= Uridine diphosphate
\b[Uridine diphosphate], conventionally abbreviated UDP in chemistry, is a <nucleotide> consisting of <uridine> with a linked diphosphate group at the <ribose> 5′ position. The <uridine> portion contains <uracil> and <ribose>. ChEBI records the neutral acid representation with <molecular formula> $\mathrm{C_9H_{14}N_2O_{12}P_2}$ and distinguishes it from its deprotonated forms.<Uridine diphosphate in ChEBI>[CHEBI:17659, structure, Formula, IUPAC Name and conjugate-base relation.]{ref}
A useful reaction example is <glycosyltransferase> action. <UDP-N-acetylgalactosamine> contains an additional sugar-derived portion; transferring that portion to an acceptor can release <uridine diphosphate>. The free nucleotide product and the complete <nucleotide sugar> donor are therefore not interchangeable names for one molecule.<Enzyme Nomenclature by the IUBMB>[EC 2.4.1.41, Reactions 1–2: nucleotide-sugar donor and UDP product.]{ref}
For a simple connectivity check, count the phosphorus atoms in the free product and in the linked donor: both contain two. That count alone cannot reveal whether the transferable sugar is still attached. Distinguishing the <chemical structures> explains why observing a nucleotide product can inform a transfer reaction, whereas measuring total phosphorus cannot establish the same conclusion.
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