Epimerization uronic acids

Residues of alginate [/3-d-(1— 4)-mannuronan or poly-(D-mann-uronic acid] are converted into L-guluronic acid residues by D-mannuronan 5-epimerase, with incorporation of tritium from water-f. For this epimerization, in which C-4 cannot be oxidized, an enolization of the carbonyl group of the carboxylic acid has been proposed.148"... 

Uronic acids are rapidly epimerized at G5 by heating their salts in neutral soln. The epimers can be separated by ion exchange resins.—E The calculated... 

This forms a repeating disaccharide of [ 4)-a-L-idopyranosyl uronic acid-(1 -> 3)-P-D-A-acetyl-2-amino-2-deoxy-4-sulfato- 3-D-galactopyranosyl-(l ]. The epimerization reaction is usually incomplete so that there are also some d-glucuronic acid residues (see Fig. 6.14C) [131]. 

This uronic acid (IdoA), the 5-epimer of GlcA, is found only in glycosaminogly-cans. IdoA is not directly synthesized from a sugar nucleotide donor. Instead, it arises by epimerization of GlcA after its incorporation into the GAG chain. 

Epimerization at C-5 of hexuronic acids is a reaction that proceeds both on the polymer and on the sugar nucleotide level. In addition to the three pairs of parent acids, namely, the u-glucolL-ido-, D manno/L-gulo-, and D-ga-lacto/L-altro-he uron c acids, the 2-amino-2-deoxy acids belonging to the last two and the 2,3-diamino-2,3-dideoxy acids belonging to the middle pair have been found. 

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