 Glucosamine

Glucosamine is an amino sugar, conventionally 2-amino-2-deoxy-D-glucose. Relative to the open-chain glucose reference structure, its carbon-2 hydroxy group is replaced by an amino group, with the corresponding stereochemical arrangement retained.^[ref]

Replacing $\mathrm{OH}$ by $\mathrm{N}{\mathrm{H}}_2$ removes one oxygen and adds one nitrogen and one hydrogen. Thus the neutral free base has molecular formula ${\mathrm{C}}_6{\mathrm{H}}_{13}\mathrm{N}{\mathrm{O}}_5$, compared with ${\mathrm{C}}_6{\mathrm{H}}_{12}{\mathrm{O}}_6$ for glucose. This is structural bookkeeping, not a claim that every preparation is made by one replacement reaction.

N-acetylglucosamine further modifies that nitrogen. Distinguishing the two names matters when identifying a polysaccharide's building blocks: an acetyl group changes chemical identity even though the six-carbon framework remains recognizable.^[ref]