ChemBio Spotlight #3
Paper: Catalytic Mechanism of a Retinoid Isomerase Essential for Vertebrate Vision
RPE65 is a membrane-bound retinoid isomerase that is important in the retinoid cycle pathway for vision in vertebrates. It converts fatty acid all-trans-retinyl ester into 11-cis-retinol for rod and cone opsins, which are receptors that receive light and turn it into an electrical signal used for vision. However, a catalytic mechanism for RPE65 has yet to be determined since it is unclear how retinoids bind to its active site. Previous research has led to the discovery of RPE65 active site residues, however possible suggestions for how RPE65 and retinoid interact place retinoid far from these residues in the active site pocket. Within the past year, Kiser et. al have proposed a catalytic mechanism for RPE65 in the retinoid cycle pathway.
Since it has been difficult to obtain a crystal structure of an RPE65 complex (RPE65 + substrate) due to the poor aqueous solubility of retinoids, the authors began their study by finding a molecule that is similar to a retinoid, but also has better solubility. The chemical structure of emixustat was studied, and it was determined that is was a good retinoid mimic along with another molecule, MB-001, that is a hybrid of emixustat.
The authors then tested the whether emixustat and MB-001 bind to RPE65 and inhibit production of 11-cis-retinol using an activity assay, which the authors found that they do. This led to an investigation of RPE65’s active site, binding sites, and hydrophobic interactions with these molecules, which allowed the authors to propose a catalytic mechanism for RPE65, which included acyl and substrate binding, van der Waals, hydrophobic, and electrostatic interactions, ester group polarization, and carbocation stabilization. This showed that derivatives of substrates that have increased solubility, therefore interacting better with active site polar pockets, can be used in other structural studies to determine catalytic mechanisms.