Physiological, Biochemical, and Structural Bioinformatic Analysis of the Multiple Inositol Dehydrogenases from Corynebacterium glutamicum

Obesity-enriched gut microbe degrades myo-inositol and promotes lipid absorption

Numerous studies have reported critical roles for the gut microbiota in obesity. However, the specific microbes that causally contribute to obesity and the underlying mechanisms remain undetermined. Here, we conducted shotgun metagenomic sequencing in a Chinese cohort of 631 obese subjects and 374 normal-weight controls and identified a Megamonas-dominated, enterotype-like cluster enriched in obese subjects. Among this cohort, the presence of Megamonas and polygenic risk exhibited an additive impact on obesity. Megamonas rupellensis possessed genes for myo-inositol degradation, as demonstrated in vitro and in vivo, and the addition of myo-inositol effectively inhibited fatty acid absorption in intestinal organoids. Furthermore, mice colonized with M. rupellensis or E. coli heterologously expressing the myo-inositol-degrading iolG gene exhibited enhanced intestinal lipid absorption, thereby leading to obesity. Altogether, our findings uncover roles for M. rupellensis as a myo-inositol degrader that enhances lipid absorption and obesity, suggesting potential strategies for future obesity management.

Efficient pathway-driven scyllo-inositol production from myo-inositol using thermophilic cells and mesophilic inositol dehydrogenases: a novel strategy for pathway control

Mesophilic enzymes, which are active at moderate temperatures, may dominate enzymatic reactions even in the presence of thermophilic crude enzymes. This study was conducted to investigate this hypothesis with mesophilic inositol dehydrogenases (IolG and IolX) produced in Geobacillus kaustophilus HTA426. To ensure the efficient production of mesophilic enzymes, we first screened for promoters induced at moderate temperatures using transcriptome analysis and identified four genes highly expressed at 30°C in the thermophile. We further characterized these promoters using fluorescent reporter assays to determine that the mti3 promoter could direct efficient gene expression at 40°C. We cloned the promoter into an Escherichia coli-Geobacillus shuttle plasmid and confirmed that the resulting vector functioned in G. kaustophilus and other thermophiles. We then used this vector for the cooperative expression of the iolG and iolX genes from Bacillus subtilis 168. G. kaustophilus cells carrying the expression vector were incubated at 60°C for cellular propagation and then at 40°C for the production of IolG and IolX. When the cells were permeabilized, IolG and IolX acted as catalysts to convert exogenous myo-inositol into scyllo-inositol at 30°C. In a scaled-up reaction, 10 g of myo-inositol was converted to 1.8 g of scyllo-inositol, which was further purified to yield 970 mg of pure powder. Notably, myo-inositol was degraded by intrinsic enzymes of G. kaustophilus at 60°C but not at 30°C, supporting our initial hypothesis. We indicate that this approach is useful for preparing enzyme cocktails without the need for purification.

IMPORTANCE

Enzyme cocktails are commonly employed for cell-free chemical synthesis; however, their preparation involves cumbersome processes. This study affirms that mesophilic enzymes in thermophilic crude extracts can function as specific catalysts at moderate temperatures, akin to enzyme cocktails. The catalyst was prepared by permeabilizing cells without the need for concentration, extraction, or purification processes; hence, its preparation was considerably simpler compared with conventional methods for enzyme cocktails. This approach was employed to produce pure scyllo-inositol from an economical substrate. Notably, this marks the first large-scale preparation of pure scyllo-inositol, holding potential pharmaceutical significance as scyllo-inositol serves as a promising agent for certain diseases but is currently expensive. Moreover, this approach holds promise for application in pathway engineering within living cells. The envisioned pathway is designed without chromosomal modification and is simply regulated by switching culture temperatures. Consequently, this study introduces a novel platform for both whole-cell and cell-free synthetic systems.

Microbial synthesis of health-promoting inositols

D-chiro-inositol and scyllo-inositol are known for their health-promoting properties and promising as ingredients for functional foods. Strains of Bacillus subtilis and Corynebacterium glutamicum were created by metabolic engineering capable of inexpensive production of these two rare inositols from myo-inositol, which is the most common inositol in nature. In addition, further modifications have enabled the synthesis of the two rare inositols from the much-cheaper carbon sources, glucose or sucrose.

Metabolic engineering Corynebacterium glutamicum for D-chiro-inositol production

D-chiro-inositol (DCI) is a potential drug for the treatment of type II diabetes and polycystic ovary syndrome. In order to effectively synthesize DCI in Corynebacterium glutamicum, the genes related to inositol catabolism in clusters iol1 and iol2 were knocked out in C. glutamicum SN01 to generate the chassis strain DCI-1. DCI-1 did not grow in and catabolize myo-inositol (MI). Subsequently, different exogenous and endogenous inosose isomerases were expressed in DCI-1 and their conversion ability of DCI from MI were compared. After fermentation, the strain DCI-7 co-expressing inosose isomerase IolI2 and inositol dehydrogenase IolG was identified as the optimal strain. Its DCI titer reached 3.21 g/L in the presence of 20 g/L MI. On this basis, the pH, temperature and MI concentration during whole-cell conversion of DCI by strain DCI-7 were optimized. Finally, the optimal condition that achieved the highest DCI titer of 6.96 g/L were obtained at pH 8.0, 37 °C and addition of 40 g/L MI. To our knowledge, it is the highest DCI titer ever reported.

A myo-inositol dehydrogenase involved in aminocyclitol biosynthesis of hygromycin A

Hygromycin A is a broad-spectrum antibiotic that contains a furanose, cinnamic acid, and aminocyclitol moieties. The biosynthesis of the aminocyclitol has been proposed to proceed through six enzymatic steps from glucose 6-phosphate through myo-inositol to the final methylenedioxy-containing aminocyclitol. Although there is some in vivo evidence for this proposed pathway, biochemical support for the individual enzyme activities is lacking. In this study, we verify the activity for one enzyme in this pathway. We show that Hyg17 is a myo-inositol dehydrogenase that has a unique substrate scope when compared to other myo-inositol dehydrogenases. Furthermore, we analyze sequences from the protein family containing Hyg17 and discuss genome mining strategies that target this protein family to identify biosynthetic clusters for natural product discovery.

The implication of viability and pathogenicity by truncated lipopolysaccharide in Yersinia enterocolitica

The fast envelope stress responses play a key role in the transmission and pathogenesis of Yersinia enterocolitica, one of the most common foodborne pathogens. Our previous study showed that deletion of the waaF gene, essential for the biosynthesis of lipopolysaccharide (LPS) core polysaccharides, led to the formation of a truncated LPS structure and induced cell envelope stress. This envelope stress may disturb the intracellular signal transduction, thereby affecting the physiological functions of Y. enterocolitica. In this study, truncated LPS caused by waaF deletion was used as a model of envelope stress in Y. enterocolitica. We investigated the mechanisms of envelope stress responses and the cellular functions affected by truncated LPS. Transcriptome analysis and phenotypic validation showed that LPS truncation reduced flagellar assembly, bacterial chemotaxis, and inositol phosphate metabolism, presenting lower pathogenicity and viability both in vivo and in vitro environments. Further 4D label-free phosphorylation analysis confirmed that truncated LPS perturbed multiple intracellular signal transduction pathways. Specifically, a comprehensive discussion was conducted on the mechanisms by which chemotactic signal transduction and Rcs system contribute to the inhibition of chemotaxis. Finally, the pathogenicity of Y. enterocolitica with truncated LPS was evaluated in vitro using IPEC-J2 cells as models, and it was found that truncated LPS exhibited reduced adhesion, invasion, and toxicity of Y. enterocolitica to IPEC-J2 cells. Our research provides an understanding of LPS in the regulation of Y. enterocolitica viability and pathogenicity and, thus, opening new avenues to develop novel food safety strategies or drugs to prevent and control Y. enterocolitica infections. KEY POINTS: • Truncated LPS reduces flagellar assembly, chemotaxis, and inositol phosphate metabolism in Y. enterocolitica. • Truncated LPS reduces adhesion, invasion, and toxicity of Y. enterocolitica to IPEC-J2 cells. • Truncated LPS regulates intracellular signal transduction of Y. enterocolitica.

Alternative routes for production of the drug candidate d-chiro-inositol with Corynebacterium glutamicum using endogenous or promiscuous plant enzymes

d-chiro-Inositol (DCI) is a promising drug candidate for treating insulin resistance and associated diseases such as type 2 diabetes or polycystic ovary syndrome. In this study, we developed two production processes for DCI using Corynebacterium glutamicum as host. In the first process, myo-inositol (MI) is oxidized to 2-keto-myo-inositol (2KMI) by the inositol dehydrogenase (IDH) IolG and then isomerized to 1-keto-d-chiro-inositol (1KDCI) by the isomerases Cg0212 or Cg2312, both of which were identified in this work. 1KDCI is then reduced to DCI by IolG. Overproduction of IolG and Cg0212 in a chassis strain unable to degrade inositols allowed the production of 1.1 g/L DCI from 10 g/L MI. As both reactions involved are reversible, only a partial conversion of MI to DCI can be achieved. To enable higher conversion ratios, a novel route towards DCI was established by utilizing the promiscuous activity of two plant-derived enzymes, the NAD+-dependent d-ononitol dehydrogenase MtOEPa and the NADPH-dependent d-pinitol dehydrogenase MtOEPb from Medicago truncatula (barrelclover). Heterologous production of these enzymes in the chassis strain led to the production of 1.6 g/L DCI from 10 g/L MI. For replacing the substrate MI by glucose, the two plant genes were co-expressed with the endogenous myo-inositol-1-phosphate synthase gene ino1 either as a synthetic operon or using a novel, bicistronic T7-based expression vector. With the single operon construct, 0.75 g/L DCI was formed from 20 g/L glucose, whereas with the bicistronic construct 1.2 g/L DCI was obtained, disclosing C. glutamicum as an attractive host for of d-chiro-inositol production.