# Metabolic Engineering _Escherichia coli_ for the Production of Lycopene
## 2 Metabolic Engineering of Two Major Pathways in _E. coli_ for Lycopene Production
### Metabolic Engineering of the Heterogeneous MVA Pathway  
Although the MEP pathway is natively present in _E. coli_[16], the utilization of this pathway has not shown high efficiency and exhibits low metabolite flux [23]. In contrast, the MVA pathway, an alternative pathway for lycopene production, is more energy-saving than the MEP pathway [30], and its introduction paved a new way for the formation of IPP from acetyl-CoA, resulting in a high-efficient lycopene production in _E. coli_. The MVA pathway is composed of the upper and lower pathway, separate from MVA [53]. When the whole MVA pathway from acetyl-CoA to IPP was engineered into _E. coli_, lycopene production increased by over 2-fold compared with the control strain [35]. Similarly, when only the lower pathway was employed, and mevalonate was supplied as a substrate, a significant increase in lycopene production was also observed [54]. These results indicated that the introduction of either the complete or the partial MVA pathway could reach an effective lycopene yield when a carbon source or carbon source with mevalonate were supplied, respectively, both through increasing lycopene precursors IPP and DMAPP. The heterologous MVA pathway was transferred into _E. coli_ DH5\(\alpha\), combined with the overexpression of a type 2 IDI from _Bacillus licheniformis_ significantly elevating lycopene production [36]. However, these studies were all implemented at the shake-flask fermentation level, not yet scalable for industrial production. Therefore, [PERSON] et al. adopted a new, targeted engineering strategy to reconstitute the MVA pathway in _E. coli_ and establish a highly efficient platform which was employed for lycopene production. The fed-batch fermentation process was scaled up to 100 L, reaching 1.23 g/L of lycopene concentration with a maximum productivity of 74.5 mg/L/h [47]. In addition, [PERSON] et al. had conducted metabolomics analysis to reveal the toxic effects of lycopene production and the metabolic differences caused by induction time variation of the MVA pathway in the engineered _E. coli_ strain. They reported that overnight induction of the MVA pathway was toxic to cells, which could recover if the lycopene pathway was not heterologously expressed simultaneously. Further, they validated that the intermediate homocysteine could contribute to the growth inhibition and the antagonistic effect between the mevalonate and lycopene pathways, resulting in the homocysteine-induced toxicity in lycopene production. This work indicated that metabolomics would be beneficial to reveal the mechanisms of the metabolite toxicity, and subsequently help to improve the metabolic engineering for the biosynthesis of carotenoid [37].  
The introduction of the MVA pathway into _E. coli_ caused dramatic improvement in lycopene production and could serve as a platform for the production of carotenoid compounds. This was attributed to the better elucidation of the MVA pathway than the MEP pathway. Moreover, the endogenous MEP pathway could be influenced by native regulation. In contrast, the introduction of the exogenous MVA pathway could play its role without regulation. However, because of the common intermediates (IPP and DMAPP), the two pathways are not totally independent, which provides potential optimization strategies for balancing two different pathways for carotenoid compounds production, including lycopene.