The enzyme catalyses the degradation of alginate by a β-elimination reaction. It cleaves the (1→4) bond between α-L-guluronate and either α-L-guluronate or β-D-mannuronate, generating oligosaccharides with 4-deoxy-α-L-erythro-hex-4-enuronosyl groups at their non-reducing ends and α-L-guluronate at the reducing end. Depending on the composition of the substrate, the enzyme produces oligosaccharides ranging from two to six residues, with preference for shorter products. cf. EC 126.96.36.199, mannuronate-specific alginate lyase.