**1. Introduction**

Carotenoids are natural red, orange or yellow pigments widely distributed in nature. The vivid color of carotenoids contributes to the beauty of many flowers, fruits and animals. For

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

example, the loveliness of yellow marigolds comes mainly from lutein, a yellow carotene; the redness of watermelons and tomatoes is because they are rich in lycopene, a red carotene; and the scarlet plumage of flamingos stems from another red carotenoid, astaxanthin. The beautiful colors of plants are also responsible for attracting insects and animals for their pollination and seed dispersal [1]. The carotenoid color originates in the structure of multiple conjugated double bonds. This unique structure enables two essential features of carotenoids: the lightharvesting capability and a powerful anti-oxidant effect by the quenching of free radicals, singlet oxygen and reactive oxygen species. In photosynthetic organisms, carotenoids are indispensable for photosynthesis and photo-protection [2]. In non-photosynthetic organisms including animals, the anti-oxidant activity not only protects cells from oxidative damages (e.g., oxidative DNA damage [3]) but can provide additional benefits for humans such as antiinflammatory and anti-cancer effect [4]. In addition, carotenoids play an important health role as pro-vitamin A compounds. About 30–50 carotenoids are believed to have vitamin A activity including two well-known compounds: β-carotene and α-carotene [2].

these molecules. Chemically synthetized carotenoids, despite being less expensive, have been reported to have hazardous effects to human health and are increasingly unpopular with consumers [19]. Currently, microbial-derived commercial carotenoids are those derived from native producer strains which have not been genetically engineered but in some cases have undergone classical mutagenesis followed by selection to screen for improved production characteristics. These include the β-carotene production strains of the microalga *Dunaliella* [21] and the fungus *Blakeslea trispora* [2]. Recent advances in microbial biotechnology have made the microbial production of carotenoids and apocarotenoids potentially more efficient and cost-effective, using metabolic engineering strategies in industrial workhorse strains such as *Escherichia coli* and

Biosynthesis of Carotenoids and Apocarotenoids by Microorganisms and Their Industrial Potential

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To date, 1117 natural carotenoids and apocarotenoids have been reported, which consist of C30, C40, C45 and C50 carotenoids [22]. Among them, C40 carotenoids and their derived apocarotenoids are the most abundant with 1093 different structures. In this chapter, I will cover only a few of the commercially interesting C40 carotenoids and apocarotenoids that will

To understand how carotenoids and apocarotenoids can be produced in microbes, it is essen-

Carotenoids are a subclass of terpenoids (or isoprenoids); thus, as other terpenoids, they share the common C5 building blocks, isopentenyl pyrophosphate (IPP) and its isomer dimethylallyl pyrophosphate (DMAPP). In nature, there exist two independent biosynthetic pathways to produce IPP/DMAPP: the mevalonate (MVA) pathway [23] and the 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway, also referred to as the 1-deoxy-D-xylulose 5-phosphate (DXP)

The MEP pathway starts from the condensation of pyruvate and glyceraldehyde-3-phosphate, which are catalyzed by DXP synthase (*dxs*), to produce DXP, which is subsequently reduced into MEP by DXP reductase (*dxr*). MEP is converted into 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDPME) by CDPME synthase (*ispD*). CDPME is subsequently transformed into 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate (HMBPP) through two intermediates, 4-(cytidine 5′-diphospho)-2-C-methyl-D-erythritol (CDPMEP) and 2-C-methyl-D-erythritol-2,4-cyclodiphosphate (MEC) by CDPME kinase (*ispE*), MEC synthase (*ispF*) and HMBPP synthase (*ispG*), respectively. Finally, HMBPP reductase catalyzes HMBPP into a 5-6:1 ratio of IPP and DMAPP, while IPP isomerase (*idi*) inter-converts IPP and DMAPP to adjust the ratio

In the MVA pathway, two molecules of acetyl-CoA are condensed into one molecule of acetoacetyl-CoA by acetyl-CoA acetyltransferase (*atoB*). Acetoacetyl-CoA is converted into mevalonate via an intermediate (S)-3-hydroxy-3-methylglutaryl-CoA (HMG-CoA) by HMG-CoA synthase and HMG-CoA reductase, respectively. IPP is produced from mevalonate by another three enzymes, mevalonate kinase (*mk*), phosphomevalonate kinase (*pmk*) and

tial to elucidate the biosynthetic enzymes which constitute these metabolic pathways.

*Saccharomyces cerevisiae,* for which the fermentation strategies are well established.

illustrate the challenges and potentials of this biosynthetic alternative supply chain.

**2. Biosynthesis of carotenoids and apocarotenoids in nature**

or the non-MVA pathway [24].

according to the cellular requirements (**Figure 1**).

Vitamin A includes retinol, retinal and retinoic acid, which are all apocarotenoids. Apocarotenoids are a group of oxidative products of carotenoids. While carotenoids contribute to the visual beauty of flowers and fruits, apocarotenoids are famous for the pleasant aromas and give rise to fragrance and palatable flavors of many flowers and fruits (such as rose, violet, tomato and raspberry) [5–7]. These apocarotenoid aromas, in a similar manner to the colored carotenoids, attract pollinators and promote plant-insect interactions [8]. In addition, some apocarotenoids act as hormones. For example, the plant growth hormone, abscisic acid, has multiple functions in plant development processes including bud dormancy and response to environmental stress and plant pathogens [5]. Strigolactones are another important subclass of apocarotenoids, functioning as shoot-branching inhibitors and promoting the formation of symbiotic association between plants and fungi [9, 10].

Due to the color, aroma, remarkable nutrition and health benefits, carotenoids and apocarotenoids have been widely used in food, feed, nutritional, pharmaceutical and personal care industries. The market demands for carotenoids and apocarotenoids are rising rapidly as increasing clinical research studies report various health and pharmaceutical benefits [11–13]. The global carotenoid market is projected to reach 1.53 billion USD by 2021 [14]. The regular uptake of food with a high content of carotenoids (e.g., β-carotene) or retinoids is vital to alleviate vitamin A deficiency. Vitamin A deficiency can lead to severe aftermath including blindness, decreased immune function and even death [15]. Lutein and zeaxanthin are critical for eye health by preventing age-related macular degeneration [16]. Astaxanthin has even more benefits such as potent anti-oxidant activities, promoting immune response, reducing eye fatigue, enhancing muscle performance and so on [11]. Because of low exceptional fragrance property, α-ionone and β-ionone are widely used in cosmetics such as perfumes [17]. Crocin is another valuable apocarotenoid and is responsible for the red pigmentation of saffron, a high-value spice with retail prices ranging between 2000 and 7000 euros/kg [18].

Despite carotenoids and apocarotenoids being widely distributed in nature, their cellular contents are extremely low. For example, 100 tons of raspberries, or 20 hectares of agricultural area, could only yield 1 g of α-ionone [19]. Similarly, it requires the manual harvest of stigmas from as many as 110,000–170,000 flowers to obtain 1 kg of saffron [20], justifying the high cost of these molecules. Chemically synthetized carotenoids, despite being less expensive, have been reported to have hazardous effects to human health and are increasingly unpopular with consumers [19]. Currently, microbial-derived commercial carotenoids are those derived from native producer strains which have not been genetically engineered but in some cases have undergone classical mutagenesis followed by selection to screen for improved production characteristics. These include the β-carotene production strains of the microalga *Dunaliella* [21] and the fungus *Blakeslea trispora* [2]. Recent advances in microbial biotechnology have made the microbial production of carotenoids and apocarotenoids potentially more efficient and cost-effective, using metabolic engineering strategies in industrial workhorse strains such as *Escherichia coli* and *Saccharomyces cerevisiae,* for which the fermentation strategies are well established.

To date, 1117 natural carotenoids and apocarotenoids have been reported, which consist of C30, C40, C45 and C50 carotenoids [22]. Among them, C40 carotenoids and their derived apocarotenoids are the most abundant with 1093 different structures. In this chapter, I will cover only a few of the commercially interesting C40 carotenoids and apocarotenoids that will illustrate the challenges and potentials of this biosynthetic alternative supply chain.
