Catalyzes the epimerization of beta-D-mannuronate to alpha-L-guluronate during the synthesis of the linear polysaccharide alginate (PubMed:11707327). 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:12775688). [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Glycan biosynthesis; alginate biosynthesis. Deletion mutant has a non-mucoid phenotype (PubMed:11707327, PubMed:12775688). Mutant produces predominantly an unsaturated disaccharide originating from degradation by AlgL (PubMed:12775688). Belongs to the D-mannuronate C5-epimerase family.