 Uridine diphosphate

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 ${\mathrm{C}}_9{\mathrm{H}}_{14}{\mathrm{N}}_2{\mathrm{O}}_{12}{\mathrm{P}}_2$ and distinguishes it from its deprotonated forms.^[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.^[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.