Spectral and Thermal Properties of Rhodopsins: Closely Related but not Tightly Coupled

Abstract

Rhodopsins, the primary molecules of vision in all seeing animals, can be activated not only by photon energy (light) but also by thermal energy (heat). Spectral absorbance is evolutionarily tuned by critical residues in the amino acid sequence of the protein part (opsin), which affect the energy needed for 11-cis → all-trans isomerization of the covalently bound chromophore. Already in the 1940’s it was suggested that high sensitivity to long-wavelength light, being indicative of a low energy barrier for activation, should correlate with high probability for thermal activation, and that randomly occurring thermal activations would constitute an irreducible noise setting absolute constraints for the detection of weak light signals. This idea has received strong experimental as well as theoretical support over the last 40 years. Most of the experimental evidence comes from physiological studies of light responses and dark noise in the light-sensitive current of vertebrate photoreceptor cells. Here I review this work, which has firmly established the correlation of spectral sensitivity and thermal noise and led to new theoretical insights. On the other hand, there remains significant freedom for independent adjustment of the two variables by tinkering with the opsin. This is a question of fundamental evolutionary as well as practical interest.