BIOSYNTHESIS AND APPLICATIONS OF ALGINATES

Alginate is a family of linear polysaccharides composed of mannuronic acid (M) and guluronic acid (G). The polymer is used as a gel-former a nd viscosifier in a wide range of industrial applications. It is also used for encapsulation of cells and enzymes. The viscosity of alginate is mainly dependent on the polymer length, while the gel-forming and water-binding properties and the degree of immunogenicity are determin ed by the fraction and distribution of G-residues. Alginates are curre ntly manufactured by harvesting brown algae, but in nature the polymer is also produced by some bacteria belonging to the genera Azotobacter and Pseudomonas. The biosynthesis of alginate has been mostly studied in Pseudomonas aeruginosa, where many of the involved proteins and ge nes have also been identified. In both algae and bacteria the polymer is first produced as mannuronan, which is then epimerized by the enzym e mannuronan C-5-epimerase. A gene encoding a periplasmic epimerase ha s been identified in the alginate gene clusters of P. aeruginosa and A zotobacter vinelandii. The A. vinelandii genome also encodes a family of at least five secreted epimerases, each of which introduces differe nt distributions of G in the alginate. These enzymes can therefore be used to modify alginates in vitro to obtain polysaccharides with the d esired content and distribution pattern of G. Such alginates may becom e useful in applications where reproducible and specific physical prop erties are required. (C) 1998 Elsevier Science Limited. All rights res erved.