Visual chromophore recycling
Krzysztof Palczewski group (Case Western Reserve University) and collaborators
The group of Krzysztof Palczewski at Case Western Reserve
University made major insights into the mechanism of bovine RPE65, a key
enzyme in the visual cycle. Data from GM/CA 23ID-D and from the NSLS were
used in the 2.14-Å structure determination (Kiser et al., Proc. Natl.
Acad. Sci. USA 106 17325-17330 (2009)), and from 23ID-D in work at
1.9-Å (Golczak et al., J. Biol. Chem. 285 9667-9682 (2010)). The
visual cycle is initiated when the 11-cis-retinylidene chromophore of
rhodopsin is photo-isomerized to all-trans-retinylidene.
All-trans-retinylidene is re-cycled through the retinal pigment epithelium
(RPE) as an all-trans-retinyl ester of phosphatidyl choline. RPE65 catalyzes
ester hydrolysis and the key isomerization from trans to cis before
re-incorporation into rhodopsin. Mutations in RPE65 cause a hereditary
childhood blinding disease called Leber congenital amaurosis, and also
retinitis pigmentosa, the degenerative blinding disease that affects 2
million people worldwide. RPE65 has a seven-bladed beta-propeller in which
one residue from each propeller is in the first (4 His) or second (3 Glu)
coordination shell of the catalytic iron. Like other enzymes with substrates
in the membrane bilayer, a hydrophobic tunnel leads to the protein surface at
an amphipathic helix. Most of the disease-related mutations are at residues
that maintain the structure of the active site. The structures also laid to
rest a high-profile hypothesis that membrane association and activity are
regulated by reversible S-palmitoylation at certain Cys residues, which are
neither surface exposed nor palmitoylated.
Figure: RPE65 topology relative to a
phosphatidylcholine-containing bilayer. [This figure was originally published
in the Journal of
Biological Chemistry (full citation below), copyright of the American
Society for Biochemistry and Molecular Biology."]
Kiser, PD, Golczak, M, Lodowski, DT, Chance, MR, Palczewski, K. Crystal
structure of native RPE65, the retinoid isomerase of the visual cycle, Proc.
Natl. Acad. Sci. USA 106, 17325-17330 (2009). DOI: 10.1073/pnas.0906600106.|
Golczak, M, Kiser, PD, Lodowski, DT, Maeda, A, Palczewski, K. Importance of
Membrane Structural Integrity for RPE65 Retinoid Isomerization Activity, J.
Biol. Chem. 285, 9667-9682 (2010). DOI: 10.1074/jbc.M109.063941.|