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.^[ref]

The neutral free sugar has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{O}}_5$. As an original atom-counting exercise, begin with galactose, ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$. Replacing $\mathrm{OH}$ by $\mathrm{N}{\mathrm{H}}_2$ 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.^[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.