Retinal opsin binding site

In view of the high specificity usually encountered in binding sites, the leniency of opsin in accepting all four diastereomers and enantiomers was surprising. Therefore, adamantyl allenic retinals, 31, were synthesized [87]. The structure of these com-... 

A series of hydroretinals were synthesized and were bound to bovine opsin to form visual pigment analogs, the hydrorhodopsins [92]. Model protonated Schiff bases were also prepared from each of these retinals, and the magnitudes of the opsin shifts were determined. Table 1 summarizes the data obtained and shows the opsin shift for bovine rhodopsin for comparative purposes. Binding studies were also carried out to ascertain that the hydroretinals occupy the same binding site as 11-cK-retinal in rhodopsin. This is required to show the relevance of the data to the natural system. 

The bottom of the binding pocket is in a position equivalent to the binding site of small cationic neurotransmitters (dopamine, adrenaline, serotonin, acetylcholine,etc.) and to the ionone ring of retinal in opsins. 

The all-fran retinal does not fit into the 11-cm retinal binding site on opsin therefore, npon isomerization the trans isomer separates from the protein. At this point an electrical impulse is generated and transmitted to the brain. In the absence of light, enzymes mediate the isomerization of all-fran retinal back to 11-cm retinal, and rhodopsin is regenerated by the binding of the cis isomer to opsin, as described above. With the completion of this step, the vision cycle can begin again. 

In the primary event of visual excitation, light iso-merizes the 11-cis-retinal of the opsin to all-tronj-ret-inal. Since all-tranr-retinal does not fit the binding site for 11-cu-retinal, the opsin molecule beeomes unstable and undergoes a series of conformational changes, followed by hydrolysis of the Schiff s base linkage between all-rranr-retinal and opsin (Fig.). Further events in the process are known with some confi-... 

The retinal binds to the opsin protein by a protonated Schiff base. Through site-directed mutagenesis, the counterion for the Schiff base has been shown to be the glutamate residue at... 

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