Question

Describe the sequence of events after rhodopsin absorbs light. (pages \(312-14\) ).

Short answer

After rhodopsin absorbs light, the 11-cis-retinal molecule within its structure undergoes isomerization and changes into all-trans-retinal, causing a conformational change in the opsin protein, activating it into metarhodopsin II. Metarhodopsin II interacts with transducin, a G protein, leading to the activation of phosphodiesterase (PDE) which hydrolyzes cGMP into 5'-GMP, causing hyperpolarization of the rod cell and generating an electrical signal for the brain. The all-trans-retinal molecule is eventually released, converted back to 11-cis-retinal, and recombined with opsin to regenerate rhodopsin in a process called the visual cycle.

Step-by-step solution

Introduction to Rhodopsin and its Structure

Rhodopsin is a light-sensitive protein found in the rod cells of the retina, responsible for the visual process at low light levels. Rhodopsin consists of two components: opsin (a protein) and 11-cis-retinal (a covalently attached chromophore).

Absorption of Light

When light enters the eye, it is absorbed by the 11-cis-retinal molecule embedded in the protein opsin. The absorption of a photon causes the 11-cis-retinal molecule to undergo isomerization, and it changes its shape to all-trans-retinal.

Photoactivation and Conformational Changes

The isomerization of 11-cis-retinal to all-trans-retinal causes a conformational change in the opsin protein. This change in conformation activates the protein and turns it into metarhodopsin II.

G Protein Pathway Activation

The activated metarhodopsin II protein interacts with a G protein called transducin, a process that ultimately leads to the activation of the enzyme phosphodiesterase (PDE), which then hydrolyzes cGMP into 5’-GMP. The decrease in cGMP levels leads to the hyperpolarization of the rod cell, creating an electrical signal sent to the brain.

Deactivation and Regeneration of Rhodopsin

To restore rhodopsin to its original state, the all-trans-retinal molecule is first released from opsin. The all-trans-retinal is then converted back to 11-cis-retinal in a series of enzymatic reactions, and it eventually recombines with opsin to regenerate rhodopsin. This process, known as the visual cycle, is essential for maintaining vision at low light levels.