Biochemistry and Molecular Biology - Belyasova N.A. 2002

Metabolism. Processes Leading to Energy Storage
Capture of Light Energy by Biomolecules
Visual Perception

The visual Organs are highly developed receptors that function on The basis of photochemical reactions of light-sensitive pigments (photoreceptors). To capture the maximum amount of light, the light-sensitive Cells of the eye (retinal rods and cones) have a specialized Structure: their outer segments are literally packed with specialized membranes that house the receptor molecules (Fig. 13.8). Each such Cell is connected via an intermediary to the axons of the Optic nerve, where membrane-bound nerve impulses are triggered through cascade reactions in response to the photochemical transformations of the photoreceptors. Thus, light of varying wavelengths, reflected by objects at different distances, is perceived by THE EYE AND converted into nerve impulses. Via the optic nerve axons, these impulses travel to a dedicated region of the Brain—the visual cortex—where they are interpreted to generate an image.

In rods (highly sensitive retinal cells responsible for black-and-white Vision), approximately 500 parallel stacked discs with a diameter of ~ 2 µm are located in their outer segment facing the external environment (Fig. 13.8). Each disc is formed by a pair of membranes separated by a narrow space. The membranes consist of 60% protein, the bulk of which is rhodopsin, a light-sensitive chromoprotein. The chromophore of rhodopsin is 11-cis-retinal, which is attached to the protein moiety, opsin, via the e-amino group of a Lysine residue (Fig. 13.8). Rhodopsin is present in the disc membranes as a transmembrane protein.

When a rhodopsin molecule absorbs a quantum of light, 11-cis-retinal isomerizes into all-trans-retinal (Fig. 13.8). As a result of this photochemical reaction, rhodopsin transitions into its active form and stimulates the G protein circulating in the rod Cytoplasm. In turn, the G protein triggers a signal Transduction cascade that ultimately generates a Nerve Impulse in the nerve cell membranes.

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Fig. 13.8. Cytology/cytology/92.html">SCHEMATIC STRUCTURE OF a rod. Rhodopsin. Isomerization of retinal from the cis form to the trans form

Trans-retinal exhibits a low affinity for opsin, and shortly thereafter, activated rhodopsin dissociates into opsin and all-trans-retinal. A specialized retinal isomerase catalyzes The conversion of trans-retinal back to its cis form, after which spontaneous association of cis-retinal with opsin occurs—driven by the high mutual affinity of these structural components—thus regenerating rhodopsin. All these events repeat cyclically and are collectively known as the visual cycle (Fig. 13.9).

To understand The Mechanism of nerve impulse generation during the cascade events triggered by light-induced retinal isomerization, certain functional features of light-sensitive Cells must be taken into account. The rod membrane contains cation pumps that continuously extrude Na⁺ and Ca²⁺ ions from The Cell, thereby establishing an ionic gradient. In the dark, a steady inward current of sodium and Calcium Ions flows into the rods, utilizing cGMP-gated channels to re-enter the cell in accordance with Facilitated Diffusion. These channels remain open when cyclic guanosine monophosphate (cGMP) is bound to them.

Fig. 13.9. The visual cycle and cascade reactions associated with the light activation of rhodopsin, leading to the generation of a nerve impulse in neuronal membranes (rhodopsin* — activated form of rhodopsin; cGMP — cyclic guanosine monophosphate)

Upon illumination, the following events take place. Active rhodopsin, in which cis-retinal has undergone photochemical conversion to all-trans-retinal, binds to a G protein. Concurrently, within the G protein complex, GDP is exchanged for GTP, causing the protein to dissociate into subunits, one of which is the active GTP-a subunit. This structure activates the enzyme cGMP phosphodiesterase, which catalyzes the Hydrolysis of cyclic guanosine monophosphate. The cGMP level drops rapidly, causing the cation channels to close. As a result, the concentration of Na⁺ and Ca²⁺ ions on the inner surface of the rod membrane plummets while the cation pumps continue to operate, leading to cell hyperpolarization. Hyperpolarization slows down the release of the excitatory neurotransmitter (glutamate) from the rods, which in turn indirectly triggers an Action Potential in the optic nerve axons (Chapter 4).

The active GTP-a subunit of the G protein has a very short lifespan and is inactivated via GTP hydrolysis. This yields a free a subunit that associates with the remaining subunits to reconstitute the G protein, which can then be reactivated by binding to active rhodopsin.

Bacteriorhodopsin. A protein structurally similar to ocular rhodopsin (bacteriorhodopsin) has been discovered in the cells of certain archaebacteria, where it also serves for photoreception. A physiologically highly specialized group of halobacteria (Halobacterium, Halococcus, Natrococcus, Natrobacterium) contains unusual purple membranes colored in bright orange to bright red shades due to their bacteriorhodopsin content. These Bacteria are unique in their ability to harness light energy absorption to generate a proton gradient across the membrane, which serves as the driving force for ATP synthesis. Another distinctive feature of halobacteria is their ability to thrive in concentrated salt solutions (halophily).

Bacteriorhodopsin is an integral protein of the purple membrane, within which it forms a hollow cylinder. The chromophore of bacteriorhodopsin is retinal. In the dark, retinal exists in the all-trans form, and its aldimine group (= N+H -) is protonated (Fig. 13.8). Upon illumination, retinal rearranges into the 13-cis form, and the aldimine group releases a proton, which is channeled to the outer surface of the membrane. This is how the proton gradient is established across the membrane. The principles of ATP synthesis driven by the energy of the proton gradient are identical to those of Oxidative Phosphorylation. Thus, bacteriorhodopsin in the purple membranes acts as a light-driven proton pump.

The energy stored through this unusual mode of "Photosynthesis" supplements the energy that halobacteria store via Respiration.