Bi-directional switch regulation of three metals for cleaner bio-production of gluconic acid by whole-cell catalysis

Abstract

Gluconic acid (GA) has been widely applied as a value-added platform biochemical, and the whole-cell catalysis of glucose for GA bioproduction by Gluconobacter sp. NL71 has shown great commercial competitiveness. In bioprocess, GA will be further dehydrogenated by Gluconobacter sp. NL71 to generate the by-products of 2-/5-/2,5-ketogluconic acid (keto-GA) that leads yield decrease and troublesome impurity problems. In this paper, the bi-directional switch regulation strategy of three metals on whole-cell catalysis was explored and developed to cleaner production of GA without by-products. With the couple dance performance of three metals of Ca2+, Mg2+ and Cu2+, a precise two-switch controlling on whole-cell catalysis and bio-oxidation was successfully achieved that results in the complete bio-conversion of glucose to GA without any keto-GA. Ca2+ and Mg2+ plays a positive activator to glucose dehydrogenation to GA, while Cu2+ works as a strong negative inhibitor to GA further oxidation to 2-/5-/2,5-keto-GA. Under the intensification with the superimposition of 100 mmol/L Mg2+, 60 mmol Ca2+ and the auxiliary regulation with 40 mmol Cu2+, 52.3 g/L glucose was transformed into 48.9 g/L GA by whole-cell catalysis with the yield of 93.5%. 58.4 g/L glucose obtained by enzymatic hydrolysate of sulfuric acid-pretreated corncob could effectively produce 54.2 g/L GA without by-product. The results verify that bi-directional switch regulation of three metals could vigorously promote the industrial bioproduction of GA, as well as provide some technical approach for fermentation engineering controlling.