Structure of RPE65 isomerase in a lipidic matrix reveals roles for phospholipids and iron in catalysis.
Publication Type:Journal Article
Source:Proc Natl Acad Sci U S A, Volume 109, Issue 41, p.E2747-56 (2012)
Keywords:Amino Acid Sequence, Animals, Binding Sites, Catalysis, Cattle, cis-trans-Isomerases, Crystallography, X-Ray, Iron, Lipids, Microsomes, Models, Molecular, Molecular Sequence Data, Phospholipids, Protein Conformation, Protein Structure, Tertiary, Retinal Pigment Epithelium, Sequence Homology, Amino Acid, X-Ray Absorption Spectroscopy
<p>RPE65 is a key metalloenzyme responsible for maintaining visual function in vertebrates. Despite extensive research on this membrane-bound retinoid isomerase, fundamental questions regarding its enzymology remain unanswered. Here, we report the crystal structure of RPE65 in a membrane-like environment. These crystals, obtained from enzymatically active, nondelipidated protein, displayed an unusual packing arrangement wherein RPE65 is embedded in a lipid-detergent sheet. Structural differences between delipidated and nondelipidated RPE65 uncovered key residues involved in substrate uptake and processing. Complementary iron K-edge X-ray absorption spectroscopy data established that RPE65 as isolated contained a divalent iron center and demonstrated the presence of a tightly bound ligand consistent with a coordinated carboxylate group. These results support the hypothesis that the Lewis acidity of iron could be used to promote ester dissociation and generation of a carbocation intermediate required for retinoid isomerization.</p>