Mannuronic residues

This enzyme [EC 4.2.2.3], also referred to as poly(j8-D-mannuronate) lyase and poly(mana) alginate lyase, catalyzes the eliminative cleavage of polysaccharides containing /3-D-mannuronate residues to give oligosaccharides with 4-deoxy-o -L-eryt/iro-hex-4-enopyranuro-nosyl groups at their ends. 

For example, the biosynthesis of alginate involves GDP-mannuronic acid (GDP-ManA) as NDP-Glx, bactoprenol as the lipid, and a glycosyltransferase that inserts a second mannuronate residue (as Z). 

An additional transferase that uses acetyl-CoA as a substrate sometimes acetylates one mannuronate unit. The disaccharide irnits are then inserted into the growing chain. An additional modification, which occurs after polymerization, is random C5 epimeriza-tion of unacetylated D-mannuronate residues to l-guluronate.136 182 Formation of alginate is of medical interest because infections by alginate-forming bacteria are a major cause of respiratory problems in cystic fibrosis.182... 

Polysaccharides. The most important member of this class of polysaccharides is alginate, a polysaccharide produced by seaweed and bacteria and widely used in the food industry. It is biosynthesised initially as a p-(1 4)-linked poly-mannuronic acid, by sequential transfer from GDP mannuronate. Some of the individual mannuronate residues are then epimerised at C5 to yield a-L-guluronate residues. The conformation of these guluronate residues is disfavoured by the axial OH on C2, the anomeric effect at Cl and above all by the now-axial carboxylate at C5 the C4 conformation, with the intersaccharide linkages now 1,4-diaxial, is adopted by the guluronate residues. 

Figure 1 Structure of alginate. A section of the polysaccharide with an arbitrary sequence is shown. Residues iabeied M are mannuronate residues and those labeled G are guluronate residues.

GDP-mannuronate is the activated precursor that donates M residues to mannuronan. The proteins required for this step are localized in the inner membrane and subsequent transformations of the incipient alginate occur in the periplasmic space. The final steps of alginate synthesis are epimerization and acetylation. These transformations are not carried out at every residue. Epimerization occurs at C5, converting a /3-D-mannuronate residue to a ci-L-guluronate residue. The number and pattern of distribution of G residues in mature alginate differs across species. Acetylation occurs at 02 or 03, and only on M residues. Acetylation and epimerization appear to be mutually exclusive, such that acetylation precludes epimerization and epimerization prevents acetylation. Secretion of mature alginate is mediated by AlgE, a porin-like protein. 

Figure 2. Structural properties of alginate are shown, with the linear array of Haworth structures given at the top, the conformational structure given next, and the effect of calcium on the formation of complexes between two polymeric strands of alginate given at the bottom. The epimerase catalyzed conversion of / ( —4) linked D-mannuronate to a(l—4) linked L-guluronate residues of poly(ManA) to the catenated structure of poly(GulA) and the formation of the eggbox structure upon the complexing of two polymer strands with Ca. (Reproduced with permission from reference 7. Copyright 1988 Elsevier.)...

Figure 7.2 Schematic representation of alginate cross-linking with divalent cations and the chemical structure of the constituent repeat units, guluronate (G) and mannuronate (M). Carboxylate groups present along the backbone (largely from the G residues) interact with multivalent cations to yield metal ion cross-linked gels.

GDP-D-mannuronic acid has been detected in Fucus gardneri, and incorporation into alginate by a particulate preparation was demonstrated.169 a-L-Gulosyluronic residues are formed by epimerization of /S-D-mannosyluronic residues, after polymerization170 172 (see Scheme 2). 

D-mannuronic acid residues joined by /3-1,4-gIycosidic linkages,84 and to have a chain length of about 100 hexuronic acid units.86... 

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