pH regulatory divergent point for the selective bio-oxidation of primary diols during resting cell catalysis

Abstract

Background: Hydroxyl acid is an important platform chemical that covers many industrial applications due to its dual functional modules. At present, the traditional technology for hydroxyl acid production mainly adopts the petroleum route with benzene, cyclohexane, butadiene and other non-renewable resources as raw materials which violates the development law of green chemistry. Conversely, it is well-known that biotechnology and bioengineering techniques possess several advantages over chemical methods, such as moderate reaction conditions, high chemical selectivity, and environmental-friendly. However, compared with chemical engineering, there are still some major obstacles in the industrial application of biotechnology. The critical issue of the competitiveness between bioengineering and chemical engineering is products titer and volume productivity. Therefore, based on the importance of hydroxyl acids in many fields, exploring a clean, practical and environmental-friendly preparation process of the hydroxyl acids is the core purpose of this study.

Results: To obtain high-purity hydroxyl acid, a microbiological regulation for its bioproduction by Gluconobacter oxydans was constructed. In the study, we found a critical point of chain length determine the end-products. Gluconobacter oxydans catalyzed diols with chain length ≤ 4, forming hydroxyl acids, and converting 1,5-pentylene glycol and 1,6-hexylene glycol to diacids. Based on this principle, we successfully synthesized 75.3 g/L glycolic acid, 83.2 g/L 3-hydroxypropionic acid, and 94.3 g/L 4-hydroxybutyric acid within 48 h. Furthermore, we directionally controlled the products of C5/C6 diols by adjusting pH, resulting in 102.3 g/L 5‑hydroxyvaleric acid and 48.8 g/L 6-hydroxycaproic acid instead of diacids. Combining pH regulation and cell-recycling technology in sealed-oxygen supply bioreactor, we prepared 271.4 g 5‑hydroxyvaleric acid and 129.4 g 6-hydroxycaproic acid in 6 rounds.

Conclusions: In this study, a green scheme of employing G. oxydans as biocatalyst for superior-quality hydroxyl acids (C2-C6) production is raised up. The proposed strategy commendably demonstrated a novel technology with simple pH regulation for high-value production of hydroxyl acids via green bioprocess developments.

Keywords: Hydroxyl acid; Oxidation; Sealed-oxygen supply; Whole-cell catalysis; pH regulation.

Fig. 1

Bioreactor operation model of sealed oxygen-supplied bioreactor (SOS-BR) for the whole-cell catalysis

Fig. 2

Whole-cell catalysis of diols by G. oxydans NL71 with pH control at 5.5. A: EG; B: 1,3-PG; C: 1,4-BG; D: 1,5-PG; E: 1,6-HG. Line: diol content (blue); accumulated hydroxyl acid content (red); accumulated diacid content (green); diol consumption rate (purple)

Fig. 3

Comparison of the whole-cell catalysis for GA, 3-HPA and 4-HBA production by G. oxydans with SOS technology. The symbols indicated accumulated GA content (■), and accumulated 3-HPA content (●) and accumulated 4-HBA content (▲) in the broth. The columns represented the average productivity of the three hydroxyl acids

Fig. 4

Comparison of the whole-cell catalysis of 1,5-PG by G. oxydans with different pH regulation. A: pH = 2.5; B: pH = 3.5; C: pH = 4.5; D: pH = 5.5; E: pH = 6.5. The symbols indicated the concentration of 1,5-PG (■), and accumulated 5-HVA content (●) and accumulated GTA content (▲) in the reaction

Fig. 5

Whole-cell catalysis of 1,5-PG by G. oxydans with pH control at 5.5. A: SOS technology, the symbols indicated the concentration of 1,5-PG (■), accumulated 5-HVA content (●), accumulated GTA content (▲) and the dotted line represented the DO level. B: cell recycling technology, the symbols indicated the accumulation of 5-HVA (■) and the columns represented the 5-HVA production achieved in each round

Fig. 6

Comparison of the whole-cell catalysis of 1,6-HG by G. oxydans with different pH regulation. A: pH = 3; B: pH = 4; C: pH = 5; D: pH = 6; E: pH = 7. The symbols indicated the concentration of 1,6-HG (■), and accumulated 6-HCA content (●) and accumulated AA content (▲) in the reaction

Fig. 7

Whole-cell catalysis of 1,6-HG by G. oxydans with pH control at 7. A: SOS technology, the symbols indicated the concentration of 1,6-HG (■), accumulated 6-HCA content (●), accumulated AA content (▲) and the dotted line represented the DO level. B: cell recycling technology, the symbols indicated the accumulation of 6-HCA (■) and the columns represented the AA production achieved in each round