The Azotobacter vinelandii mannuronan C-5-epimerase AlgE1 consists of two separate catalytic domains

The Azotobacter vinelandii enzyme AlgE1 is a member of a family of secreted mannuronan C-5-epimerases. These enzymes convert beta-n-mannuronic acid residues (M) to alpha-L-guluronic acid residues (G) at the polymer level in the industrially important polysaccharide alginate, leading to altered physical and immunological properties of the polymer. The reaction product of AIgE1 was found to be a mixture of blocks of continuous G residues (G-blocks) and blocks containing alternating M and G residues (MG-blocks), The enzyme is dependent on Ca2+ for activity, and only Sr2+ of those tested was able to replace Ca2+. Zn2+ blocked the activity even at low concentrations. algE1 has been divided into two parts based on the modular type of structure previously reported to be a characteristic of the secreted epimerases, and each part has been expressed in Escherichia coli, These experiments showed that AlgE1 contains two catalytic domains, AlgE1-1, which introduces both G-blocks and MG-blocks, and AlgE1-2, which only introduces MG-blocks, AlgE1-1 has a much lower specific activity than both AlgE1-2 and AlgE1, However, the two halves of AlgE1 seem to cooperate in such a way that they contribute approximately equally to the overall epimerization reaction.