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Structure of the all-trans isomer of pharaonis halorhodopsin in the absence of halide ionsStructure of the all-trans isomer of pharaonis halorhodopsin in the absence of halide ions
Structural highlights
Publication Abstract from PubMedLike other microbial rhodopsins, the light driven chloride pump halorhodopsin from Natronomonas pharaonis (pHR) contains a mixture of all-trans/15-anti and 13-cis/15-syn isomers in the dark adapted state. A recent crystallographic study of the reaction states of pHR has shown that reaction states with 13-cis/15-syn retinal occur in the anion pumping cycle that is initiated by excitation of the all-trans isomer. In this study, we investigated interconversions among different isomeric states of pHR in the absence of chloride ions. The illumination of chloride free pHR with red light caused a large blue shift in the absorption maximum of the retinal visible band. During this "red adaptation", the content of the 11-cis isomer increased significantly, while the molar ratio of the 13-cis isomer to the all-trans isomer remained unchanged. The results suggest that the thermally activated interconversion between the 13-cis and the all-trans isomers is very rapid. Diffraction data from red adapted crystals showed that accommodation of the retinal chromophore with the 11-cis/15-syn configuration was achieved without a large change in the retinal binding pocket. The measurement of absorption kinetics under illumination showed that the 11-cis isomer, with a lambdamax at 565 nm, was generated upon excitation of a red-shifted species (lambdamax = 625 nm) that was present as a minor component in the dark adapted state. It is possible that this red-shifted species mimics an O-like reaction state with 13-cis/15-syn retinal, which was hypothesized to occur at a late stage of the anion pumping cycle. Crystal Structure of the 11-cis Isomer of Pharaonis Halorhodopsin: Structural Constraints on Interconversions among Different Isomeric States.,Chan SK, Kawaguchi H, Kubo H, Murakami M, Ihara K, Maki K, Kouyama T Biochemistry. 2016 Jul 26;55(29):4092-104. doi: 10.1021/acs.biochem.6b00277. Epub, 2016 Jul 13. PMID:27352034[1] From MEDLINE®/PubMed®, a database of the U.S. National Library of Medicine. References
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