title: Structure of RPE65 isomerase in a lipidic matrix reveals roles for phospholipids and iron in catalysis.
abstract: 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.
authors.affiliation: Department of Pharmacology, School of Medicine, Case Western Reserve University, Cleveland, OH 44106, USA. pdk7@case.edu
authors.forename: Philip D
authors.initials: PD
authors.lastname: Kiser
authors.forename: Erik R
authors.initials: ER
authors.lastname: Farquhar
authors.forename: Wuxian
authors.initials: W
authors.lastname: Shi
authors.forename: Xuewu
authors.initials: X
authors.lastname: Sui
authors.forename: Mark R
authors.initials: MR
authors.lastname: Chance
authors.forename: Krzysztof
authors.initials: K
authors.lastname: Palczewski
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D001665:Binding Sites / Q000235:genetics; D002384:Catalysis; D002417:Cattle; D018360:Crystallography, X-Ray; D007501:Iron / Q000737:chemistry* / Q000378:metabolism; D008055:Lipids / Q000737:chemistry*; D008861:Microsomes / Q000201:enzymology; D008958:Models, Molecular; D008969:Molecular Sequence Data; D010743:Phospholipids / Q000737:chemistry* / Q000378:metabolism; D011487:Protein Conformation; D017434:Protein Structure, Tertiary; D055213:Retinal Pigment Epithelium / Q000201:enzymology; D017386:Sequence Homology, Amino Acid; D056928:X-Ray Absorption Spectroscopy; D019745:cis-trans-Isomerases / Q000737:chemistry* / Q000235:genetics / Q000378:metabolism
chemical_list: D008055:Lipids; D010743:Phospholipids; D007501:Iron; D019745:cis-trans-Isomerases