Catalyzes the epimerization of beta-D-mannuronate to alpha-L-guluronate during the synthesis of the linear polysaccharide alginate (PubMed:8830682). 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 (By similarity). [(1->4)-beta-D-mannuronosyl](n) = [alginate](n) Inhibited by zinc. Glycan biosynthesis; alginate biosynthesis. Produced during vegetative growth. This epimerase does not require calcium for its activity. Probably cannot form G blocks. Belongs to the D-mannuronate C5-epimerase family.