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

Two NADPH-dependent 2-ketogluconate reductases involved in 2-ketogluconate assimilation in Gluconobacter sp. strain CHM43

Incomplete oxidation of glucose by Gluconobacter sp. strain CHM43 produces gluconic acid and then 2- or 5-ketogluconic acid. Although 2-keto-D-gluconate (2KG) is a valuable compound, it is sometimes consumed by Gluconobacter itself via an unknown metabolic pathway. We anticipated that 2KG reductase (2KGR) would be a key enzyme in 2KG metabolism. GLF_0478 and GLF_1777 were identified in the genome of strain CHM43, which encode proteins with 70% and 48% amino acid sequence identity, respectively, to the 2KGR of Gluconobacter oxydans strain 621H. Constructed mutant derivatives of strain CHM43 lacking GLF_0478, GLF_1777, or both were examined for their 2KG consumption ability. Strains ∆GLF_0478 and ∆GLF_1777 consumed 2KG like the parental strain. However, the double-deletion (∆∆) strain did not consume 2KG at all, although it produced 2KG like the parental strain. Strains ∆GLF_0478 and ∆GLF_1777 each showed decreased 2KGR activity compared with the parental strain, and strain ΔΔ lost 2KGR activity. These results suggest that reduction of 2KG catalyzed by GLF_0478 and GLF_1777 is the committed step in 2KG metabolism in Gluconobacter sp. strain CHM43. The two 2KGRs showed high activity at neutral pH and lower KM values for NADPH than NADH. Results of induction experiments suggest that GLF_0478 is constitutively expressed at a low level but induced by 2KG, and GLF_1777 is also inducible by 2KG but repressed in the absence of an inducer. Our study that characterizes the key genes for 2KG consumption in Gluconobacter gives insights for improvement of biological 2KG production systems.

IMPORTANCE

2-Keto-D-gluconate (2KG), a product of incomplete oxidation of glucose by acetic acid bacteria including Gluconobacter spp., is used for various purposes, including in the food industry. Gluconobacter also consumes 2KG via an unclear metabolic pathway. It is reported that Pseudomonas spp. and Cupriavidus necator phosphorylate 2KG in the first step of 2KG metabolism, but some enteric bacteria including Escherichia coli reduce 2KG. This study evaluated the 2KG consumption ability of a mutant derivative of a strain of Gluconobacter that lacks two putative 2KGR-encoding genes. The mutant strain did not consume 2KG at all; the two 2KGRs were each found to catalyze 2KG reduction. It is concluded that reduction of 2KG is the committed step in 2KG metabolism in Gluconobacter. The results presented here give insights that might facilitate improvement of 2KG production systems that use Gluconobacter.

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 and Transcriptional Regulation of the 2-Ketogluconate Utilization Operon in Pseudomonas plecoglossicida

Pseudomonas plecoglossicida JUIM01 is an industrial 2-keto-d-gluconate (2KGA)-producing strain. However, its regulation mechanism of 2KGA metabolism remains to be clarified. Among other reported Pseudomonas species, the 2-ketogluconate utilization operon (kgu operon) plays key roles in 2KGA catabolism. In this study, the structural genes of the kgu operon and its promoter in P. plecoglossicida JUIM01 were identified using reverse transcription PCR and lacZ reporter gene fusion. The results showed the kgu operon in P. plecoglossicida was composed of four structural genes: kguE, kguK, kguT, and kguD. The ptxS gene located upstream of kguE was excluded from the kgu operon. Then, the knockout and corresponding complementation strains of kguE, kguK, kguT, and kguD were constructed, respectively, to prove the kgu operon was involved in 2KGA catabolism of P. plecoglossicida. The knockout stains, especially JUIM01ΔkguE, showed potential as industrial production strains for 2KGA. Moreover, the transcriptional regulation mechanism of PtxS on the kgu operon was elucidated using multiple methods. In P. plecoglossicida, the LacI-family transcription regulator PtxS could recognize a 14 bp palindrome (5'-TGAAACCGGTTTCA-3') within the promoter region of the kgu operon and specifically bind to a 26 bp region where the palindrome was located. As the binding sites overlapped with the transcription start site of the kgu operon, the binding of PtxS possibly hindered the binding of RNA polymerase, thus repressing the transcription of the kgu operon and further regulating 2KGA catabolism. 2KGA bound to PtxS as an effector to dissociate it from the kgu operon promoter region, so as to relieve the transcription repression. The results will provide strategies for improving the product accumulation in 2KGA industrial production and theoretical bases for the construction of a Pseudomonas chassis.

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.