A 2-ketogluconate kinase KguK in Pseudomonas plecoglossicida JUIM01: Enzymatic characterization and its role in 2-keto-d-gluconic acid metabolism

Can cadmium-contaminated rice be used to produce food additive sodium erythorbate?

Cadmium (Cd) in rice is a significant concern for its quality and safety. Currently, there is a crucial need to develop cost-effective and efficient ways to remove Cd or re-utilize Cd-contaminated rice. The food additive sodium erythorbate is produced via 2-ketogluconic acid (2KGA) fermentation by Pseudomonas plecoglossicida and lactonization using starch-rich raw materials, such as rice. We aimed to determine whether cadmium-contaminated rice can be used to produce sodium erythorbate. To achieve this aim, the migration of cadmium during the production of sodium erythorbate from Cd-contaminated rice was studied. Five rice varieties with different Cd contents from 0.10 to 0.68 mg/kg were used as raw materials. The results indicated the presence of Cd in rice and CaCO3 did not have a notable impact on the fermentation performance of 2KGA. The acidification of 2KGA fermentation broth, the addition of K4Fe(CN)6·3H2O and ZnSO4, and 2KGA purification using cation exchange effectively removed >98% of the Cd in the fermentation broth, but the 2KGA yield remained high at approximately 94%. The sodium erythorbate synthesized from Cd-contaminated rice was of high quality and free from Cd, meeting the requirements of the Chinese National Standard, GB 1886.28-2016. The study provided a safe and effective strategy for comprehensively utilizing Cd-contaminated rice to produce high value-added food additive.

Characterization of a transcriptional regulator PtxS from Pseudomonas plecoglossicida for regulating 2-ketogluconic acid metabolism

Homologs of PtxS are ubiquitous transcriptional regulators controlling the expression of the glucose dehydrogenase and kgu operon to globally regulate the 2-ketogluconic acid (2KGA) metabolism in Pseudomonas. In the present study, a PtxS from a 2KGA industrial producer Pseudomonas plecoglossicida JUIM01 (PpPtxS) was heterologously expressed in E. coli BL21(DE3), then structurally and functionally characterized. The obtained results showed that PpPtxS was a 36.65-kDa LacI-family transcriptional regulator. 2KGA was the sole effector of PpPtxS. Glucose negatively affected the molecular binding of PpPtxS and 2KGA, and gluconic acid inhibited the PpPtxS-2KGA binding reaction. PpPtxS in water solution mainly existed as a dimer and bound to two molecules of 2KGA. The effector 2KGA mainly bound to the region close to the C-terminal of PpPtxS by interacting with the 299th to the 301st amino acids (Ala, Gln, Pro, Thr, Glu and Arg). PpPtxS specifically recognized and bound to a 14-bp palindrome sequence (5'-TGAAACCGGTTTCA-3') due to its conserved HTH motif at the N-terminal. The characterization of PpPtxS in this study would provide a theoretical guidance for the industrial production of 2KGA.