Scholarly article on topic 'Metabolic engineering for the microbial production of isoprenoids: Carotenoids and isoprenoid-based biofuels'

Metabolic engineering for the microbial production of isoprenoids: Carotenoids and isoprenoid-based biofuels Academic research paper on "Industrial Biotechnology"

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Carotenoids / Isoprenoid-based biofuel / Metabolic engineering / Synthetic biology

Abstract of research paper on Industrial Biotechnology, author of scientific article — Fu-Xing Niu, Qian Lu, Yi-Fan Bu, Jian-Zhong Liu

Abstract Isoprenoids are the most abundant and highly diverse group of natural products. Many isoprenoids have been used for pharmaceuticals, nutraceuticals, flavors, cosmetics, food additives and biofuels. Carotenoids and isoprenoid-based biofuels are two classes of important isoprenoids. These isoprenoids have been produced microbially through metabolic engineering and synthetic biology efforts. Herein, we briefly review the engineered biosynthetic pathways in well-characterized microbial systems for the production of carotenoids and several isoprenoid-based biofuels.

Academic research paper on topic "Metabolic engineering for the microbial production of isoprenoids: Carotenoids and isoprenoid-based biofuels"

KeAi

Synthetic and Systems Biotechnology

Synthetic and Systems Biotechnology xxx (2017) 1—9

ADVANCING RESEARCH EVOLVING SCIENCE

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Synthetic and Systems Biotechnology

journal homepage: http://www.keaipublishing.com/en/journals/synthetic-and-systems-biotechnology/

Metabolic engineering for the microbial production of isoprenoids: Carotenoids and isoprenoid-based biofuels

Fu-Xing Niu a'b'1, Qian Lu a'b'1, Yi-Fan Bu a'b, Jian-Zhong Liu a'b' *

a Biotechnology Research Center and Biomedical Center, School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China b South China Sea Bio-Resource Exploitation and Utilization Collaborative Innovation Center, School of Life Sciences, Sun Yat-sen University, Guangzhou 510275, China

ARTICLE INFO ABSTRACT

Isoprenoids are the most abundant and highly diverse group of natural products. Many isoprenoids have been used for pharmaceuticals, nutraceuticals, flavors, cosmetics, food additives and biofuels. Carotenoids and isoprenoid-based biofuels are two classes of important isoprenoids. These isoprenoids have been produced microbially through metabolic engineering and synthetic biology efforts. Herein, we briefly review the engineered biosynthetic pathways in well-characterized microbial systems for the production of carotenoids and several isoprenoid-based biofuels.

© 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/

4.0/).

Article history: Received 28 April 2017 Received in revised form 3 August 2017 Accepted 9 August 2017

Keywords: Carotenoids

Isoprenoid-based biofuel Metabolic engineering Synthetic biology

Contents

1. Introduction.......................................................................................................................00

2. Carotenoid products ................................................................................................................00

2.1. Lycopene........................................................................................................................................................................................................................................00

2.2. b-Carotene..................................................................................................................................................................................................................................00

2.3. Zeaxanthin..................................................................................................................................................................................................................................00

2.4. Astaxanthin..................................................................................................................................................................................................................................00

3. Isoprenoid-based biofuels...........................................................................................................00

3.1. Hemiterpenoid-based biofuels..................................................................................................................................................................................................00

3.2. Monoterpenoid-based biofuels................................................................................................................................................................................................00

3.3. Sesquiterpenoid-based biofuels ..............................................................................................................................................................................................00

4. Conclusions .......................................................................................................................00

Acknowledgments ..................................................................................................................................................................................................................................00

References................................................................................................................................................................................................................................................00

1. Introduction

Isoprenoids, also called terpenoids or terpenes, are the most

* Corresponding author. Biotechnology Research Centre, School of Life Science, Sun Yat-Sen University, Guangzhou 510275, China. E-mail address: lssljz@mail.sysu.edu.cn (J.-Z. Liu). Peer review under responsibility of KeAi Communications Co., Ltd.

1 Contributed equally to this work.

abundant and highly diverse (structurally and functionally) group of natural products synthesized in almost all living organisms. Many isoprenoids have been used for pharmaceuticals, nutraceuticals, flavors, cosmetics, food additives and biofuels. Isoprenoids are usually classified into groups according the number of carbons: hemiterpenes (C5), monoterpenes (C10), sesquiterpenes (C15), diterpenes (C20), triterpenes (C30) and tetraterpenes (carotenoids, C40).

All isoprenoids derive from isopentenyl diphosphate (IPP) and

http://dx.doi.org/10.1016/j.synbio.2017.08.001

2405-805X/© 2017 The Authors. Production and hosting by Elsevier B.V. on behalf of KeAi Communications Co. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

its isomer dimethylallyl diphosphate (DMAPP) (Fig. 1). They can be produced by two metabolic pathways, the mevalonate pathway (MVA or MEV) [1] and the 1-deoxy-D-xylulose-5-phosphate (DXP) pathway (also called the 2-C-methyl-D-erythritol 4-phosphate pathway, MEP pathway) [2]. The MVA pathway is mainly present

in archaea, fungi, plant cytoplasm and other eukaryotes. The DXP pathway is mostly found in bacteria and plant plastids. The MVA pathway initiates with the condensation of two acetyl-CoAs by thiolase to produce acetoacetyl-CoA. Subsequently, another acetyl-CoA is condensed with acetoacetyl-CoA to synthesize 3-hydroxy-3-

Fig. 1. Isoprenoid biosynthetic pathway. G3P: Glyceraldehyde 3-phosphate; DXP: 1-deoxy-D-xylulose-5-phosphate; MEP: 2-C-methyl-D-erythritol-4-phosphate; CDP-ME: 4-diphosphocytidyl-2-C-methyl-D-erythritol; CDP-MEP: 4-diphosphocytidyl-2-C-methyl-D-erythritol-2-phosphate; MEcPP: 2-C-methyl-D-erythritol-2,4-cyclodiphosphate; HMB-PP: 4-hydroxy-3-mehtyl-butenyl 1-diphosphate; HMG-CoA: 3-hydroxy-3-methyl-glutaryl-CoA; Mev-P: Mevalonate 5-phosphate; Mev-PP: Mevalonate diphosphate; IPP: Iso-pentenyl diphosphate; DMAPP: Dimethylallyl diphosphate; GPP: Geranyl diphosphate; FPP: Farnesyl diphosphate; GGPP: Geranylgeranyl diphosphate; Dxs: 1-deoxy-D-xylulose-5-phosphate synthase; IspC: 1-D-deoxy-D-xylulose 5-phosphate reductoisomerase; IspD: 2-C-methyl-D-erythritol-4-phosphate cytidylyltransferase; IspE: 4-diphosphocytidyl-2-C-methyl-D-erythritol kinase; IspF: 2-C-methyl-D-erythritol-2,4-cyclodiphosphate synthase; IspG: 1-hydroxy-2-methyl-2-(E)-butenyl-4-diphosphate synthase; IspH: 4-hydroxy-3-methyl-2-(E)-butenyl-4-diphosphate reductase; Idi: Isopentenyl diphosphate isomerase; AtoB: Acetoacetyl-CoA synthase; HMGS: Hydroxymethylglutaryl-CoA synthase; HMGR: Hydroxymethylglutaryl-CoA reductase; MK: Mevalonate kinase; PMK: Phosphomevalonate kinase; PMD: Mevalonate diphosphate decarboxylase; GPPS: GPP synthase; FPPS: FPP synthase; GGPPS: GGPP synthase; TS: terpene synthase.

methyl-glutaryl-CoA (HMG-CoA) by HMG-CoA synthase. Then, mevalonic acid is formed from HMG-CoA using NADPH as a cofactor by HMG-CoA reductase. Two kinases, mecalonate kinase (MK) and phosphomevalonate kinase (PMK), sequentially catalyze the phosphorylation of mevalonate to produce mevalonate 5-diphosphate (Mev-PP). The final step of the MVA pathway to form IPP is the ATP-driven decarboxylation catalyzed by mevalo-nate diphosphate decarboxylase (PMD). The isomerization of IPP by isopentenyl diphosphate isomerase (Idi) leads to DMAPP formation and the initiation of isoprenoid biosynthesis (Fig. 1). The overall stoichiometry of the MVA pathway for synthesizing IPP from glucose is given by equation (1).

1.5 Glucose + 2 NADPH + 6 NAD = IPP + CO2 + 2 ADP + 2 NADP + 6 NADH (1)

The DXP pathway consists of seven enzymatic steps that convert glyceraldehyde 3-phosphate (G3P) and pyruvate to IPP and DMAPP in a ratio of 5:1 [3]. The DXP pathway starts with the condensation of G3P and pyruvate to produce DXP by DXP synthase (Dxs). This step is crucial and is known as the rate-limiting step of the DXP pathway. DXP is then converted to 2-C-methyl-D-erythritol-4-phosphate (MEP), 4-diphosphocytidyl-2-C-methyl-D-erythritol (CDP-ME), 4-diphosphocytidyl-2-C-methyl-D-erythritol-2-phosphate (CDP-MEP), 2-C-methyl-D-erythritol-2,4-cyclodiphosphate (MEcPP), 4-hydroxy-3-mehtyl-butenyl 1-diphosphate (HMB-PP), and IPP and DMAPP via the series of enzymatic reactions. The overall stoichi-ometry of the DXP pathway for synthesizing IPP from glucose is given by equation (2).

Glucose + 2 ATP + 3 NADPH + NAD = IPP + CO2 + 2 ADP + 3 NADP + NADH (2)

From equations (1) and (2), it can be found that the theoretical maximum IPP yield on glucose via the DXP pathway (5/6 = 0.83 C-mol/C-mol) is higher than that via the MVA pathway (5/9 = 0.56 C-mol/C-mol). However, the DXP pathway needs one mol more NADPH and 2 mol more ATP than the MVA pathway, indicating that the DXP pathway requires more energy and reducing equivalents.

As most isoprenoids were originally discovered in plants, their extraction from plant materials remains a major production route. However, it becomes increasingly difficult to meet their growing demand from plant extraction because of the slow growth rate and low isoprenoid content of plants. Microbes are an excellent alternative for overcoming this limitation, as they grow fast, require little land/water resources, and naturally produce the building blocks of all isoprenoids: IPP and DMAPP (Fig. 1). In recent years, quite a few isoprenoid compounds were overproduced in engineered microorganisms, including carotenoids, isopentanol, pinene, farnesene, limonene and bisabolene, etc. (Table 1).

Herein, we briefly reviewed recent advances in the metabolic engineering of microorganisms for isoprenoid production, focusing on carotenoids and isoprenoid-based biofuels.

2. Carotenoid products

Carotenoids are an important group of natural and liposoluble pigments with multiple physiological and nutritional functions. They were found in plants, fungi, algae and bacteria, displaying yellow, orange or red color. Carotenoids are widely used as food colorants, food and cosmetics additives, health supplements, animal feeds and nutraceuticals. At least 700 carotenoids have been characterized [4]. Carotenoids can be classified into C30, C40 and C50 carotenoids [5]. More than 95% of known carotenoids are C40 carotenoids [6]. The global carotenoids market in 2015 was $1.23

billion, with an expected increase to $1.81 billion by 2022 [7]. 2.1. Lycopene

Lycopene is one of the most widely used carotenoids in the healthcare product market owing to its excellent performance as an antioxidant and its great potential in the reduction of prostate cancer risk in humans. With the development of metabolic engineering, the heterologous expression of the lycopene biosynthetic pathway in Escherichia coli and Saccharomyces cerevisiae has become a promising strategy for lycopene production.

The following strategies have been used to improve lycopene production in E. coli 1) overexpression of the rate-limiting enzyme's genes; 2) removal of the competing pathways; 3) introduction of a heterologous MVA pathway; 4) cofactor engineering; 5) genome modifications, including promoter replacement and chromosomal evolution [8]. A shot-gun approach was used to screen native genes that should be overexpressed in lycopene production. Overexpression of the dxs, appY, crl and rpoS genes improved lycopene production [9]. Flux scanning based on enforced objective flux (FSEOF) was successfully employed for the identification of gene amplification targets for improving lycopene production. Co-overexpression of the dxs, idi and mdh genes enhanced lycopene production [10]. The deletions of gdhA and gpmAB improved lycopene production [10]. Flux balance analysis also revealed that the deletions of aceF, fdhF and gdhA enhanced lycopene production by 40% [11]. Zhou et al. reported that the zwf knockout increased lycopene production by 130% [12]. Introduction of a heterologous MVA pathway into E. coli increased the IPP supply, leading to an increase in lycopene production [13—16]. Introduction of a heterologous MVA pathway increased lycopene content up to 198 mg/g dry cell weight (DCW) from 68 mg/g DCW [15]. Our group also reported that the chromosomal heterologous expression of the optimized S. cerevisiae MVA pathway can further improve lycopene production [16]. The promoter engineered E. coli LYCOP 20 produced lycopene at 529.45 mg/L and 20.25 mg/g DCW in a fed-batch culture [16]. Combined modulating expression of sucAB, sdhABCD and talB with the regulatory part for increasing ATP and NADPH availability, and dxs, idi and crtB with the RBS libraries resulted in a significant increase in lycopene production to 3.52 g/L with a content of 50.6 mg/g DCW [17]. Zhu et al. applied the targeted engineering strategy to construct an engineered E. coli harboring the MVA and DXP pathway that produced lycopene at 1.23 g/L (34.3 mg/g DCW) in a 100-L fed-batch fermentation [18]. Kim et al. constructed an engineered E. coli co-expressing the DXP and MVA pathway that produced lycopene at1.35g/L(32 mg/g DCW) in a 2-L fed-batch fermentation [19]. Plasmid-based overexpression of genes has been the principal strategy for metabolic engineering. However, plasmid-based expression systems are not suitable because of genetic instability and the requirement for constant selective pressure to ensure plasmid maintenance. Thus, a chemically induced chromosomal evolution (CIChE), which is a plasmidfree and high gene copy expression system for engineering E. coli, was first developed to overcome these drawbacks by Tyo [20]. We applied the CIChE strategy to construct a lycopene hyper-producer E. coli that does not carry a plasmid or an antibiotic marker. The CIChE E. coli produced lycopene at 33.43 mg/g DCW [21]. De Mey's group developed a new combinatorial multigene pathway assembly approach based on Single Strand Assembly methods and Golden Gate Assembly, and they applied this approach to optimize the lycopene biosynthetic genes in E. coli overexpressing the MEP pathway to obtain a lycopene hyper-producer E. coli that produced 448 mg/g DCW of lycopene in a shake flask fermentation [22]. This yield is the highest value reported so far.

S. cerevisiae is another host strain of metabolic engineering for

F.-X. Niu et al. / Synthetic and Systems Biotechnology xxx (2017) 1—9

Table 1

Production of isoprenoids by engineered microorganisms.

Isoprenoids produced

Approach

Culture conditions

Yield/Titer

References

Lycopene E. coli Systematic (model-based) methods;

Combinatorial (transposition-based) methods; Gene knockout.

Lycopene E. coli Central metabolic genes knockout;

Amplification of MEP pathway genes Lycopene E. coli Overexpression of native dxs; Other

optimization methods (promoters, vectors, strains)

Lycopene E. coli Introduction of a heterologous MVA pathway;

Overexpressing Bacillus licheniformis idi Lycopene E. coli Optimization of MVA pathway; Promoter

engineering

Lycopene E. coli Increase ATP and NADPH; Engineering TCA

modules; Overexpression of dxs\idi\crtE Lycopene E. coli Application of the targeted engineering strategy

Lycopene E. coli Co-expression of the DXP and MVA pathway

Lycopene E. coli Application of CIChE

Lycopene E. coli Optimization of the lycopene biosynthetic

genes; Overexpressing the MEP pathway (dxs-idi-ispDF)

Lycopene S. cerevisiae Combination of directed evolution and

metabolic engineering strategy Lycopene S. cerevisiae Combination of host engineering and pathway

engineering

Lycopene Y.lipolytica Deletion of POX1 and GUT2

Lycopene S. avermitilis Activation of the silent lycopene synthetic gene

cluster

ß-cartoene E. coli Plasmid-expressing the lower MVA pathway

and idi from S. cerevisiae, Plasmid-expressing the upper MVA pathway from Enterococcus faecalis, Bacillus subtilis dxs and fni, and GPPS2 from Abies grandis; Plasmid-expressing the ß-cartoene synthetic pathway. ß-cartoene E. coli Combined engineering of the MEP, the ß-

carotene synthetic, the TCA and the pentose phosphate (PP) modules by artificial modulation parts

ß-cartoene E. coli Optimizing the biosynthetic pathway

ß-cartoene S. cerevisiae Decentralized assemble strategy ß-cartoene S. cerevisiae Using the inducer/inhibiter-free sequential

control strategy to sequentially control the expression of the carotenoid pathway, the MVA pathway and the competitive squalene pathway by glucose in the culture broth, Zeaxanthin E. coli Optimization of the zeaxanthin biosynthetic

pathway

Zeaxanthin E. coli Introduction of a dynamically controlled TIGR-

mediated MVA pathway Astaxanthin E. coli Chromosomal expressing the optimized

synthetic pathway

Astaxanthin E. coli RBS-modulated expression of the astaxanthin

biosynthetic genes

Astaxanthin E. coli Plasmid-overexpression of Pantoea ananatis

crtEIB, Pantoea agglomerans crtYZ, Brevundimonas sp. SD212 crtW and E. coli idi Astaxanthin S. cerevisiae Introduction of codon-optimized

Haematococcuspluvialis crtZ and bkt Astaxanthin S. cerevisiae combinatorial metabolic engineering and

protein engineering Astaxanthin C. glutamicum Balanced expression of crtW and crtZ Isoprene E. coli Introduction of MVA pathway, codon and RBS

optimization, deleted nine relevant genes to express ispS

Isoprene E. coli Chromosomal expressing the MVA lower

pathway; Plasmid-expression of the MVA upper pathway; Plasmid-expression of mvk from Methanosarcina mazei and isoprene synthase gene from Populus alba Isoprene E. coli Overexpression of MEP and MVA pathway;

Plasmid-expressing mvk from Methanosarcina mazei and isoprene synthase gene from Populus alba

Isoprene S. cerevisiae

Shake-flask fermentation

Shake-flask fermentation Shake-flask fermentation

Shake-flask fermentation

Fed-batch fermentation

Fed-batch fermentation

Fed-batch fermentation Fed-batch fermentation Shake-flask fermentation Shake-flask fermentation

Fed-batch fermentation

Fed-batch fermentation

Shake-flask fermentation Shake-flask fermentation

Fed-batch fermentation

Fed-batch fermentation

Fed-batch fermentation Shake-flask fermentation Fed-batch fermentation

Shake-flask fermentation Fed-batch fermentation Shake-flask fermentation Shake-flask fermentation

Shake-flask fermentation

Shake-flask fermentation

Shake-flask fermentation Shake-flask fermentation

14 L fed-batch fermentation

Fed-batch fermentation

Fed-batch fermentation

18 mg/g DCW

7.55 mg/g DCW 16.8 mg/L

198 mg/g DCW

20.25 mg/g DCW

50.602 mg/g DCW

34.3 mg/g DCW 32 mg/g DCW 33.43 mg/g DCW 448 mg/g DCW

24.41 mg/g DCW

55.56 mg/g DCW

16 mg/g DCW 82 mg/g DCW

60 mg/g DCW

3.2 g/L

11.95 mg/g DCW 23.16 mg/g DCW 7.50 mg/g DCW 5.8 mg/g DCW 8.64 mg/g DCW

4.7 mg/g DCW

8.10 mg/g DCW

0.4 mg/L/h 1832 mg/L

60 g/L

24 g/L

2527 mg/L

[12] [13]

[18] [19] [21] [22]

2.0 g/L [30]

7.41 mg/g DCW [31]

20.79 mg/g DCW, 1156 mg/L [32]

F.-X. Niu et al. / Synthetic and Systems Biotechnology xxx (2017) 1—9

Table 1 (continued )

Isoprenoids produced

Approach

Culture conditions

Yield/Titer

References

Dual metabolic engineering of cytoplasmic and mitochondrial acetyl-CoA utilization Isoprene S. cerevisiae Combining the two-level expression system

and directed evolution of ISPS Isopentenol E. coli Introduction of MVA pathway; Expressing

BsNudF gene

Isopentenol E. coli Constructing the MVA IPP-bypass pathway

Isopentenol E. coli RBS engineering of nudB; Expressing the Idi-

NudB fusion protein

Myrcene E. coli Co-overexpression of MVA pathway, AgGPPS

and ms from Quercus ilex L. Myrcene E. coli Introducing the MVA lower pathway;

Expressing the MVA upper pathway in combination with AgGPPS and SabSl Pinene E. coli Introduction of MVA pathway; Expressing

AgGPPS-Pt30 fusion protein Pinene E. coli Introduction of MVA pathway; Expressing

AgPS-AgGPPS fusion protein Pinene E. coli Introduction of MVA pathway; Expressing

PSmut-AgGPPS fusion protein Limonene E. coli Introduction of MVA pathway, Expressing the

AgGPPS—LS fusion protein Farnesene E. coli Introduction of MVA pathway; Expressing the

codon-optimized FS-lspA fusion protein Farnesene E. coli Application of In vitro reconstitution and

targeted proteomics; Overexpression of Idi with IspA and AFS in E. coli expressing synthetic MVA pathway

Farnesene S. cerevisiae Introduction of the artificial acetyl coenzyme

biosynthetic pathway (contained Dickeya zeae aldehyde dehydrogenase (acylating), Leuconostoc mesenteroides xylulose-5-phosphate specific phosphoketolase and Clostridium kluyveri phosphotransacetylase) with the NADH-consuming HMG-CoA reductase from Silicibacter pomeroyi Bisabolene E. coli Co-expressing the codon-optimized AgBIS and

the optimized MVA pathway Bisabolene S. cerevisiae Co-expressing the codon-optimized AgBIS and

the optimized MVA pathway Bisabolene S. cerevisiae Screening the yeast knockout libraries; Co-

expressing the MVA pathway and BIS gene Farnesol E. coli Co-expressing ispA and the MVA pathway

Farnesol E. coli Overexpressing ispA, pgpB and the MVA

pathway

Farnesol S. cerevisiae Overexpressing the truncated HMG-CoA

reductase

Fed-batch fermentation

Shake-flask fermentation

Shake-flask fermentation Shake-flask fermentation

Shake-flask fermentation

Fed-batch fermentation

Fed-batch fermentation Shake-flask fermentation Shake-flask fermentation Shake-flask fermentation Shake-flask fermentation Shake-flask fermentation

200,000 L bioreactor fed-batch fermentation

Shake-flask fermentation

Shake-flask fermentation

Fed-batch fermentation

Shake-flask fermentation Shake-flask fermentation

5L fed-batch fermentation

3.7 g/L

1.3 g/L

705 mg/L 2.23 g/L

58.19 mg/L

2.65 g/L

0.97 g/L 32.4 mg/L 150 mg/L 435 mg/L 380 mg/L 1.1 g/L

130 g/L

912 mg/L

994 mg/L

5.2 g/L

135.5 mg/L 526.1 mg/L

145 mg/L

[52] [54]

[60] [61] [62] [63]

[70] [70]

carotenoid production. Xie et al. applied a combined directed evolution and metabolic engineering strategy to construct an engineered S. cerevisiae that produced 1.61 g/L (24.41 mg/g DCW) of lycopene in a fed-batch fermentation [23]. Some distantly located genetic loci may have potential interactions with the target pathway. The deletions of these distant genes (YPL062W, YJL064W, ROX1 and DOS2) improved carotenoid production in S. cerevisiae [24,25]. Chen et al. constructed an engineered S. cerevisiae by combining host engineering (distant genetic loci and cell mating types) with pathway engineering (enzyme screening and gene fine tuning) for lycopene production, which produced 1.65 g/L (55.56 mg/g DCW) of lycopene in a 5-L bioreactor fed-batch fermentation [25]. Yarrowia lipolytica is another yeast that has been successfully used for lycopene production. The deletion of POX1 and GUT2, which led to an increase in the size of lipid bodies, significantly enhanced lycopene production (16 mg/g DCW) in Y lipolytica [26].

Streptomyces avermitilis has also been successfully used for lycopene production. After activating the silent lycopene synthetic gene cluster in S. avermitilis, 82 mg/g DCW of lycopene was produced in a shake flask fermentation [27].

2.2. ¡3-Carotene

b-Carotene is a carotenoid compound that has been widely used in the industrial production of not only pharmaceuticals but also nutraceuticals, animal feed additives, functional cosmetics, and food colorants. b-Carotene functions as provitamin A, and it is responsible for the synthesis of retinoids. b-Carotene is the cycli-zation product of lycopene by lycopene b-cyclase (CrtY) (Fig. 1 ). The heterologous expression of the b-carotene biosynthetic genes in non-carotenogenic microbiology, e.g., E. coli and S. cerevisiae, has become a main alternative means of b-carotene production. E. coli co-overexpressing the optimized MEP pathway (Bacillus subtilis dxs and fni, and GPPS2 from Abies grandis) and the MVA pathway produced 3.2 g/L of b-carotene in a fed-batch fermentation [28]. ATP and NADPH are two important cofactors for terpenoid compounds. Combined engineering of the MEP, the b-carotene synthetic, the TCA and the pentose phosphate (PP) modules by artificial modulation parts resulted in a significant increase in the b-carotene yield. The final strain, E. coli CAR005, produced 2.1 g/L b-carotene with a yield of 60 mg/g DCW [29]. After integrating the b-carotene biosynthetic pathway into the E. coli genome and optimizing the

MEP, central metabolic pathway and b-carotene biosynthetic pathway, the engineered E. coli produced 2.0 g/L of b-carotene in fed-batch fermentation [30]. Yu's group developed a decentralized assembly strategy to construct a controllable multigene pathway, and then they applied this strategy to construct a controllable b-carotene biosynthetic pathway in S. cerevisiae. The resulting strain produced 7.41 mg/g DCW of b-carotene [31]. They then established an inducer/inhibiter-free sequential control strategy in S. cerevisiae by combining a modified GAL regulation system and a HXT1 promoter-controlled squalene synthetic pathway [32]. They applied this strategy to sequentially control the expression of the carotenoid pathway, the MVA pathway and the competitive squa-lene pathway by glucose in the culture broth, resulting in marked increase in b-carotene production, which reached 20.79 mg/g DCW [32].

2.3. Zeaxanthin

Zeaxanthin (3,3'-dihydroxyl-b-carotene) is a yellow oxygenated carotenoid composed of 40 carbon atoms that is used as a food additive and as a feed additive for fish (color enhancement for the flesh) and poultry (yolk and skin pigmentation) [33]. Zeaxanthin plays a critical role in preventing age-related macular degeneration and cancer and may protect against age-related cataract formation [34,35]. The hydroxylation of each ring of b-carotene by b-carotene hydroxylase (CrtZ) produces zeaxanthin (Fig. 1). Co-overexpression of the dsx and idi genes in engineered E. coli harboring the zeax-anthin biosynthetic pathway had an additive effect on zeaxanthin production, which reached 1.6 mg/g DCW [36]. It has been reported that CrtZ is the rate-limiting step in zeaxanthin biosynthesis and a higher expression level of crtZ should be required for zeaxanthin production [37,38]. We compared Pantoea ananatis, Pantoea agglomerans and Haematococcus pluvialis crtZ and reported that P. ananatis crtZ is superior to those from P. agglomerans or H. pluvialis for zeaxanthin production [38]. E. coli BETA-1 containing pZSBA-2(P37-crtZPAN) produced 11.95 mg/g DCW of zeaxanthin

[38]. To balance the expression of the multigene, the tunable intergenic region (TIGR)-mediated MVA pathway was introduced into the zeaxanthin-producing strain, E. coli ZEAX, leading to an increase in zeaxanthin production [39]. However, IPP and FPP are toxic when they accumulate in E. coli. To avoid the accumulation of IPP or FPP, a dynamically controlled TIGR-mediated MVA pathway was introduced into the zeaxanthin producing strain, E. coli ZEAX, markedly enhancing its zeaxanthin production, which achieved 722.46 mg/L (23.16 mg/g DCW) in a 5.0-L fed-batch fermentation

2.4. Astaxanthin

Astaxanthin is a highly valued keto-carotenoid with strong antioxidant activity and singlet oxygen quenching ability. The pathway from b-carotene to astaxanthin is a crucial step in the synthesis of astaxanthin. This pathway requires two bifunctional enzymes: b-carotene hydroxylase CrtZ to add hydroxyl functional groups to carbons 3 and 3' of b-carotene and b-carotene ketolase CrtW to add keto functional groups to carbons 4 and 4' of b-carotene (Fig. 1). The two enzymes are bifunctional proteins with respect to their substrate specificity. CrtZ can convert not only b-carotene to zeaxanthin but also canthaxanthin to astaxanthin. CrtW is capable of converting not only b-carotene but also zeaxanthin. Consequently, the heterologous expression of crtZ and crtW in a b-carotene-producing strain results in the accumulation of eight intermediates (echinenone, canthaxanthin, adonirubin, b-cryptox-anthin, zeaxanthin, adonixanthin, 3-hydroxyechinenone and 3'-hydroxyechinenone), which affects the percentage of astaxanthin

that is produced relative to the total carotenoid content. The CrtW and CrtZ enzymes from different sources show different activities and substrate specificities. Thus, optimal astaxanthin biosynthesis requires careful control of the carbon flux along a cooperative function of these two proteins. It has been suggested that astax-anthin biosynthesis proceeds from b-carotene through hydroxyl-ation first, and then onto ketolation [40]. To increase the astaxanthin percentage relative to the total carotenoid content, we compared the conversion efficiency to astaxanthin in four CrtWs, which had higher efficiency for astaxanthin production reported in literature, with recombinant E. coli cells that synthesizes zeax-anthin due to the presence of the P. ananatis crtEBIYZ and found that the Brevundimonas sp. SD212 crtW and P. ananatis crtZ genes are the best combination for astaxanthin production [41]. After tune-fining the crt genes, an astaxanthin producer E. coli ASTA-1 that does not carry a plasmid or antibiotic marker was constructed. The engineered strain E. coli ASTA-1 produced 7.50 mg/g DCW of astaxanthin with an astaxanthin ratio of 96.6% relative to the total carotenoid content in a shake flask fermentation [41]. The ratio of astaxanthin to the total carotenoids (96.6%) is the highest value reported to date. Balanced expression of the astaxanthin biosyn-thetic genes with a compact set of ribosome binding sites led to an astaxanthin accumulation of 5.8 mg/g DCW in E. coli [42]. Ma et al. identified and characterized the astaxanthin-producing ability of Sphingomonas sp. ATCC 55669 by complete genome sequencing, and then compared the astaxanthin biosynthetic efficiency of the crt genes from different microorganisms in E. coli. The resulting E. coli plasmid-expressing P. ananatis crtEIB, P. agglomerans crtYZ, B. sp. SD212 crtW and E. coli idi produced 8.64 mg/g DCW [43].

An astaxanthin producing S. cerevisiae was constructed by integrating two copies of the codon-optimized H. pluvialis crtZ and bkt in b-carotene producing S. cerevisiae. The engineered S. cerevisiae produced 4.7 mg/g DCW of astaxanthin in a shake-flask culture [44]. The group recently applied combinatorial metabolic engineering and protein engineering to markedly enhance astax-anthin production S. cerevisiae, which reached 8.10 mg/g DCW in shake-flask cultures [45].

Recently, Corynebacterium glutamicum has been engineered for astaxanthin production, and it reached 1.6 mg/g DCW [46].

In addition, the titer of astaxanthin is much lower than that of other carotenoids (lycopene, b-carotene and zeaxanthin). Because very few carotenoids were detected in our engineered strain E. coli ASTA-1, we guess that the lower astaxanthin yield may be because the recombinant enzyme (b-carotene hydroxylase and ketolase) or product of their enzymatic reaction affects the formation of the carotenoid precursors upstream of phytoene. Therefore, further efforts focused on astaxanthin production should be carried out.

3. Isoprenoid-based biofuels

Methyl branching and cyclic structures are commonly observed in isoprenoids. The methyl branching structure lowers the freezing point significantly. The cyclic structures increase the energy density and are generally considered valuable features for jet fuels. In recent years, some isoprenoids have been tested and produced as potential diesel and gasoline fuel alternatives because of their lower hygroscopy, higher energy content and good fluidity at low temperatures.

3.1. Hemiterpenoid-based biofuels

Isoprene (C5H8) is the simplest isoprenoid. It is used to produce millions of tons of rubber annually and has been suggested as a liquid fuel [47]. Co-overexpression of Populus trichocarpa codon-optimized isoprene synthase gene ISPS and the MVA pathway

genes in the 9-gene knockout E. coli AceCo improved isoprene production, reaching 1832 mg/L in a shake-flask culture [48]. Plasmid-expression of the upper pathway of MVA in concert with the P. alba isoprene synthase gene ISPS plus the mevalonate kinase and phosphogluconolactonase gene in E. coli integrated the lower pathway of MVA from S. cerevisiae and resulted in production of 60 g/L isoprene with a mass yield of isoprene from glucose in a 14-L fed-batch fermentation [49]. Fed-culture of the engineered E. coli overexpressing the synergistic dual pathway of MVA and MEP resulted in the production of 24.0 g/L isoprene with a yield of 0.267 g/g [50]. The isoprene synthase gene has also been introduced in S. cerevisiae for isoprene production. In recent years, organelle engineering of yeast has attracted increasing attention in the biosynthesis of chemicals. Dual metabolic engineering of the cytoplasmic and mitochondrial acetyl-CoA increased isoprene production in S. cerevisiae, reaching 2527 mg/L in a fed-batch fermentation [51]. A two-level expression system was developed for the PGAL!-controlled ISPS by overexpression of GAL4 [52]. Combining the two-level expression system and directed evolution of ISPS in S. cerevisiae led to the production of 3.7 g/L in a fed-batch fermentation [52].

Ester of isoprenoid alcohols (C5, C10 and C15) have the potential to be used as replacements for petroleum-based diesels. B. subtilis nudF and E. coli nudB have been introduced into E. coli for iso-prentol/isoprenol production [53,54]. Overexpression of some isopentenol tolerance-enhancing genes, such as metR and mdlB, improved the production of isopentenol in E. coli [55]. A novel IPP-bypass MVA pathway was reported for isopentenol production in E. coli. The IPP-bypass MVA pathway contains the decarboxylation of mevalonate phosphate by PMD and the hydroxylation of iso-pentenyl monophosphate (IP) by E. coli phosphatase AphA [56]. George et al. constructed an E. coli with a high yield in 3-methyl-3-buten-1-ol production [57]. A titer of 2.23 g/L isoprenol was obtained by using an oleyl alcohol overlay in the engineered E. coli. This is the highest yield achieved from an engineered stain.

3.2. Monoterpenoid-based biofuels

Monoterpenoids are C10 compounds built from two isoprenoid units (one IPP and one DMAP). Monoterpenoids can be divided into three major subgroups based on their structural features: 1) acyclic monoterpenes, such as myrcene and ocimene; 2) monocyclic monoterpenes, such as limonene, menthol, and carvone; 3) bicyclic monoterpenes, such as pinene, sabinene, and camphor.

Co-overexpression of the MVA pathway, A. grandis GPPS2 and the Quercus ilex L. myrcene synthase gene in E. coli resulted in the production of 58.19 mg/L myrcene [58]. E. coli harboring the MVA pathway, A. grandis GPPS2 and the Salvia pomifera sabinene syn-thase gene sabs1 produced 2.65 g/L sabinene in a fed-batch fermentation [59].

A novel biosynthetic pathway of a-pinene was assembled in E. coli BL21(DE3) with the heterologous MVA pathway, codon-optimized GGPS from A. grandis and codon-optimized a-pinene synthase Pt30 from Pinus taeda [60]. The final producing strain YJM28 produced 5.44 mg/L in a shake-flask fermentation and 0.97 g/L a-pinene in a fed-batch fermentation. Sarria et al. combi-natorically expressed three pinene synthase (PS) and three GPPS from conifers in engineered E. coli harboring the MVA pathway. They achieved approximately 28 mg/L pinene using the best combination (PS and GGPS from A. grandis). Furthermore, they designed GPPS-PS protein fusions to reduce GPP product inhibition and toxicity by substrate channeling, producing 32.4 mg/L pinene in a shake-flask fermentation [61]. PS is the rate-limiting enzyme for pinene biosynthesis. To significantly improve the activity of PS, a-pinene synthase Pt1 from P. taeda was evolved to obtain a PS

mutant PSD380A. They expressed the PS mutant and GPPS from A. grandis in the engineered E. coli harboring the MVA pathway and achieved 150 mg/L pinene in a shake-flask fermentation [62]. An engineered E. coli expressing the MVA pathway, codon-optimized GGPS from A. grandi and codon-optimized limonene synthase from Mentha spicata on one plasmid produced 400 mg/L limonene in a shake-flask fermentation [63].

Monoterpenes have been reported to be highly toxic, resulting in low microbial production of monoterpene. Overexpression of efflux pump or tolerance-enhancing genes has become a common strategy for improving monoterpene production [64,65]. Overexpression of the efflux pump gene (YP-692684) from Alcanivorax borkumensis significantly improved tolerance and enhanced limonene production [64]. This tolerance engineering strategy has also successfully been applied for improving the production of iso-pentenol, olefin and other biofuels [55,64—66].

3.3. Sesquiterpenoid-based biofuels

Sesquiterpenoids are one of the largest groups of isoprenoid natural products and have a wide range of activities from antimicrobial agents (such as phytoalexins capsidiol) to alarm phero-mones (such as farnesene). Structurally, sesquiterpenoids can be acyclic, monocyclic, bi- or even tricyclic with different TPS that catalyze FPP into a large variety of sesquiterpenes. Sesquiterpenoids are 15 carbons, close to the average length of diesel (C16), but with a branched, rather than a straight-chain structure. Among sesquiterpenoids, farnesol, farnesene and bisabolane have been proposed as diesel fuels and produced from IPP.

Combinational expression of the heterologous MVA pathway and the fused proteins of IspA/AFS led to an approximate 317-fold increase over the initial production of farnesene in E. coli. The final engineered E. coli produced approximately 380 mg/L farne-sene in a shake-flask fermentation [67]. In vitro studies on the purified protein components of MVA and the downstream FPP pathway have revealed that Idi played a key role in a-farnesene synthesis in vitro [68]. Based on the in vitro studies, farnesene production was optimized through overexpression of Idi with IspA and AFS in E. coli expressing the synthetic MVA pathway. After 96 h of induction, farnesene production reached a concentration of approximately 1.1 g/L in a shake-flask fermentation [68]. Meadows et al. constructed an artificial cytosolic acetyl coenzyme biosyn-thetic pathway with a reduced ATP requirement, which contained Dickeya zeae aldehyde dehydrogenase (acylating) (ADA), Leuco-nostoc mesenteroides xylulose-5-phosphate specific phosphoketo-lase (PK) and Clostridium kluyveri phosphotransacetylase (PTA) [69]. Combining the artificial acetyl coenzyme biosynthetic pathway with the NADH-consuming HMG-CoA reductase from Silicibacter pomeroyi in S. cerevisiae enhanced farnesene production, which reached 130 g/L in 200, 000-L bioreactor fed-batch fermentation [69].

Bisabolene, a monocyclic sequiterpene, has been identified as a precursor to a potential D2 diesel fuel. To obtain higher titers of bisabolene, bisabolene synthase (BIS) genes from Arabidopsis thaliana, Pseudotsuga menziesii, A. grandis and Picea abies have been screened in E. coli (harboring the entire MVA pathway in a single plasmid) and S. cerevisiae [70]. Overexpression of the codon-optimized AgBIS in an engineered E. coli expressing the optimized MVA pathway resulted in production of 912 mg/L of bisabolene. The same level of bisabolene was also obtained in the engineered S. cerevisiae with an overproduction of FPP [70]. Kirby et al. reported a novel route from ribulose 5-phosphate (Ru5P) to DXP (nDXP) and uncovered two nDXP genes: ribBG108S and yajO. Expression of a Dxr-RibB(G108S) fusion improved bisabolene titers more than 4-fold [71]. Using a carotenoid-based phenotypic screen of the yeast

deletion collection, the genes that affected isoprenoid synthesis in yeast were identified. Combinations of these deletions and other MVA pathway modifications improved the titers of bisabolene more than 20-fold to 800 mg/L in a flask and 5.2 g/L in a fermentation process [72].

Farnesol is an important C15 isoprenyl alcohol. Co-overexpression of the heterologous MVA pathway and ispA in E. coli led to the production of 135 mg/L farnesol [73]. Overexpression of IspA and the membrane phosphatase PgpB, along with a heterologous MVA pathway in E. coli, increased farnesol production to 526.1 mg/L in a shake flask fermentation [74]. A farnesol production of 145 mg/L was attained by S. cerevisiae ATCC 200589 with overexpression of HMG-CoA reductase (Hmgl) in a fermentation culture for 7 days [75].

4. Conclusions

Carotenoids and isoprenoid-based biofuels are two classes of important isoprenoids. With advances in metabolic engineering and synthetic biology, engineered microorganisms have become a primary alternative for their production. Most studies on carot-enoid production are focused on the regulation of carbon flux. Our results from the comparative proteomes demonstrate that zeax-anthin overproduction may be associated with not only precursor availability but also cofactor availability, oxidative stress response, and membrane storage capacity [39]. Morphology engineering for increasing member storage capacity may be another strategy for improving carotenoid production.

Some isoprenoids have been proposed as biofuels. However, the levels of isoprenoid-based biofuels are lower than the order of magnitude of those of carotenoids. Moreover, the titers of mono-terpenes are lower than those of hemiterpenes and sesquiterpenes. Toxicity and enzyme activity may be major factors. Tolerance engineering and the evolution of enzymes may be effective strategies for improving yields.

Acknowledgments

We are grateful to the National Natural Science Foundation of China (Grant NO. 21276289), the Natural Science Foundation of Guangdong Province (NO. 2015A030311036), the Project of the Scientific and Technical Program of Guangdong Province (NO. 2015A010107004) and the Project of the Scientific and Technical Program of Guangzhou (NO. 201607010028) for their financial support.

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