all-trans retinal binding

Definition

Target type: molecularfunction

Binding to all-trans retinal, a compound that plays an important role in the visual process in most vertebrates. All-trans retinal (trans r., visual yellow) results from the bleaching of rhodopsin by light, in which the 11-cis form is converted to the all-trans form. Retinal is one of the forms of vitamin A. [GOC:curators]

All-trans retinal binding is a crucial molecular function in the process of vision. It involves the interaction of the light-absorbing molecule all-trans retinal with specific proteins, primarily rhodopsin in the rod cells of the retina. This interaction triggers a cascade of events that ultimately leads to the transmission of visual signals to the brain.

Here is a detailed description of the molecular function of all-trans retinal binding:

1. **Retinal Conformation Change:** In the dark, all-trans retinal is bound to rhodopsin in a specific conformation. Upon exposure to light, all-trans retinal absorbs a photon, causing it to undergo a conformational change from its extended all-trans form to a bent, cis-retinal form. This change in retinal conformation triggers a series of structural changes within rhodopsin.

2. **Rhodopsin Activation:** The conformational change in retinal alters the shape of the rhodopsin protein, causing it to become activated. This activation involves a shift in the orientation of transmembrane helices within rhodopsin, exposing a binding site for another protein, transducin.

3. **Transducin Activation:** Activated rhodopsin interacts with transducin, a heterotrimeric G protein. This interaction causes the alpha subunit of transducin to release GDP and bind GTP, leading to the activation of transducin.

4. **Phosphodiesterase Activation:** Activated transducin binds to and activates phosphodiesterase, an enzyme that hydrolyzes cyclic GMP (cGMP), a second messenger molecule.

5. **cGMP Reduction and Channel Closure:** The breakdown of cGMP by phosphodiesterase reduces its concentration in the cytoplasm of the rod cell. This reduction in cGMP concentration leads to the closure of cGMP-gated cation channels in the plasma membrane of the rod cell.

6. **Hyperpolarization:** Closure of the cation channels causes the rod cell to become hyperpolarized, which is a change in its membrane potential to a more negative value. This hyperpolarization signal is transmitted along the optic nerve to the brain.

7. **Signal Amplification:** The binding of a single photon to retinal initiates a cascade of events that results in the amplification of the signal, leading to the detection of very faint light. This amplification is achieved through a series of enzymatic reactions that involve transducin and phosphodiesterase.

8. **Adaptation:** To ensure proper functioning in varying light conditions, rhodopsin and other proteins involved in the signaling pathway are subject to regulatory mechanisms that control their activity. These mechanisms include phosphorylation of rhodopsin by rhodopsin kinase and the activation of arrestin, a protein that binds to phosphorylated rhodopsin, preventing further activation.

In summary, all-trans retinal binding is an essential molecular function that enables the detection of light and the transmission of visual information to the brain. It involves a complex series of events that include retinal conformation change, protein activation, and signal amplification. This intricate process ensures that we can perceive the world around us in all its visual glory.'
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Proteins (2)

Compounds (8)