EC 4.2.2.3; recommended name: poly(β-d-mannuronate) lyase; systematic name: poly(β-d-1,4-mannuronide) lyase. An enzyme that catalyses a reaction resulting in eliminative cleavage of polysaccharides containing β-d-mannuronate residues to give oligosaccharides with 4-deoxy-α-l-erythro-hex-4-enopyranuronosyl groups at their ends.