Membrane phospholipids and the dark side of vision

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Abstract
The key step in the visual pigment regeneration process is an enzyme-catalyzedtrans tocis retinoid isomerization reaction. This reaction is of substantial general interest, because it requires the input of metabolic energy. The energy is needed because the 11-cis-retinoid reaction products are approximately 4kcal/mol higher in energy than their all-trans congeners. In the retinal pigment epithelium a novel enzymatic system has been discovered which is capable of converting all-trans-retinol into all-trans retinyl esters, by means of a lecithin retinol acyl transferase (LRAT), followed by the direct processing of the ester into 11-cis-retinol. In this process the free energy of hydrolysis of a retinyl ester, estimated to be approximately −5kcal/mol, is coupled to the endothermic (+4kcal/mol) isomerization reaction, resulting in an overall exothermic process. The overall process is analogous to ATP-dependent group transfer reactions, but here the energy is provided by the membrane phospholipids. This process illustrates a new role for membranes: they can serve as an energy source.