448: The Structure of a Retinal-Forming Carotenoid Oxygenase -- Kloer et al. 308 (5719): 267 -- Science
Retinal and its derivatives participate in numerous cellular activities; they are crucial for vision and the immune system and are therefore of nutritional importance. Retinal-forming carotenoid oxygenases constitute a sequence-related family of more than 100 currently known members. The family was discovered through a 9'-cis-epoxycarotenoid oxygenase that participates in the biosynthesis of the important plant hormone abscisic acid. A prominent family member is ß-carotene-15,15'-oxygenase from animals, which cleaves ß-carotene symmetrically to two molecules of retinal. Another member is ß-carotene-9',10'-oxygenase, cleaving ß-carotene asymmetrically to form apo-10'-ß-carotenal, which is thought to be converted to retinoic acid, a key actor in developmental processes. The family includes the retinal pigment epithelial protein RPE65, mutations of which cause Leber's congenital amaurosis, a severe blinding disease. In plants, the genome of Arabidopsis thaliana codes for as many as nine family members, several of which have been established as carotenoid oxygenases. Some of these genes yield products regulating growth and development. Other plant members catalyze the biosynthesis of pigments. Cyanobacterial retinal-producing members have been proposed and recently identified in Synechocystis. Here, we report the crystal structure of the Synechocystis enzyme at 2.4 Å resolution, revealing the reaction geometry and establishing a solid base for modeling all other family members.
These folks are interested in retinal because it occurs in animals and plants, and in the cyanobacteria which may be ancestral to modern plant chloroplasts. This particular retinal may inform us on the evolution of photosynthetic plants.