 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 ${\mathrm{C}}_6{\mathrm{H}}_{10}{\mathrm{O}}_7$; loss of its carboxyl proton gives iduronate.^[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.^[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.^[ref]