= Galactosamine
<Galactosamine> is an <amino sugar> derived from <galactose> by replacing its carbon-2 <hydroxy group> with an <amino group>. The conventional D compound is 2-amino-2-deoxy-D-galactose. Specifying the D configuration does not by itself specify the alpha or beta <anomer> of a cyclic form.<Nomenclature of Carbohydrates (Recommendations 1996) by Alan D. McNaught>[Section 2-Carb-14: amino-sugar definition and accepted D-galactosamine name]{ref}
The neutral free sugar has <molecular formula> $\mathrm{C_6H_{13}NO_5}$. As an original atom-counting exercise, begin with <galactose>, $\mathrm{C_6H_{12}O_6}$. Replacing $\mathrm{OH}$ by $\mathrm{NH_2}$ removes one oxygen and adds one nitrogen and one hydrogen overall. This gives the stated formula; it is structural bookkeeping, not a claim that this substitution is a one-step laboratory synthesis.<D-galactosamine pyranose in ChEBI>[CHEBI:60312, formula and pyranose identity; CHEBI:148869, explicitly alpha form]{ref}
<Galactosamine> and <glucosamine> differ in the configuration at carbon4 of their corresponding D-sugar frameworks. Adding an <acetyl group> to the nitrogen instead produces <N-acetylgalactosamine>, a different structural modification. Neither relationship is determined by a formula alone: <epimers> can share a formula, while nitrogen substitution can change it.
In <chondroitin>, the chain contains <N-acetylgalactosamine>-derived residues rather than simply a collection of free <galactosamine> molecules. Distinguishing the free sugar, its nitrogen-modified derivative and its residue within a <polysaccharide> prevents confusing composition with molecular identity.
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