Biosynthetic Mechanism for L -Gulose in Main Polar Lipids of Thermoplasma acidophilum and Possible Resemblance to Plant Ascorbic Acid Biosynthesis

Advances in Novel Animal Vitamin C Biosynthesis Pathways and the Role of Prokaryote-Based Inferences to Understand Their Origin

Vitamin C (VC) is an essential nutrient required for the optimal function and development of many organisms. VC has been studied for many decades, and still today, the characterization of its functions is a dynamic scientific field, mainly because of its commercial and therapeutic applications. In this review, we discuss, in a comparative way, the increasing evidence for alternative VC synthesis pathways in insects and nematodes, and the potential of myo-inositol as a possible substrate for this metabolic process in metazoans. Methodological approaches that may be useful for the future characterization of the VC synthesis pathways of Caenorhabditis elegans and Drosophila melanogaster are here discussed. We also summarize the current distribution of the eukaryote aldonolactone oxidoreductases gene lineages, while highlighting the added value of studies on prokaryote species that are likely able to synthesize VC for both the characterization of novel VC synthesis pathways and inferences on the complex evolutionary history of such pathways. Such work may help improve the industrial production of VC.

GDP-Mannose 3,5-Epimerase: A View on Structure, Mechanism, and Industrial Potential

GDP-mannose 3,5-epimerase (GM35E, GME) belongs to the short-chain dehydrogenase/reductase (SDR) protein superfamily and catalyses the conversion of GDP-d-mannose towards GDP-l-galactose. Although the overall reaction seems relatively simple (a double epimerization), the enzyme needs to orchestrate a complex set of chemical reactions, with no less than 6 catalysis steps (oxidation, 2x deprotonation, 2x protonation and reduction), to perform the double epimerization of GDP-mannose to GDP-l-galactose. The enzyme is involved in the biosynthesis of vitamin C in plants and lipopolysaccharide synthesis in bacteria. In this review, we provide a clear overview of these interesting epimerases, including the latest findings such as the recently characterized bacterial and thermostable GM35E representative and its mechanism revision but also focus on their industrial potential in rare sugar synthesis and glycorandomization.

Efficient whole-cell biosynthesis of l-gulose by coupling mannitol-1-dehydrogenase with NADH oxidase

L-Gulose is a rare aldohexose to serve as a building block for anticancer drug bleomycin and nucleoside-based antivirals. However, preparative inaccessibility and high cost have hindered its pharmaceutical application. Despite a regio- and stereo-selective enzymatic synthesis of l-gulose from d-sorbitol using a variant of NAD⁺-dependent mannitol-1-dehydrogenase from Apium graveolens (mMDH) was explored, low efficiency and productivity caused by NADH accumulation or insufficient amount of NAD⁺ limited the practical utility of this process. In this study, a stable and efficient NADH oxidase from Bacillus cereus (bcNOX) was found to be more compatible with mMDH to recycle NAD⁺ in E. coli cells for l-gulose biosynthesis. After a systematic optimization of the whole-cell system, efficient biosynthesis of l-gulose was achieved. Starting with 70 g/L of readily available and cheap d-sorbitol resulted in a volumetric productivity of 5.5 g/L/d. This whole-cell approach enables practical, efficient and environmentally friendly biosynthesis of l-gulose and exhibits the potential of becoming a biocatalytic strategy for various enzymatic oxidative transformations.

GDP-altrose as novel product of GDP-mannose 3,5-epimerase: Revisiting its reaction mechanism

GDP-mannose 3,5-epimerase (GM35E) catalyzes the double epimerization of GDP-mannose to yield GDP-l-galactose. GDP-l-gulose (C5-epimer) has previously been detected as a byproduct of this reaction, indicating that C3,5-epimerization occurs through an initial epimerization at C5. Given these products, GM35E constitutes a valuable bridge between d- and l-hexoses. In order to fully exploit this potential, the enzyme might be subjected to specificity engineering for which profound mechanistic insights are beneficial. Accordingly, this study further elucidated GM35E's reaction mechanism. For the first time, the production of the C3-epimer GDP-altrose was demonstrated, resulting in an adjustment of the acknowledged reaction mechanism. As GM35E converts GDP-mannose to GDP-l-gulose, GDP-altrose and GDP-l-galactose in a 72:4:4:20 ratio, this indicates that the enzyme does not discriminate between the C3 and C5 position as initial epimerization site. This was also confirmed by a structural investigation. Based on a mutational analysis of the active site, residues S115 and R281 were attributed a stabilizing function, which is believed to support the reactivation process of the catalytic residues. This paper eventually reflected on some engineering strategies that aim to change the enzyme towards a single specificity.