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Nat Chem Biol. 2015 Aug;11(8):598-605. doi: 10.1038/nchembio.1840. Epub 2015 Jun 15.

Control of carotenoid biosynthesis through a heme-based cis-trans isomerase.

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1] Department of Biological Sciences, Lehman College, City University of New York (CUNY), Bronx, New York, USA. [2] Graduate School and University Center, CUNY, New York, New York, USA.
New York Structural Biology Center, New York, New York, USA.
Department of Biochemistry, University of Mississippi Medical Center, Jackson, Mississippi, USA.
Department of Chemistry, Georgia State University, Atlanta, Georgia, USA.
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina, USA.
Department of Biological Sciences, Lehman College, City University of New York (CUNY), Bronx, New York, USA.


Plants synthesize carotenoids, which are essential for plant development and survival. These metabolites also serve as essential nutrients for human health. The biosynthetic pathway for all plant carotenoids occurs in chloroplasts and other plastids and requires 15-cis-ζ-carotene isomerase (Z-ISO). It was not known whether Z-ISO catalyzes isomerization alone or in combination with other enzymes. Here we show that Z-ISO is a bona fide enzyme and integral membrane protein. Z-ISO independently catalyzes the cis-trans isomerization of the 15-15' carbon-carbon double bond in 9,15,9'-cis-ζ-carotene to produce the substrate required by the subsequent biosynthetic-pathway enzyme. We discovered that isomerization depends upon a ferrous heme b cofactor that undergoes redox-regulated ligand switching between the heme iron and alternate Z-ISO amino acid residues. Heme b-dependent isomerization of a large hydrophobic compound in a membrane was previously undescribed. As an isomerase, Z-ISO represents a new prototype for heme b proteins and potentially uses a new chemical mechanism.

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