The mannuronan C-5-epimerase AlgE2 is one of a family of Ca2+-dependent epimerases secreted by Azotobacter vinelandii. These enzymes catalyze the conversion of beta -D-mannuronic acid residues (M) to alpha -L-guluronic acid residues (G) in alginate. AlgE2 had a pH optimum between 6.5 and 7 and a temperature optimum around 55 degreesC. Addition of low molecular weight organic compounds, including buffers, amino acids and osmoprotective compounds, affected the activity of the enzyme. The charge, size and stereochemistry of the added compounds were important. The activity of AlgE2, dissolved in various buffers (same pH), decreased with increasing fraction of positively Ca2+ was omitted only Si2+, of the metal ions tested, charged buffer ions. Mono- and divalent metal ions also influenced the activity. When Ca supported some activity of AlgE2. At high concentration of Ca2+ (3.3 mM) these ions had a negative effect on the activity, whereas at low Ca2+ concentration (0.58 mM) the activity was enhanced by addition of Sr2+, and to some degree also by addition of Mg2+ and Mn2+ During epimerization AlgE2 occasionally causes cleavage of the alginate chain. These chain breaks could not be prevented by changes in the conditions during the epimerization. The composition and sequential structure of epimerized alginate was not altered by changes in the epimerization conditions. (C) 2001 Elsevier Science Inc. All rights reserved.