized plant photoreceptors. Whereas plant phyto-chromes perform their photoisomerization exclusively via a covalently bound bilin chromophore, Bphs are variable in their chromophore selection. This is demonstrated in the cyanobacterium CalothrixPCC7601 that expresses two Bphs, CphA and CphB. CphA binds phycocyanobilin (PCB) covalently, whereas CphB, lacking the covalently binding cysteine of the plant phytochromes, carries biliver-din IXa (BV) as the chromophore | Eur. J. Biochem. 271 1117-1126 2004 FEBS 2004 doi Chromophore selectivity in bacterial phytochromes Dissecting the process of chromophore attachment Benjamin Quest1 and Wolfgang Gartner2 1 Max-Planck-Institute for Biochemistry Martinsried Germany 2Max-Planck-Institute for Bioinorganic Chemistry Mulheim an der Ruhr Germany Bacterial phytochromes Bphs are ancestors of the well characterized plant photoreceptors. Whereas plant phytochromes perform their photoisomerization exclusively via a covalently bound bilin chromophore Bphs are variable in their chromophore selection. This is demonstrated in the cyanobacterium Calothrix PCC7601 that expresses two Bphs CphA and CphB. CphA binds phycocyanobilin PCB covalently whereas CphB lacking the covalently binding cysteine of the plant phytochromes carries biliverdin IXa BV as the chromophore. Our experiments elucidate the different modes of chromophore-protein interaction in CphA and CphB and offer a rationale for their chromophore selectivity. The tight binding of BV by CphB prevents PCB from competing for the binding cavity. Even when the chromophore-binding cysteine has been inserted CphB-mutant L266C PCB replaces BV very slowly indicating the tight but not irreversible binding of BV. The mutant CphB L266C showed a redox-sensitivity with respect to its PCB binding mode under reducing conditions the chromoprotein assembly leads to spectra indicative for a covalent binding whereas absence of dithiothreitol or its removal prior to assembly causes spectra indicative for noncovalent binding. Regarding the CphB-type Bphs lacking the covalently binding cysteine our results support the involvement of the succeeding histidine residue in chromophore fixation via a Schiff base-like bond between the bilin A-ring carbonyl and the histidine imidazole group. The assembly process and the stability of the holo-proteins were strongly influenced by the concentration of added imidazole mimicking the histidine

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