The Autotrophic Biorefinery by Robert Kourist Sandy Schmidt

Author:Robert Kourist, Sandy Schmidt
Language: eng
Format: epub
Publisher: De Gruyter
Published: 2021-10-25T05:37:48.909000+00:00

7.2.2.3 Genetic engineering for improved production of astaxanthin

Isopentenyl pyrophosphate (IPP) is the key intermediate for the synthesis of all carotenoids, including astaxanthin [26]. IPP can be originated from glyceraldehyde-3-phosphate and pyruvate by two different pathways: the cytosolic mevalonate (MVA) and the 2-C-methyl-D-erythritol 4-phosphate (MEP), also known as the non-mevalonate or 1-deoxy-D-xylulose 5-phosphate (DXP) pathway, located in the chloroplast. [26, 130 151, 152, 153, 154] In H. pluvialis, IPP is most likely synthesized using the DXP pathway, as this microalga lacks three key enzymes of the MVA pathway [26, 155]. IPP undergoes isomerization to dimethylallyl pyrophosphate (DMAPP) and, subsequently, IPP and DMAPP are condensed into geranylgeranyl pyrophosphate (GGPP) by farnesyl pyrophosphate (FPP) and GGPP synthases [26, 130]. GGPP is a shared precursor with other isoprenoids [26]. The first committed step to the synthesis of carotenoids is the conversion of GGPP into phytoene by phytoene synthase, with is then converted into lycopene by desaturases [156]. Lycopene is then transformed into β-carotene by lycopene β-cyclase. Until this point, the biosynthetic pathway is linear and essentially the same in different organisms. However, after formation of β-carotene, the biosynthetic routes differ among different organisms [130]. In algae, β-carotene can be oxidized into 4-keto intermediates or 3-hydroxyl intermediates by β-carotene ketolases and hydroxylases respectively, in a total of two hydroxylation and two ketolation reactions that can occur sequentially or non-sequentially (Figure 7.4) [130]. Consequently, there are seven intermediates between β-carotene and astaxanthin. In algae, the β-carotene ketolases and hydrolyases are encoded by crtO (or bkt) and chyb (or crtR-b), respectively [130]. The final two oxygenation steps catalyzed by Bkt and CrtR-b are rate limiting steps of astaxanthin biosynthesis [26, 157].

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