Catalytic properties and specificity of a recombinant, overexpressed D-mannuronate lyase

Lysis of alginates and of their saturated and unsaturated fragments was monitored by H-1 NMR spectroscopy. AlxM(B) alginate lyase performs beta-elimination on the mannuronic acid (M) residues. It does not cleave the guluronic acid (G) sequences, nor the M-G or the G-M diads. In consequence, it is a true mannuronate lyase. The end product of the reaction is O-(4-deoxy-alpha-L-erythro-hex-4-enopyranosyl-uronic acid)-(1-->(4)-O-(beta-D-mannopyranosyluronic acid)-(1-->4)-O-beta-D-mannopyranuronic acid. Viscosity measurements made during degradation of a polymannuronate alginate showed that AlxM(B) behaves as an endo-enzyme. HPLC analysis of the degradation products of oligomannuronates and oligoalginates suggested that the beta-elimination requires the interaction of the enzyme with at least three sequential mannuronic acid residues. The catalytic site may possess 5 sub-sites and accommodate pentamers with different M/G ratio. Kinetic measurements showed that the specificity constant V-m/K-m increased with the number of mannuronic acid residues. AlxM(B) may be reversibly inhibited by heteropolymeric blocks in a competitive manner. (C) 1998 Elsevier Science Ltd. All rights reserved.