Abstract
Retinal proteins play significant roles in light-induced protons/ions transport across the cell membrane. A recent studied retinal protein, gloeobacter rhodopsin (gR), functions as a proton pump, and binds the carotenoid salinixanthin (sal) in addition to the retinal chromophore. We have studied the interactions between the two chromophores as reflected in the circular dichroism (CD) spectrum of gR complex. gR exhibits a weak CD spectrum but following binding of sal, it exhibits a significant enhancement of the CD bands. To examine the CD origin, we have substituted the retinal chromophore of gR by synthetic retinal analogues, and have concluded that the CD bands originated from excitonic interaction between sal and the retinal chromophore as well as the sal chirality induced by binding to the protein. Temperature increase significantly affected the CD spectra, due to vanishing of excitonic coupling. A similar phenomenon of excitonic interaction lose between chromophores was recently reported for a photosynthetic pigment-protein complex (Nature Commmun, 9, 2018, 99). We propose that the excitonic interaction in gR is weaker due to protein conformational alterations. The excitonic interaction is further diminished following reduction of the retinal protonated Schiff base double bond. Furthermore, the intact structure of the retinal ring is necessary for obtaining the excitonic interaction.
Highlights
Retinal proteins play significant roles in light-induced protons/ions transport across the cell membrane
We have studied the circular dichroism (CD) and corresponding absorption spectroscopies of gloeobacter rhodopsin (gR) and its artificial pigments derived from synthetic retinal analogues following sal binding, to shed light on the origin of the CD spectra in the visible region, and its enhancement by sal binding
The gR CD spectrum bands in the 300–700 nm region are attributed to the retinal induced chirality, and/or to excitonic interaction between retinal chromophores located in different proteins which are arranged in pentamers as previously reported[9,13,39]
Summary
Retinal proteins play significant roles in light-induced protons/ions transport across the cell membrane. A recent studied retinal protein, gloeobacter rhodopsin (gR), functions as a proton pump, and binds the carotenoid salinixanthin (sal) in addition to the retinal chromophore. Abbreviations gR Gloeobacter rhodopsin Apo-gR/apo Apo-protein of gR Sal Salinixanthin CD Circular Dichroism xR Xanthorhodopsin bR Bacteriorhodopsin wt-bR Wild type bacteriorhodopsin DDM N -Dodecyl β-d-maltoside SDS Sodium dodecyl sulphate EC Excitonic coupling CE Cotton effect NaBH4 Sodium borohydride PBS Protonated Schiff base. The CD spectrum of native xR, which contains two chromophores (retinal and sal), exhibits sharp positive bands at 513, 478, and 455 nm, and a negative band at 530 nm, which formed a biphasic shaped spectrum[18,19] These CD bands arise only when the carotenoid sal is bound to the protein and forms the protein-carotenoid complex.
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