Sandbox 173: Difference between revisions

There is positive enthalpy associated with the formation of Metarhodopsin II. This formation of the active state, also linked with the increase in entropy, is suggested to release the constraints in the helices and expose the cytoplasmic binding sites<ref>Article 9</ref>. An important part of this process includes the 9-methyl group of retinal, which is suggested to provide a scaffold for proton transfers essential for the formation of the active state<ref>Article 9</ref>.

====Signalling Cascade and Polarization of the Cell Membrane====

The excited rhodopsin interacts with a large number of transducin molecules, found in the cytoplasic face of the disk membrane. Transducin is a member of the heterotrimeric GTP-binding proteins family, and it binds to GDP in the dark. This interaction generates a signaling cascade where transducin molecules are activated through the trigger of GDP-GTP nucleotide exchange in the α subunit<ref>Article 6</ref>.  Each activated transducin dissociates into Tα-GTP and Tβγ subunits, and Tα-GTP activates cGMP-specific phosphodiesterase by binding and removing its inhibitory subunit<ref>Textbook</ref>.  

The cGMP phosphodiesterase is an integral protein of the retina with its active site on the cytoplasmic side of the disk. Its inhibitory subunit tightly binds to it in the dark and suppresses its activity.  The now activated phosphodiesterase degrades many molecules of cGMP, efficiently decreasing the concentration of cGMP <ref>Textbook</ref>. This results in the closing of the cGMP-gated cation channels in the plasma membrane of the outer segment. The cell hyperpolarizes due to the decrease in the influx of sodium and calcium ions, results in the decrease in the release of glutamate into the synaptic cleft. This electric signal is sent to the brain through ranks of interconnecting neurons and then through the optic nerve of this hyperpolarization<ref>Article 6</ref>.