Sandbox Reserved 895: Difference between revisions

In order to stimulate the phototransduction pathway, 11-''cis''-retinal must be absorbed into the body. 11-''cis''-retinal often referred as Vitamin A, is an important dietary vitamin that starts the cascade of phototransduction. Once absorbed into the body and brought to the photoreceptor outer segment (POS), 11-''cis''-retinal is conjugated to opsin to form the rhodopsin complex. Upon light incidence, 11-''cis''-retinal is converted though a photo-isomerization reaction into all-''trans''-retinal. The conversion of 11-''cis''-retinal to all-''trans''-retinal causes a conformation change in the rhodopsin complex. This conformation change activates a G-protein coupled protein transducing which triggers the subsequent phototransduction cascade. The generated all-''trans''-retinal then dissociates from the rhodopsin complex which frees the opsin protein to bind another 11-''cis''-retinal molecule and restart the phototransduction cascade. All-''trans''-retinal must be converted back into 11-''cis''-retinal to serve as the chromophore for the phototransduction cascade. To regenerate 11-''cis''-retinal a series of enzyme catalyzed chemical reactions must take place. These reactions take place within the photoreceptor outer segment (POS) as well as the retinal pigment epithelium (hRPE).  

[[Image:Visual_Cycle_RPE65_2.jpg|thumb|center|512 px|alt=Figure 2: Human Visual Cycle| '''Figure 2:''' A schematic representation of the Retinoid (Visual) Cycle in Humans. (Abbreviations: all-''trans''-ROL, all-''trans''-retinol; LRAT, lecithin retinol acyltransferase; RPE65, retinal pigment epithelium 65; 11-''cis''-ROL, 11-''cis''-retinol; 11-''cis''-RDH, 11-''cis''-retinol dehydrogenase; 11-''ci''s-RAL, 11-''cis''-retinal; ''hV'', light energy; all-''trans''-RAL, all-''trans''-retinal; atRDH, all-''trans''-retinol dehydrogenase) <ref> DOI 10.1016/j.bbadis.2018.04.014 </ref>]]

Zhang and colleagues determined the binding interactions between RPE65 and emixustat by looking at the protein-ligand complex under three different considerations. The three considerations were RPE65 with a racemic emixustat, RPE65 with pure (''R'')-emixustat and RPE65 with pure (''S'')-emixustat. With all three cases, residual maps showed the presence of bound palmitate in the active site with the carboxylate oxygen forming a monodentate coordinate bond with the iron ion. The hydroxyl moiety in emixustat interacted via a hydrogen bond with the hydroxyl moiety in Thr147 whereas the primary amine was involved in a polar interaction with the carboxylate groups of Glu148 as well as the bound palmitate ligand. Crystal structure that was obtained in the presence of the racemic mixture of emixustat showed that the electron density was consistent with exclusive binding by the (''R'')-isomer ('''Figure 1B'''). Using pure (''R'')-isomer and the pure (''S'')-isomer as a follow up study to the experiment confirmed the stereoselectivity for the (''R'')-isomer of emixustat. Stereoselectivity was present because of the interaction between the hydroxyl moieties in the enantiomers. Looking at the electron density of the 3-amino-1-phenylpropan-1-ol moiety of (''R'')-emixustat showed that when the racemic mixture was used the binding was that of the (''R'')-isomer. This showed that (''R'')-emixustat had a higher binding affinity when compared to (''S'')-emixustat. The difference in binding affinity and potency was likely because of the less favorable polar interaction observed in the (''S'')-isomer. Bond lengths for the hydroxyl-Thr137 Oγ , amine-Glu148 Oε2 and amine-palmitate O1 interactions were 3.1, 3.2 and 2.8 angstroms for the (''S'')-isomer and 3, 2.7 and 2.6 angstroms for the (''R'')-isomer respectively ('''Figure 9''').

The difference in bond lengths also confirms the increase in binding affinity and potency of the (R)-isomer. <ref> DOI 26075817 </ref>