FIGURE 3.

Proposed pathway for astaxanthin synthesis in green microalgae. Asterisks denote enzymes which have been targeted for metabolic engineering, as shown in Table 2. PSY, phytoene synthase; PDS, phytoene desaturase; ZDS, ζ-carotene desaturase; LYC-B, lycopene β-cyclase; BKT, β -carotene ketolase; CrtR-b, β-carotene hydroxylase. Modified from (Grünewald et al., 2001; Ye et al., 2008). Synthesis of β -carotene occurs in the chloroplast, while there is evidence that the subsequent steps could also take place in the cytoplasm (Grünewald et al., 2001; Guedes et al., 2011).