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 and 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 determined by the fraction and distribution of G-residues. Alginates are currently 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 genes have also been identified. In both algae and bacteria the polymer is first produced as mannuronan, which is then epimerized by the enzyme mannuronan C-5-epimerase. A gene encoding a periplasmic epimerase has been identified in the alginate gene clusters of P. aeruginosa and Azotobacter vinelandii. The A. vinelandii genome also encodes a family of at least five secreted epimerases, each of which introduces different distributions of G in the alginate. These enzymes can therefore be used to modify alginates in vitro to obtain polysaccharides with the desired content and distribution pattern of G. Such alginates may become useful in applications where reproducible and specific physical properties are required. (C) 1998 Elsevier Science Limited. All rights reserved.