= UDP-N-acetylgalactosamine
{c}
\b[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.<Enzyme Nomenclature by the IUBMB>[EC 2.4.1.41, Reactions 1–2: UDP-N-acetyl-α-D-galactosamine donor and UDP product.]{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>.<Enzyme Nomenclature by the IUBMB>[EC 2.4.1.175, Reactions and Comments: chondroitin-chain extension and complementary glucuronosyltransferase activity.]{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.<Identification and molecular cloning of a chondroitin synthase from Pasteurella multocida type F by P. L. DeAngelis and A. J. Padgett-McCue>[Abstract: sugar-donor specificity and chemically tested polymer product.]{ref}
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