Retinal isomerase

There ensues a series of dark reactions or conformational changes that have the effect of greatly activating the imine linkage of the all-frans-rhodopsin towards hydrolysis. On hydrolysis, all-frawj-retina] is released and is unable to recombine with opsin until it is reconverted to the 11-cis isomer. The trans-to-cis rearrangement is a thermal rather than a photochemical reaction and is catalyzed by the enzyme retinal isomerase. The cycle of reactions is summarized in Figure 28-13. 

Hubbard " found in the retina an enzjnne, retinal isomerase, which changes all trans- to 11-CTS-retinal. In the dark, retinal isomerase converts all trans- or 11-m-retinal to a mixture of about 5% 11-as and 95% all trans. In the light, the rate of isomerization of all trans is in-... 

Retinal isomerase converts dX -trans retinal to 1 -cis retinal. 

Vitamin A is the isoprenoid alcohol retinol, which is required for a number of body processes including vision, growth, maintenance of mucous membranes, reproduction and proper growth of the cartilage matrix upon which bone is deposited. Of these, the molecular basis of its role in the visual cycle is best understood. In addition to the alcohol retinol, both retinal and retinoic acid (Fig. 2.4) also act to restore some of the deficiency symptoms. In rod vision, retinol is oxidised to retinal. 11-ds-Retinal combines with the protein opsin to form rhodopsin. After the absorption of a photon, rhodopsin undergoes a series of changes, eventually dissociating to opsin and all-fraMS-retinal. This last compound is converted back to 11-ds-retinal by retinal isomerase (EC.5.2.1.3) Retinoic acid is able to replace retinol in all functions except the visual cycle and reproduction. 

If this were all that happened, we would be able to see for only a few moments, because all of the 11 -c/s-retinal present in the rod cells would be quickly consumed. Fortunately, the enzyme retinal isomerase, in the presence of light, converts the traws-retinal back to the 11 -cis isomer so that the cycle can be repeated. Calcium ions in the cell and its membrane control how fast the visual system recovers after exposure to light. They also mediate the way in which cells adapt to various... 

Another set of significant photochemical reactions in human biochemistry is contained in the chemistry of vision. These reactions involve vitamin A (retinol), which is a C20 compound belonging to a class of compounds known as diterpenes. These compounds are molecules formally constructed by the biopolymerization of four isoprene, CH2=C(CH3)—CH=CH2, molecules. Retinol (an all-trans pentaene) is first oxidized via liver enzymes (biological catalysts) to vitamin A aldehyde (frans-retinal). The frans-retinal, which is present in the light-sensitive cells (the retina) of the eye, undergoes further enzymatic transformation (retinal isomerase) to give cfs-retinal (a second form of vitamin A aldehyde) in which one of the double bonds of the aU-trans compound is isomerized. 

Many polyenes are important in biological processes, and as we might imagine, polyenes that absorb in the dsible spectra are important in human dsion. Vitamin A is trans-xtX no, for example (Fig. 12.33). We have two en mes, one of which, retinol dehydrogenase, oxidizes /rawr-retinol to /rawr-retinal and another, retinal isomerase, which isomerizes one double bond of /rawr-retinal to produce m-retinal. 

---