Retinal interconversions

Figure 15.9 (a) The ds- and trans-retinal interconversions in the detection of light Within the photoreceptor cell, light is detected by the conversion of c/s-retinal to trans-retinal, components of the light-sensitive pigment rhodopsin. This apparently small chemical change is sufficient for trans-retinal to dissociate from rhodopsin. (b) The cis/trans q/cte. To continue the process, trans-retinal must be converted back to c/s-retinal. 

A number of geometric isomers of retinol exist because of the possible cis-trans configurations around the double bonds in the side chain. Fish liver oils contain mixtures of the stereoisomers synthetic retinol is the all-trans isomer. Interconversion between isomers readily takes place in the body. In the visual cycle, the reaction between retinal (vitamin A aldehyde) and opsin to form rhodopsin only occurs with the 11 -cis isomer. 

Absorption of a photon by the purple iill-trans retinal chro-mophore (with a single broad absorption band with a maximum at 568 nm) initiates the reaction sequence BR-/zv K L Ml M2 N -o- N - O BR (7, 8), where each state and substate is well defined by spectroscopic and crystallographic means. Although a kinetic scheme that rigorously fits all data into a linear sequence has not yet been produced, the proton transport mechanism can be understood by the molecular properties of the intermediate states and by their interconversions. 

Figure 2 summarizes the interconversions of the forms of vitamin A and retinal believed to occur in the eye. ... 

This type of isomerization still seems to be of limited importance. For example, a mixture of unseparable valence-bond isomers 75 and 76 was obtained by thermolysis of an appropriate cyclobutene precursor (92TL883). Another 2//-pyran intermediate postulated in the Z-E interconversion of stereoisomeric retinals could be neither isolated nor identified (92JOC1110). 

The retina of the eye contains cone cells and rod cells. The cone cells are responsible for color vision and for vision in bright light. The rod cells are responsible for vision in dim light. In rod cells, vitamin A is oxidized to an aldehyde and the trans double bond at C-11 is isomerized to a cis double bond. The mechanism for the enzyme-catalyzed interconversion of cis and trans double bonds is discussed in Section 18.15. The protein opsin uses a lysine side chain (Lys 216) to form an imine with (llZ)-retinal, resulting in a complex known as rhodopsin. When rhodopsin... 

The chemical changes that occur when light impinges on the retina of the eye involve several of the phenomena that we have studied. Central to an understanding of the visual process at the molecular level are two phenomena in particular the absorption of light by conjugated polyenes and the interconversion of cis-trans isomers. The conjugated polyene, derived from a compound called retinal, is a part of a molecule called rhodopsin. 

Our ability to see depends in part on an interconversion of cis and trans isomers that takes place in our eyes. A protein called opsin binds to cA-retinal (formed from vitamin A) in photoreceptor cells (called rod cells) in the retina to form rhodopsin. When rhodop-sin absorbs light, a double bond interconverts between the cis and trans configurations, triggering a nerve impulse that plays an important role in vision. fran -Retinal is then released from opsin. trawi-Retinal isomerizes back to cA-retinal and another cycle begins. To trigger the nerve impulse, a group of about 500 rod cells must register five to seven rhodopsin isomerizations per cell within a few tenths of a second. 

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