Enhanced Coenzyme Q10 Yield by Blocking Hopanoids Pathway

Coenzyme Q10 (CoQ10), otherwise known as ubiquinone (UQ), is a valuable bioactive compound used both medically and cosmetically [1]. Many studies focus on enhancing CoQ10 production in Escherichia coli (E.coli) by genetic engineering [29]. Agrobacterium tumefaciens also have been employed as the producers of CoQ10 [10,11]. Typical purple non-sulfur bacteria (PNSB) generally have a relatively high content of CoQ10, as ideal CoQ10 producers [12-16]. R. Palustris (Rhodopseudomonas palustris) is a PNSB with a high content of inherent CoQ10, whose fermentation processes have been well researched over the years [17,18]. Therefore, R. Palustris could be a potential host for CoQ10 production by metabolic engineering.

R. palustris TIE-1 contains more than 30mg/l DCW (dry cell weight) of hopanoids that are not required for growth under normal conditions, although they play a role in membrane integrity and pH homeostasis [19]. The deletion of squalenehopene cyclase protein (Shc), which cyclizes squalene to the basic hopene structure, can make the strain no longer produce any hopanoids [19]. As the synthesis of hopanoids occurs as a branched pathway of CoQ10 synthesis in R. palustris TIE- [22] by increasing cyclic photophosphorylation and photosynthesis, We hypothesized that the photophosphorylation could be increased by NaHSO3 in R. palustris, and the accumulation of CoQ10 might be improved because of that CoQ10 functions as an electron-transfer agent in photophosphorylation [23] (Figure 1).

Medium, culture conditions and general methods
The bacteria seed cultivation was performed at 30 0 c in YP medium (3g/l peptone, 3/l yeast extract, pH 6.5) for 48h under aerobic condition. The engineered R. palustris strains for squalene overproduction were cultured at 30 0 c in a designed medium (2g/l sodium succinate, 10g/l glucose, 3g/l peptone, 3g/l yeast extract, 2g/l KH 2 PO 4 , 2g/l K 2 HPO 4 , 2g/l MgSO 4 •7H 2 O, 0.5g/l FeSO 4 .7H 2 O, 5ml/l mineral solution, pH 6.5) under anaerobic condition. The mineral solution contained: 1g/l NaMoO 4 •2H 2 O, 2g/l CuSO 4 •5H 2 O, 1g/l ZnSO 4 , 1g/l H 3 BO 3 , 1g/l MnCl 2 . Anaerobic cultivation was achieved via static culture in the presence of nitrogen with white fluorescent light (3200 lx). Kanamycin of 350mg/l was supplemented to the culture media to retain the constructed plasmids with corresponding antibiotic selection markers. All the cell cultures were grown for 120h to be in the stationary phase, before the cells were collected for CoQ10 analysis.

Plasmid construction
For gene overexpression, the primers F_dps_EcoRI (GGAATTCGTGAATGGGATTGGACGAG GTTTCG) and R_dps_ BamHI (CGGGATCCTCAGGCGATGCGTTCGACCA) were used to amplify dps gene from Rhodobacter sphaeroides. And then, the gene segment was ligated into pMG103 between EcoR I and BamH I sites to generate the plasmid pMGD. The ubiA gene was amplified with the primers F_ubiA_BamHI (CGGGATCCATGAGTGGAATTCCGGCCAG) and R_ubiA_ HindIII (CCCAAGC TTTCACGCCATGCTGCGCGAGA) containing BamHI and HindIII sites and ligated into pMGD to generate pMGDU.

CoQ10 and lycopene assay
Cell growth was measured using a spectrophotometer at 600nm and converted to dry cell weight using a prepared standard curve of DCW versus OD600•Vol. CoQ10 was extracted with an organic solvent from the saponified liquid before being subjected to high-performance liquid chromatography (Hitachi, Tokyo, Japan), according to the procedure described before [13]. The extraction of lycopene was carried out according to previous research [24], its content was estimated via HPLC at 475nm [25].

Increased CoQ10 content in R. palustris (Δshc)
For accumulating squalene in R. palustris, the shc gene was deleted to block hopanoids pathway in our previous study [26]. Although the titer of squalene in R. palustris (Δshc) reached 3.8mg/g DCW, it was much lower than the yield of total hopanoids (about 30mg/g DCW) in the wild strain, suggesting that many FPPs had been directed into other pathways. Here, R. palustris (Δshc) was employed to study the accumulation of CoQ10 and carotenoids. Lycopene, a metabolite in the carotenoids pathway, was analyzed to represent the accumulation of carotenoids.
As shown in Figure 2, the CoQ10 content of R. palustris (Δshc) reached 3.71mg/g DCW, which corresponds to a 34.7% improvement over the parental strain R. palustris TIE-1. Moreover, R. palustris (Δshc) yielded a 46.2% improvement of the lycopene content over R. palustris TIE-1. These results suggested that shc deletion diverted the FPP flux from the production of hopanoids to the biosyntheses of both CoQ10 and carotenoids. It is noteworthy that there was not a noticeable decrease in biomass of the recombinant strain, demonstrating the hopanoids pathway blocking was an efficient strategy for improving CoQ10 production. However, carotenoids content increased more significantly, means that the increase in CoQ10 production might be restricted by the increased carotenoids yield because it consumed more FPP (Figure 2).

CoQ10 production improvement by the co-expression of dps and ubiA
To divert more FPP to the biosynthesis of CoQ10, decaprenyl diphosphate synthase (Dps) from Rhodobacter sphaeroides 2.4.1 which catalyzes FPP to form DPP was expressed. The investigation revealed that the CoQ10 content of R. palustris (Δshc)/pMGD was increased to 4.38mg/g DCW but the CoQ10 content in R. palustris TIE-1/pMGD only reached 3.15mg/g DCW (Table 1). It was clear that the increment of CoQ10 content in R.

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palustris (Δshc)/pMGD was higher than that in R. palustris TIE-1/pMGD. In the meantime the lycopene content in R. palustris (Δshc)/pMGD was decreased from 5.06mg/g DCW to 4.67mg/g DCW (Table 1). Results suggested that the Dps expression enhanced FPP flux to the biosynthesis of CoQ10 and decreased the FPP flux to carotenoids pathway. However, the increment of CoQ10 content was not as high as expected. It was hypothesized that the limited pools of pHBA and UbiA restricted the utilization of DPP for CoQ10 biosynthesis, although more DPP had been formed by Dps expression.
UbiA which catalyzes pHBA and DPP to form decaprenyl-pHBA is one of the rate-limiting enzymes in the synthetic pathway of CoQ10. And its overexpression has been shown to increase CoQ10 content, especially when accompanied with the supplementation of pHBA [2,6]. To increase the utilization of DPP for CoQ10 biosynthesis, endogenous UbiA was over expressed along with the pHBA supplementation (100mg/l). As can be seen in Table 1, the CoQ10 content was increased to 6.16mg/g DCW in R. palustris (Δshc)/pMGDU, while that was 4.42mg/g DCW for R. palustris TIE-1/pMGDU. Notably, the lycopene content was further decreased to 3.92 mg/g DCW. Obviously, the expression of UbiA and supplementation with pHBA enhanced the competitiveness of CoQ10 pathway for FPP. However, the cell mass was slightly decreased, which may caused by metabolic burden due to protein overexpression (Table 1). To study the influence of NaHSO3 on the cell growth and CoQ10 production of R. palustris TIE-1, NaHSO 3 at different levels was supplemented into the culture medium. As shown in Figure 3, both the biomass and the CoQ 10 content were increased in the presence of NaHSO 3 . The supplementation with 0.5mM NaHSO 3 resulted in 6.51mg/g DCW CoQ10 and 1.33g/l biomass, generating the highest CoQ10 production of 8.65mg/g, corresponding to a 33.2% improvement over the control ( Figure  3). The increases in biomass and CoQ10 content indicated that NaHSO 3 increased the photo-phosphorylation with intensified electron transport efficiency, and thus the accumulation of CoQ10 was enhanced as one of the electron transfer agents. In future, more studies are needed to explore the functional mechanism of NaHSO3 ( Figure 3).

Conclusion
In this study, we report several strategies, including hopanoids pathway blocking, genes co-expression and NaHSO 3 supplementation, for enhancing CoQ10 production. Based on the combination of these strategies, the content of CoQ10 reached 6.51mg/g DCW, which was 1.3-times higher than that for wildtype strain. This work enriched the strategy for metabolic engineering and showed the potential of producing coenzyme Q10 by Rhodopseudomonas palustris TIE-1.