Introduction: In recent years, the use of biomass to produce value-added chemicals has attracted much attention. Because it is found in abundance, it is not a food source, it is renewable, and in addition, it does not have the problems of using fossil resources such as environmental pollution and rapid depletion of natural resources. The decomposition of lignocellulosic biomass leads to the production of D-xylose, which is the second most abundant sugar in nature, and is not easily fermentable. Several wild microbial species or metabolically engineered strains have been isolated or engineered to ferment xylose to produce valuable chemical compounds such as 1,2,4-butanetriol. Butanetriol is a four-carbon straight-chain polyol with three hydrophilic hydroxyl groups. Butanetriol (BTO), which has attracted a lot of attention in the past few years, has wide applications in various fields, including in medicine and the pharmaceutical industry as a precursor for the synthesis of several high-value drugs such as Crestor and Zetia and the Anti-HIV drug Amprenavir. BTO is usually produced through chemical routes using glycidol, 2-butene-1, malate, or 3,4-dihydroxybutanoate as starting materials. But it has problems such as harsh reaction conditions, multiple steps, high production costs low efficiency, and severe environmental pollution. The set of problems in the chemical production of BTO has caused the development of its biological (microbial) production process to be considered in recent years, and researchers have created new processes for the production of BTO from xylose with microbial conversion. In 2003, Niu and colleagues were the first to produce butanetriol via biological pathways. In this study, the biosynthesis of butanetriol was carried out in an E. coli microbial strain using D-xylose as a substrate.
Methods: The biological production pathway of butanetriol consists of four steps. By expressing the enzymes D-xylose dehydrogenase, D-xylonate dehydratase, benzoylformate decarboxylase, and aldehyde reductase, xylose is converted to butanetriol. Since only two enzymes (XylD) dehydratase D-xylonate and (adhP) aldehyde reductase naturally exist in E. coli, the genes for the other two enzymes of this pathway, namely xylose dehydrogenase (XDH) from Pseudomonas fragii and 2-keto acid decarboxylase was extracted from Pseudomonas putida bacteria and then cloned into the target E. coli strain and led to the conversion of 10 g/L of D-Xylose to 1.6 g/L of BTO with a yield of 25%.
Results: In the past twenty years, various types of research have been carried out for the biological production of BTO in different strains under different conditions. However, none of them has led to the production of BTO with high efficiency, but in a research conducted in 2023, by combining several strategies, choosing suitable enzymes, and optimizing the reaction, the researchers succeeded in producing 125 grams per liter of BTO from 180 g/L of D-Xylose with The efficiency exceeded 97%, and also in a 2024 study, Hu et al. developed a whole-cell bioconversion system by optimizing enzymes and strains to produce BTO from renewable biomass, which resulted in the conversion of 200 g/L D-Xylose with The yield was more than 74%. Both of these researches have production value on an industrial scale.
Conclusion: Although whole-cell transformation may be the most promising strategy for the preparation of BTO due to its simple operation, environmental compatibility, and potential low cost. However, there are still problems that need to be solved to improve the bioconversion efficiency for the synthesis of BTO from D-xylose, because whole-cell production processes sometimes face the problems of toxicity, competition of metabolites, and generation of byproducts. In this case, the performance of the four enzymes should be optimized to avoid the accumulation of intermediates such as d-xylonic acid and 2-keto-3-deoxy-d-xylonate (KDX). In particular, dehydratase and decarboxylase enzymes should be optimized in terms of expression and catalytic efficiency. The results of this review article show that by using recent advances in metabolic engineering and process optimization, the bioproduction of butanetriol has been significantly improved, but there is still a need for further research and development to improve scalability and reduce cost, and increase the production.
Keywords: Biomass, 1,2,4-Butenetriol, Metabolic Engineering, Genetic Engineering, E. coli