= Amino sugar
An \b[amino sugar] is a <monosaccharide> derivative in which an alcoholic <hydroxy group> is replaced by an amino group. This commonly occurs at carbon 2, but the position is part of the chemical identity rather than a requirement that all <amino sugars> share. IUPAC describes these compounds systematically as amino-deoxy derivatives; attachment of nitrogen at the anomeric position belongs to the distinct glycosylamine naming case.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Section 2-Carb-14: amino sugars, general principles and accepted trivial names.]{ref}
For example, <glucosamine> is 2-amino-2-deoxy-D-<glucose>: comparing it with <glucose>, replace the carbon-2 $\mathrm{OH}$ by $\mathrm{NH_2}$. <N-acetylglucosamine> further modifies that nitrogen with an acetyl group. It remains an amino-sugar derivative; adding that modification does not make it a <protein> or an <amino acid>. The useful classification follows the <monosaccharide> framework and the location of the nitrogen-containing group.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Section 2-Carb-14 and cross-reference to N-substituted derivatives in 2-Carb-25.]{ref}
These modified building blocks help connect sugar chemistry to biological materials. <Hyaluronic acid> alternates <N-acetylglucosamine> with <glucuronic acid>, so every two-residue repeat contains one amino-sugar derivative. In an ideal segment with 100 complete repeats, there are 100 residues of each type, not 200 <N-acetylglucosamine> residues.<Enzymological characterization of the Pasteurella multocida hyaluronic acid synthase by P. L. DeAngelis>[Abstract: alternating two-sugar composition and use of two UDP-sugar substrates.]{ref}
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