myo-inositol oxygenase from rat kidneys. Substrate-dependent oligomerization

Effect of magnesium ions on glucaric acid production in the engineered Saccharomyces cerevisiae

Glucaric acid has been successfully produced in Escherichia coli and fungus. Here, we first analyzed the effects of different metal ions on glucaric acid production in the engineered Saccharomyces cerevisiae Bga-3 strain harboring the glucaric acid synthesis pathway. We found that magnesium ions could promote the growth rate of yeast cells, and thus, increase the glucaric acid production by elevating the glucose and myo-inositol utilization of Bga-3 strain. RNA-Seq transcriptome analysis results showed that the upregulation of genes involved in the gluconeogenesis pathway, as well as the downregulation of genes associated with the glycolysis pathway and pentose phosphate pathway in response to MgCl₂ were all benefit for the enhancement of the glucose-6-phosphate flux, which was the precursor for myo-inositol and glucaric acid. In addition, we found that MgCl₂ could also increase the activity of MIOX4, which was also crucial for glucaric acid synthesis. At last, a final glucaric acid titer of 10.6 g/L, the highest reported titer, was achieved in the fed-batch fermentation using a 5-L bioreactor by adding 100 mM MgCl₂. Our findings will provide a new way of promoting the production of other chemicals in the engineered yeast cells.

Microbial engineering for the production of C2-C6 organic acids

Covering: up to the end of 2020Organic acids, as building block compounds, have been widely used in food, pharmaceutical, plastic, and chemical industries. Until now, chemical synthesis is still the primary method for industrial-scale organic acid production. However, this process encounters some inevitable challenges, such as depletable petroleum resources, harsh reaction conditions and complex downstream processes. To solve these problems, microbial cell factories provide a promising approach for achieving the sustainable production of organic acids. However, some key metabolites in central carbon metabolism are strictly regulated by the network of cellular metabolism, resulting in the low productivity of organic acids. Thus, multiple metabolic engineering strategies have been developed to reprogram microbial cell factories to produce organic acids, including monocarboxylic acids, hydroxy carboxylic acids, amino carboxylic acids, dicarboxylic acids and monomeric units for polymers. These strategies mainly center on improving the catalytic efficiency of the enzymes to increase the conversion rate, balancing the multi-gene biosynthetic pathways to reduce the byproduct formation, strengthening the metabolic flux to promote the product biosynthesis, optimizing the metabolic network to adapt the environmental conditions and enhancing substrate utilization to broaden the substrate spectrum. Here, we describe the recent advances in producing C₂-C₆ organic acids by metabolic engineering strategies. In addition, we provide new insights as to when, what and how these strategies should be taken. Future challenges are also discussed in further advancing microbial engineering and establishing efficient biorefineries.

Molecular cloning, expression, and characterization of myo-inositol oxygenase from mouse, rat, and human kidney

myo-Inositol oxygenase (MIOX) is a non-heme iron enzyme, which catalyzes the conversion of myo-inositol to d-glucuronic acid, the first committed step in myo-inositol catabolism. Full-length cDNAs of 858bp each coding for 33kDa protein were cloned from kidney cDNA libraries of mouse, rat, and human. The individual clones were expressed in Escherichia coli and recombinant MIOX proteins were purified to electrophoretic homogeneity. A hydrophobic interaction chromatography step yielded multiple conformers, with mouse and human MIOX showing three peaks and rat enzyme revealing two peaks. Individual MIOX peaks exhibited distinct V(max) and K(m) values. Interestingly, upon storage, the 33kDa protein was degraded to a approximately 30kDa truncated protein in each species, and formed small amounts of dimers of identical subunits. While MIOX is a highly conserved enzyme in all mammalian species, the labile nature and tendency to degrade in solution may be the source of significant differences in size previously reported in the literature. Regardless of the source, our results strongly dispel previous conflicting literature reports on the size of the protein and confirm that MIOX is a 33kDa protein.

Purification, characterization and functional cloning of inositol oxygenase from Cryptococcus

The enzyme inositol oxygenase (myo-inositol : oxygen oxidoreductase; E.C. 1.13.99.1) is a monooxygenase that converts inositol into glucuronic acid in the presence of molecular oxygen. This enzyme is integrated into a pathway leading to either degradation and energy production or the biosynthesis of precursors for polysaccharides. The enzyme was purified from the yeast Cryptococcus lactativorus by a five-step chromatography procedure. The purified enzyme shows a molecular mass of 37 kDa on SDS-PAGE, similar to the estimation of the size of the native enzyme determined by size exclusion chromatography. Peptides of the inositol oxygenase protein derived from a tryptic digest were sequenced de novo by nanoelectrospray tandem mass spectrometry. Using degenerate oligonucleotides, the corresponding gene was cloned from first strand cDNA. The open reading frame encodes a 315 amino acid polypeptide with a predicted molecular mass of 36.9 kDa. Inositol oxygenase is a single copy gene in C. lactativorus. It has close homologues in other fungi such as Cryptococcus neoformans and Neurospora crassa. Biochemical characterization of the enzyme showed a pH optimum of 6-6.5 and a temperature optimum of 30 degrees C. Myo-inositol is the only accepted substrate with a Km of ca. 5 mM. The enzyme contains a Fe-centre but the enzyme activity is resistant to KCN.

myo-Inositol oxygenase: molecular cloning and expression of a unique enzyme that oxidizes myo-inositol and D-chiro-inositol

myo-Inositol oxygenase (MIOX) catalyses the first committed step in the only pathway of myo-inositol catabolism, which occurs predominantly in the kidney. The enzyme is a non-haem-iron enzyme that catalyses the ring cleavage of myo-inositol with the incorporation of a single atom of oxygen. A full-length cDNA was isolated from a pig kidney library with an open reading frame of 849 bp and a corresponding protein subunit molecular mass of 32.7 kDa. The cDNA was expressed in a bacterial pET expression system and an active recombinant MIOX was purified from bacterial lysates to electrophoretic homogeneity. The purified enzyme displayed the same catalytic properties as the native enzyme with K(m) and k(cat) values of 5.9 mM and 11 min(-1) respectively. The pI was estimated to be 4.5. Preincubation with 1 mM Fe(2+) and 2 mM cysteine was essential for the enzyme's activity. D-chiro-Inositol, a myo-inositol isomer, is a substrate for the recombinant MIOX with an estimated K(m) of 33.5 mM. Both myo-inositol and D-chiro-inositol have been implicated in the pathogenesis of diabetes. Thus an understanding of the regulation of MIOX expression clearly represents a potential window on the aetiology of diabetes as well as on the control of various intracellular phosphoinositides and key signalling pathways.