Guluronic residues

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.)...

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. 

ALII, which cleaves between guluronate residues, is a member of PL 8 and is one of the three structures in this family which have been solved.They are small enzymes (M = 20-30 kDa), Pacman-like molecules with the two jaws , which close over the substrate, being constructed from p-strands. There are no clues as to its mechanism, the structure-based guesswork being (probably erroneously) based on the assumption that the reaction is similar to the syn eliminations catalysed by M-M specific alginate lyases. 

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. 

The kind of polysaccharides that are isolated from different bacteria are as follows Alginate, a linear copolymer with (l-4)-linked p-D-mannuronate and its a-L-guluronate residues that is produced by two bacterial genera Pseudomonas species and Azotobacter vinelandii [4]. Bacterial alginates are useful for the production of micro- or nanostructures suitable for medical applications. Cellulose, a p (1—>4) linked D-glucose unit obtained from Acetobacter xylinum. Cellulose of plant origin is usually impure as it contains... 

AiginicAcids.— The secondary and tertiary structures of alginic acid, carrageenans, and agarose have been discussed. Natural-abundance C n.m.r. spectroscopy has been used for the first time to determine the C-1 doublet and C-5 triplet frequencies in alginates which contain an unbranched chain composed of blocks of (1 - 4)-linked )S-D-mannuronate and -i guluronate residues. The triplet frequencies provide information on the monomer sequences in alginates. 

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