With the growing importance of rare sugars in a myriad of industries, including food and pharmaceuticals, the search for cost-effective and efficient production intensifies. Consequently, carbohydrate epimerases active on unsubstituted sugars have sparked great industrial attention, as they allow for a one-step production of rare sugars (e.g. allulose) directly from their abundant and cheap counterparts (e.g. fructose). However, the link between abundant aldoses (e.g. glucose, galactose) and their rare C3-epimers (e.g. allose, gulose) has yet to be established. Therefore, the discovery or design of an aldose C3-epimerase would grant these enzymes a central role as biocatalysts in the rare aldose production. An understudied group of epimerases, i.e. aldolase-type epimerases, mechanistically allow for the C3-epimerisation of aldoses, but have yet to demonstrate this specificity. Therefore, this project seeks to engineer these enzymes into aldose C3-epimerases using semi-rational enzyme design. This will be achieved by identifying structure-function relationships and unlocking specificity determinants through an in-depth analysis of the mechanism, dynamics and evolution. As such, the project ventures in the uncharted territory of aldolase-type epimerases to develop new enzymes for rare sugar synthesis.