Contributed
Molecular mechanism for thermal stability of microbial rhodopsins: A spectroscopic study

Rhodposins, also called retinal proteins, are a class of photoreceptor proteins found in almost all domains of life, including, mammals, bacteria and archaea [1]. These proteins consist of an opsin apoprotein and a bound retinal chromophore that absorbs light for conversion of energy in microbial rhodopsins and initiation of intra-/ intercellular signaling in animal rhodopsin. We have studied the effect of external stimuli-dependent thermal stability of several microbial rhodopsins, including, thermophilic rhodopsin (TR), bacteriorhodopsin (bR), gloebacter rhodopsin (GR) and proteorhodopsin (PR) using UV-visible absorption and circular dichroism (CD) spectroscopies. The protein of our primary interest, TR, prepared from the genome of a thermophilic bacterium Thermus thermophilus, found in a hot spring at around 75 °C, shows resistance to high temperatures. The ultrafast photoisomerization of this protein was found to be independent of temperature to about 70 ^oC [2]. We have revealed that the thermal stability of TR is pH dependent. We also found that the thermal denaturation of the microbial rhodopsins is catalyzed by illumination of the retinal chromophore. We propose that the denaturation process is initiated by the hydrolysis of the retinal-protein protonated Schiff base (PSB) linkage, and the resulting apoprotein is much less stable than the protein that consists of the retinal-protein covalent bond. We further propose that the hydrolysis process is the rate determining step of the thermal denaturation of the studied proteins. Our studies with artificial TR pigments, derived from synthetic retinals analogs, further support this proposal.

References:

1. Ernst, O.P.; Lodowski, D.T. et al., Chem. Rev. 2014, 114, 126.

2. Iyer, E.S.S.; Misra, R. et al., J. Am. Chem. Soc. 2016, 138, 12401.

3. Misra, R.; Sudo, Y.; Sheves, M. (unpublished results)