Structural and Functional Consequences of the Weak Binding of Chlorin e6 to Bovine Rhodopsin

The chlorophyll-derivative chlorin e6 (Ce6) identified in the retinas of deep-sea ocean fish is proposed to play a functional role in red bioluminescence detection. Fluorescence and ¹ H NMR spectroscopy studies with the bovine dim-light photoreceptor, rhodopsin, indicate that Ce6 weakly binds to it with microm affinity. Absorbance spectra prove that red light sensitivity enhancement is not brought about by a shift in the absorbance maximum of rhodopsin. ¹⁹ F NMR experiments with samples where ¹⁹ F labels are either placed at the cytoplasmic binding site or incorporated as fluorinated retinal indicate that the cytoplasmic domain is highly perturbed by binding, while little to no changes are detected near the retinal. Binding of Ce6 also inhibits G-protein activation. Chemical shift changes in ¹ H-¹⁵ N NMR spectroscopy of ¹⁵ N-Trp labeled bovine rhodopsin reveal that Ce6 binding perturbs the entire structure. These results provide experimental evidence that Ce6 is an allosteric modulator of rhodopsin.