Catalyzes the epimerization of beta-D-mannuronate to alpha-L-guluronate during the synthesis of the linear polysaccharide alginate. 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. [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Glycan biosynthesis; alginate biosynthesis. Belongs to the D-mannuronate C5-epimerase family.