= Iduronic acid
<Iduronic acid> is a <uronic acid> found especially as L-iduronic-acid residues in <dermatan sulfate> and <heparin>. The neutral free acid has <molecular formula> $\mathrm{C_6H_{10}O_7}$; loss of its carboxyl proton gives <iduronate>.<Iduronic acid in ChEBI>[CHEBI:24769, Definition, Formula and conjugate-base relationship]{ref}
The biologically important L form differs from D-<glucuronic acid> at carbon 5. This <epimerization> changes stereochemistry while preserving the elemental composition. In a heparin-related biosynthetic reaction, <heparosan-N-sulfate-glucuronate 5-epimerase> converts D-<glucuronate> residues already present in a polymer into L-<iduronate> residues.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[EC 5.1.3.17, Reaction and Comments]{ref}
For a hypothetical polymer segment containing ten eligible uronic-acid residues, conversion of three produces seven remaining glucuronate residues and three iduronate residues. The segment still has ten uronic-acid residues; chemical modification has changed their identities without adding three new sugar units. This is a distinction between modifying a chain and extending it.
Substrate context matters experimentally. The official enzyme record specifies a requirement for neighboring N-sulfated sugar residues and restrictions associated with O-sulfation. Supplying the free sugar alone therefore does not reproduce the stated polymer reaction.<Heparosan-N-sulfate-glucuronate 5-epimerase in IUBMB Enzyme Nomenclature>[Comments, substrate requirements]{ref}
Back to article page
