Membrane phospholipids and the dark side of vision.
The key step in the visual pigment regeneration process is an enzyme-catalyzed trans to cis 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 4 kcal/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 -5 kcal/mol, is coupled to the endothermic (+4 kcal/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.
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Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School, Boston, Massachusetts 02115.
D000818:Animals; D008563:Membrane Lipids / Q000378:metabolism*; D010743:Phospholipids / Q000378:metabolism*; D010777:Photochemistry; D012243:Rhodopsin / Q000378:metabolism; D014785:Vision, Ocular / Q000502:physiology*
D008563:Membrane Lipids; D010743:Phospholipids; D012243:Rhodopsin