Mannuronan C5-epimerases for tailored seaweed alginate modification

Alginate is a linear polysaccharide composed of (1 → 4)-linked β-D-mannuronate (M) and its C5-epimer α-L-guluronate (G) distributed in blocks of mainly MM-, GG- or alternating MG-residues. Alginate is found in the cell walls of brown macroalgae (seaweed, class Phaeophyceae), but is also produced by certain bacterial species within the Pseudomonas and Azotobacter genera. Due to its hydrocolloid and gelation properties, alginate is widely used in foods and healthcare products, e.g. for moisture control in band-aids. The gelation properties of alginate are determined by the G-residues, i.e. the so-called GG blocks, through interactions with cations, particularly Ca²⁺. In brown macroalgae alginate is first produced as poly-M molecules (mannuronan). During cell wall adaptation and maturation, the M-residues are modified by mannuronan C5-epimerases, EC 5.1.3.37, which convert some of the M-residues to G-residues by catalyzing the “flipping” of the C5 group. While the G:M ratio varies by macroalgal species and geographic location, mature alginates usually contain more G-residues and have better gelation properties, whereas brown macroalgae cultivated in calm water, including those now farmed in the Northern Hemisphere, typically have a low G:M ratio. The interest in using microbial mannuronan C5-epimerases to improve the gelation properties and thus increase the application value of alginate from cultivated brown macroalgae is therefore increasing. The structure and catalytic properties of different microbial mannuronan C5-epimerases, especially those of Azotobacter vinelandii, have been studied for decades. Recently, extensive product profiling and recombinant production of macroalgal C5-epimerases have also been reported. In this review we present detailed descriptions of mannuronan C5-epimerases, including the microbial, macroalgal, and mutated enzymes characterized so far, focusing on reaction mechanisms, enzyme types, assays, modular structure-function properties, and not least reaction product composition. The findings provide an overview of how minor changes can shape enzyme function and alginate composition.