Enzymes that produce retinal and related apocarotenoids constitute a sequence-and thus structure-related family, a member of which was analyzed by x-ray diffraction. This member is an oxygenase and contains an Fe 2+-4-His arrangement at the axis of a seven-bladed β-propeller chain fold covered by a dome formed by six large loops. The Fe2+ is accessible through a long nonpolar tunnel that holds a carotenoid derivative in one of the crystals. On binding, three consecutive double bonds of this carotenoid changed from a straight all-trans to a cranked cis-trans-cis conformation. The remaining trans bond is located at the dioxygen-ligated Fe2+ and cleaved by oxygen.
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