While vitamin A, as retinoic acid, has important hormonal actions (which are not discussed here), its best known function is in vision. Within photoreceptor cells of the retina, and even in certain bacteria, vitamin A aldehyde (retinal, Fig. 1) forms a Schiff base with specific lysine side chains of the light receptor proteins. Two of the best known of these receptors are rhodopsin, the pigment present in the rod cells of the mammalian retina, and bacteriorhodopsin, the light receptor of the purple membranes of certain salt-tolerant bacteria. In both of these cases, the protein consists of a similar bundle of seven connected helical segments that pass through a membrane. The retinal Schiff base is inside the bundle, held rigidly in a small “box.” In both cases, a particular stereoisomer of retinal is present. In bacteriorhodopsin it is the all-trans isomer pictured in Fig. 1, but in rhodopsin it is the 11-cis isomer shown in Fig. 19. Upon absorption of light, this isomer is converted almost instantaneously into the all-trans form as shown in Fig. 19. The all-trans retinal then leaves the photoreceptor and is replaced with a new molecule of the 11-cis isomer before the photoreceptor can act again. In bacteriorhodopsin, absorption of light converts the all-trans retinal into the 13-cis isomer within about three trillionths of a second. In both cases, the change in shape of the retinal upon absorption of light induces a small alteration in the geometry and chemical properties of the photoreceptor protein that surrounds the light-absorbing molecule. This is enough to start a chain of signaling events in the retina that leads to a nerve impulse being sent to the brain. In the bacteria, the light absorption is used in a different way to pump a proton from the inside of the cell across the membrane to the outside. The resulting gradient of hydrogen ions (positive charges) across the membrane represents a store of protonic energy similar to that in an electrical condenser. It is used by these cells as a source of energy.
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