Editing Retinal (section)

===Visual cycle===
{{main|Visual cycle}}
[[File:Visual cycle.svg|thumb|right|350x350px|Visual cycle]]
The visual cycle is a circular [[enzymatic pathway]], which is the front-end of phototransduction. It regenerates 11-''cis''-retinal. For example, the visual cycle of mammalian rod cells is as follows:
#[[all-trans-retinyl ester|all-''trans''-retinyl ester]] + H<sub>2</sub>O → 11-''cis''-retinol + [[fatty acid]]; [[RPE65]] isomerohydrolases;<ref>{{cite journal |last1=Moiseyev |first1=Gennadiy |last2=Chen |first2=Ying |last3=Takahashi |first3=Yusuke |last4=Wu |first4=Bill X. |last5=Ma |first5=Jian-xing |year=2005 |title=RPE65 is the isomerohydrolase in the retinoid visual cycle |journal=Proceedings of the National Academy of Sciences |volume=102 |issue=35 |pages=12413–12418 |doi=10.1073/pnas.0503460102 |pmid=16116091 |pmc=1194921 |bibcode=2005PNAS..10212413M|doi-access=free }}</ref>
#[[11-cis-retinol|11-''cis''-retinol]] + NAD<sup>+</sup> → 11-''cis''-retinal + NADH + H<sup>+</sup>; 11-''cis''-retinol dehydrogenases;
#[[11-cis retinal|11-''cis''-retinal]] + [[aporhodopsin]] → [[rhodopsin]] + H<sub>2</sub>O; forms [[Schiff base]] linkage to [[lysine]], -CH=N<sup>+</sup>H-;
#rhodopsin + [[photon|hν]] → [[metarhodopsin]] II (i.e., 11-''cis'' [[photoisomerization|photoisomerizes]] to all-''trans''):
#:(rhodopsin + hν → photorhodopsin → bathorhodopsin → lumirhodopsin → metarhodopsin I → metarhodopsin II);
#[[metarhodopsin]] II + H<sub>2</sub>O → aporhodopsin + all-''trans''-retinal;
#[[all-trans-retinal|all-''trans''-retinal]] + NADPH + H<sup>+</sup> → all-''trans''-retinol + NADP<sup>+</sup>; all-''trans''-retinol [[dehydrogenase]]s;
#all-''trans''-retinol + fatty acid → all-''trans''-retinyl ester + H<sub>2</sub>O; [[lecithin retinol acyltransferase]]s (LRATs).<ref>{{cite journal |last1=Jin |first1=Minghao |last2=Yuan |first2=Quan |last3=Li |first3=Songhua |last4=Travis |first4=Gabriel H. |year=2007 |title=Role of LRAT on the Retinoid Isomerase Activity and Membrane Association of Rpe65 |journal=Journal of Biological Chemistry |volume=282 |issue=29 |pages=20915–20924 |doi=10.1074/jbc.M701432200 |pmid=17504753 |pmc=2747659|doi-access=free }}</ref>

Steps 3, 4, 5, and 6 occur in [[rod cell|rod cell outer segments]]; Steps 1, 2, and 7 occur in [[retinal pigment epithelium]] (RPE) cells.

RPE65 isomerohydrolases are [[homology (biology)|homologous]] with beta-carotene monooxygenases;<ref name="von Lintig"/> the homologous ninaB enzyme in ''Drosophila'' has both retinal-forming carotenoid-oxygenase activity and all-''trans'' to 11-''cis'' isomerase activity.<ref name="Oberhauser08">{{cite journal |last1=Oberhauser |first1=Vitus |last2=Voolstra |first2=Olaf |last3=Bangert |first3=Annette |last4=von Lintig |first4=Johannes |last5=Vogt |first5=Klaus |year=2008 |title=NinaB combines carotenoid oxygenase and retinoid isomerase activity in a single polypeptide |journal=Proceedings of the National Academy of Sciences |volume=105 |issue=48 |pages=19000–5 |doi=10.1073/pnas.0807805105 |pmid=19020100 |pmc=2596218 |bibcode=2008PNAS..10519000O|doi-access=free }}</ref>