Efficient expression of myo-inositol oxygenase in Escherichia coli and application for conversion of myo-inositol to glucuronic acid

Metabolic engineering for microbial production of sugar acids

Carbohydrates including sugar acids are commonly used as carbon sources in microbial biotechnology. These sugar acids are themselves desirable and often overlooked targets for biobased production since they find applications in a broad range of industries, examples include food, construction, medical, textile, and polymer industries. Different stages of oxidation for natural sugar acids can be distinguished. Oxidation of the aldehyde group yields aldonic acids, oxidation of the primary hydroxy group leads to uronic acids, and both oxidations combined yield aldaric acids. While the chemical oxidation of sugars to their acid forms often is a one-pot reaction under harsh conditions, their biosynthesis is much more delicate. Bio-based production can involve enzymatic conversion, whole-cell biotransformation, and fermentation. Generally, the in vivo approaches are preferred because they are less resource-intensive than enzymatic conversion. Metabolic engineering plays a crucial role in optimizing microbial strains for efficient sugar acid production. Strategies include pathway engineering to overexpress key enzymes involved in sugar oxidation, deletion of competing pathways to enhance the precursor availability and eliminate the product consumption, cofactor balancing for efficient redox reactions, and transporter engineering to facilitate precursor import or sugar acid export. Synthetic biology tools, such as CRISPR-Cas and dynamic regulatory circuits, have further improved strain development by enabling precise genetic modifications and adaptive control of metabolic fluxes. The usage of plant biomass hydrolysates for bio-based production further adds to the environmental friendliness of the in vivo approaches. This review highlights the different approaches for the production of C5 and C6 sugar acids, their applications, and their catabolism in microbes.

Combined application of myo-inositol and corn steep liquor enhances seedling growth and cold tolerance in cucumber and tomato

Low temperatures pose a common challenge in the production of cucumbers and tomatoes, hindering plant growth and, in severe cases, leading to plant death. In our investigation, we observed a substantial improvement in the growth of cucumber and tomato seedlings through the application of corn steep liquor (CSL), myo-inositol (MI), and their combinations. When subjected to low-temperature stress, these treatments resulted in heightened levels of photosynthetic pigments, thereby fostering enhanced photosynthesis in both tomato and cucumber plants. Furthermore, it contributed to a decrease in malondialdehyde (MDA) levels and electrolyte leakage (REP). The effectiveness of the treatment was further validated through the analysis of key gene expressions (CBF1, COR, MIOX4, and MIPS1) in cucumber. Particularly, noteworthy positive outcomes were noted in the treatment involving 0.6 mL L-1 CSL combined with 72 mg L-1 MI. This study provides valuable technical insights into leveraging the synergistic effects of inositol and maize leachate to promote early crop growth and bolster resistance to low temperatures.

Sequence-based bioprospecting of myo-inositol oxygenase (Miox) reveals new homologues that increase glucaric acid production in Saccharomyces cerevisiae

myo-Inositol oxygenase (Miox) is a rate-limiting enzyme for glucaric acid production via microbial fermentation. The enzyme converts myo-inositol to glucuronate, which is further converted to glucaric acid, a natural compound with industrial uses that range from detergents to pharmaceutical synthesis to polymeric materials. More than 2,000 Miox sequences are available in the Uniprot database but only thirteen are classified as reviewed in Swiss-Prot (August 2019). In this study, sequence similarity networks were used to identify new homologues to be expressed in Saccharomyces cerevisiae for glucaric acid production. The expression of four homologues did not lead to product formation. Some of these enzymes may have a defective "dynamic lid" - a structural feature important to close the reaction site - which might explain the lack of activity. Thirty-one selected Miox sequences did allow for product formation, of which twenty-five were characterized for the first time. Expression of Talaromyces marneffei Miox led to the accumulation of 1.76 ± 0.33 g glucaric acid/L from 20 g glucose/L and 10 g/L myo-inositol. Specific glucaric acid titer with TmMiox increased 44 % compared to the often-used Arabidopsis thaliana variant AtMiox4 (0.258 vs. 0.179 g glucaric acid/g biomass). AtMiox4 activity decreased from 12.47 to 0.40 nmol/min/mg protein when cells exited exponential phase during growth on glucose, highlighting the importance of future research on Miox stability in order to further improve microbial production of glucaric acid.