Modification of Alginate Using Mannuronan C-5-Epimerases

Abstract

The enzymatic interconversions of the various hexoses found in polysaccharides and glycoproteins normally take place either on the unsubstituted sugars, their phosphate esters, or at the sugar nucleotide levels. In a few cases, however, epimerization reactions take place after the polysaccharides are formed. This type of postpolymerization epimerization is a feature of several uronic-acid containing polysaccharides found both in eukaryotes and in bacteria. These reactions are exemplified by C-5 inversion of D-glucuronic acid into L-iduronic acid in dermatan (1) and heparin (2) synthesis and D-mannuronic acid (M) into L-guluronic acid (G) in alginates (3). The latter epimerization (Fig. 1), which is catalyzed by mannuronan C-5 epimerases is the last step in the biosynthesis of alginates. Since M prefers the 4C1-conformation, while G prefers the 1C4-conformation, the relative amount and distribution of each monomer has a profound effect on the secondary structure and thus on the properties of the polymers (4). Although stretches of consecutive M (M blocks) or alternating M and G (MG blocks) are relatively flexible, stretches of consecutive G (G blocks) are fairly stiff. Furthermore, the gel-forming properties are related mainly to the content and sequencial arrangements of guluronic acid. Two contiguous, diaxially linked G-residues form binding sites for calcium ions, and long sequences of such calcium-binding sites form crosslinks with similar sequences in other alginate molecules, giving rise to junctions in the gel network. It has also been found that long M blocks are immunogenic (5).