Galactose to tagatose isomerization by the l -arabinose isomerase from Bacillus subtilis : A biorefinery approach for Gelidium sesquipedale valorisation

Tagatose is a rare sugar with increasing commercial interest as sweetener. Biotechnological production of d -tagatose by enzymatic isomerization of d -galactose provides an alternative to chemical processes. In the last years, l -arabinose isomerases (L-AIs) from different origins have been studied to increase the effectiveness of tagatose production. In this work, the L-AI from Bacillus subtilis, previously reported to have unique substrate specificity for l -arabinose, was expressed in Escherichia coli and studied for isomerization of d -galactose to d -tagatose. The recombinant enzyme demonstrated, for the first time, tagatose bioconversion capacity, reaching ~59% conversion. Furthermore, a sustainable tagatose production strategy was developed by using Gelidium sesquipedale red seaweed and its undervalued processing residues as source of galactose. L-AI successfully converted the galactose-rich hydrolysate, obtained from direct acid hydrolysis of seaweed, to tagatose (50.9% conversion). Additionally, the process combining autohydrolysis of G. sesquipedale and acid hydrolysis of the remaining residue allowed a full integral valorisation of polysaccharides: 13.33 g of agar, an important hydrocolloid, coupled with the production of 5.97 g of tagatose. These results confirmed that seaweed biomass and waste-derived are promising substrates for tagatose production by L-AI, contributing to the advancement of circular economy and to the actual needs of food industry.