Changing Form and Function through Carotenoids and Synthetic Biology

Abstract

The diverse structures and multifaceted roles of carotenoids make these colorful pigments attractive targets for synthetic biology.

Figure 1.

Simplified plant carotenoid biosynthetic pathway to zeaxanthin and lutein. The major question of metabolon organization and assembly is highlighted by the inset showing varying localization of maize (Zea mays) phytoene synthase (PSY) allelic variants and isozymes fused to GFP and introduced into chloroplasts marked by red chlorophyll autofluorescence (adapted from Shumskaya et al., 2012). Formation of lutein actually requires the synergistic interactions of CYP97A and CYP97C (Quinlan et al., 2012). Steps catalyzed by the bacterial enzyme, CrtI, are also indicated and correspond to the four steps catalyzed by the plant enzymes PDS, Z-ISO, ZDS, and CRTISO. These reactions also require an electron transfer chain and plastoquinones to channel electrons/protons produced during desaturation steps. Redox states of the Z-ISO heme iron depict posttranslational regulation of the pathway. Redox changes cause conformational changes in Z-ISO leading to a switch of the heme ligands and activation/deactivation of Z-ISO. When the heme iron is oxidized, the pathway is interrupted whereas when the heme iron is reduced, Z-ISO is active and the pathway progresses. “Z-ISO oxidized,” refers to the Z-ISO heme iron in the oxidized Fe⁺³ state; “Z-ISO reduced,” refers to the Z-ISO heme iron in the reduced Fe⁺² state. CrtI, bacterial phytoene desaturase. Plant enzymes: PSY, phytoene synthase; PDS, phytoene desaturase; Z-ISO, 15-cis-ζ-carotene isomerase; ZDS, ζ-carotene desaturase; CRTISO, carotenoid isomerase; LCYE, lycopene ε-cyclase; LCYB, lycopene β-cyclase; CYP97A, cytochrome P450 97A (P450 type β-ring hydroxylase); CYP97C, cytochrome P450 97C (P450 type ε-ring hydroxylase); HYDB, β-ring hydroxylase (nonheme diiron type). For more details related to this figure, see Moise et al. (2014) and Beltran et al. (2015).