UDP-N-acetylgalactosamine is a nucleotide sugar combining a uridine diphosphate moiety with N-acetylgalactosamine. It serves as a donor for glycosyltransferases: the sugar-derived residue is incorporated into an acceptor, and uridine diphosphate is released. The nucleotide-linked donor and the free N-acetylgalactosamine molecule are therefore different chemical participants.[ref]
A structural example is chondroitin synthesis. Addition of an N-acetylgalactosamine residue to an appropriate growing chain uses UDP-N-acetylgalactosamine, while formation of the alternating glucuronic acid portion requires a different nucleotide sugar. The donor is not itself the finished polysaccharide.[ref]
A primary characterization of a chondroitin synthase tested donor specificity as well as product identity. Structurally related nucleotide sugars did not simply replace the required donors in that assay. This illustrates why the shared nucleotide portion does not make every nucleotide sugar interchangeable: the sugar identity and the accepting enzyme both matter.[ref]

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