Catalyzes the epimerization of beta-D-mannuronate to alpha-L-guluronate during the synthesis of the linear polysaccharide alginate (PubMed:8144447, PubMed:15968068, PubMed:16866359). In addition, is part of a periplasmic protein complex that protects alginate from degradation by AlgL by channeling the newly formed alginate polymer through a scaffold that transfers the alginate polymer through the periplasmic space to the outer membrane secretin AlgE (PubMed:12581364, PubMed:15968068). [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Inhibited by the presence of acetyl groups on the substrate. Glycan biosynthesis; alginate biosynthesis. The C-terminal region contains a right-handed beta-helix (RHbetaH) fold, which is common among proteins that bind and cleave long-chain linear polysaccharides. Deletion mutant has a non-mucoid phenotype and secretes uronic acids instead of polymerized alginate. Belongs to the D-mannuronate C5-epimerase family.