Unleashing the innate ability of Escherichia coli to produce D-Allose

D-allose is a rare monosaccharide, found naturally in low abundances. Due to its low-calorie profile and similar taste to sucrose, D-allose has the potential to become an ideal sugar substitute. D-allose also displays unique properties and health benefits that can be applied to various fields, including food and medicine. D-allose can be produced using two enzymatic steps in vitro: the epimerization of D-fructose, then the isomerization of the resulting D-psicose. This method suffers from poor yield due to the reversible nature of both reactions. We found that Escherichia coli possesses all of the required enzymes to convert D-glucose to D-allose with a thermodynamically favorable pathway, through a series of phosphorylation-epimerization-isomerization-dephosphorylation steps. To increase carbon flux toward D-allose production, the pathway genes were additionally expressed, and the competing pathways were removed. The engineered strains achieved production of D-allose, at a titer of 56.4 g L^-1, a productivity of 0.65 g L^-1 hr^-1, and a yield of 41.4% under test tube conditions.