Glucuronic acid epimerization

Feingold, D.S., Neufeld, E.F., and Hassid, W.Z. (1960) The 4-epimerization and decarboxylation of uridine diphosphate d-glucuronic acid by extracts from Phaseolus aureus seedlings. JBiol.Chem. 235 910-913. 

L-lduronic acid synthesis Synthesis of L-iduronic acid residues occurs after D-glucuronic acid has been incorporated into the carbohydrate chain. Uronosyl 5-epimerase causes epimerization of the D- to the L-sugar. 

Aldaric acids or their derivatives undergo ready epimerization in pyridine. Alkaline treatment of 4-O-methyl-D-glucuronic acid in the presence of air affords 4-O-methyl-D-glucaric and -D-mannaric acids as well as lower O-methylaldaric acids.262 The two diastereo-isomeric 3-deoxy-2-C-(hydroxymethyl)pentaric acids obtained from... 

At a relative humidity of 80%, the sodium salt of pig-mucosal heparin shows a fiber repeat of 1.65 nm, with a triclinic unit-cell containing one chain. The sodium salt of macromolecular heparin from rat skin, at 78% r.h., showed a repeat of 1.73 nm, and crystallized in an orthorhombic unit-cell. After 24 hours at 84% r.h., the repeat changed to 1.65 nm, the pattern corresponding to that of pig-mucosal heparin. The stereochemistry of the chain in terms of the chain repeat and , i/r maps was discussed. It was suggested that the chemical formula for heparin is poly[(l— 4)-/3-D-GlcpA-(l— 4)- -d-GlcNAcp], and a scheme of 5-epimerization of /3-D-glucuronic acid to a-L-iduronic acid was discussed. 

Other modification reactions include N-sulfation of the glucosamine residue, formation of iduronate by epimerization of glucuronate residues, and O-sulfation of C(, on glucosamine and C2 on iduronic acid. Epimerization may be linked to O-sulfation of iduronic acid, since experimental elimination of PAPS blocks the epimerization reaction. O-sulfation may stabilize the iduronic configuration around C5, favoring its formation. 

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