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The industrially important polysaccharide alginate is composed of the two sugar monomers β-d -mannuronic acid (M) and its epimer α-l -guluronic acid (G). In the bacteriumAzotobacter vinelandii , the G residues originate from a polymer-level reaction catalyzed by one periplasmic and at least five secreted mannuronan C-5-epimerases. The secreted enzymes are composed of repeats of two protein modules designated A (385 amino acids) and R (153 amino acids). The modular structure of one of the epimerases, AlgE1, is A1 R1 R2 R3 A2 R4 . This enzyme has two catalytic sites for epimerization, each site introducing a different G distribution pattern, and in this article we report the DNA-level construction of a variety of truncated forms of the enzyme. Analyses of the properties of the corresponding proteins showed that an A module alone is sufficient for epimerization and that A1 catalyzed the formation of contiguous stretches of G residues in the polymer, while A2 introduces single G residues. These differences are predicted to strongly affect the physical and immunological properties of the reaction product. The epimerization reaction is Ca2+ dependent, and direct binding studies showed that both the A and R modules bind this cation. The R modules appeared to reduce the Ca2+ concentration needed for full activity and also stimulated the reaction rate when positioned both N and C terminally.

journal of bacteriology

The A Modules of the <i>Azotobacter vinelandii</i> Mannuronan-C-5-Epimerase AlgE1 Are Sufficient for both Epimerization and Binding of Ca <sup>2+</sup>

The A Modules of the Azotobacter vinelandii Mannuronan-C-5-Epimerase AlgE1 Are Sufficient for both Epimerization and Binding of Ca 2+

The A Modules of the Azotobacter vinelandii Mannuronan-C-5-Epimerase AlgE1 Are Sufficient for both Epimerization and Binding of Ca ²⁺