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A nucleoside diphosphate is a nucleotide whose nucleoside carries a diphosphate chain: two phosphate groups joined through a phosphoanhydride bond. For the usual 5′ species, the phosphate nearest the sugar is attached at its 5′ position. Adenosine diphosphate is an example, with adenine as its nucleobase and ribose as its sugar. ChEBI records its acid structure as ${\mathrm{C}}_{10}{\mathrm{H}}_{15}{\mathrm{N}}_5{\mathrm{O}}_{10}{\mathrm{P}}_2$.^[ref]

Counting two phosphorus atoms is necessary for this example but does not establish that connectivity. A molecule bearing two separate phosphate esters on different sugar positions would have different connectivity from a single diphosphate chain. The chemical structure, including the bridging oxygen, distinguishes the alternatives.

The adenylate kinase reaction makes the relationship with nucleoside monophosphates and nucleoside triphosphates explicit: one adenosine triphosphate and one adenosine monophosphate interconvert with two adenosine diphosphates.^[ref] The number of nucleotide molecules stays two while their phosphate counts change from three plus one to two plus two.

An original comparison also shows why identifying a nucleoside diphosphate is more informative than measuring total phosphorus. A sample containing $1\mathrm{mmol}$ of pure adenosine diphosphate contains $2\mathrm{mmol}$ of phosphorus atoms. A different sample containing $1\mathrm{mmol}$ each of adenosine monophosphate and free phosphate has that same phosphorus total, but different molecules and reaction possibilities. Total phosphorus alone cannot discriminate the samples.

Polynucleotide phosphorylase is an enzyme that can remove nucleotides from the $3^{\prime }$ end of RNA by phosphorolysis, releasing nucleoside diphosphates. Its reversible reaction is conventionally writtenwhere ${\mathrm{P}}_{\mathrm{i}}$ is inorganic phosphate and $\mathrm{NDP}$ is the released nucleoside diphosphate. The accepted enzyme classification is EC2.7.7.8.^[ref]

For a sequence-accounting example, start with $5^{\prime }$-ACGU-$3^{\prime }$. One terminal cleavage removes the U unit as uridine diphosphate, leaving ACG. Removing the next unit releases the corresponding G-containing diphosphate. The direction is $3^{\prime }$ toward $5^{\prime }$; phosphate participation and the diphosphate product distinguish this chemistry from hydrolytic cleavage releasing a nucleoside monophosphate.

Under conditions favoring the reverse reaction, nucleoside diphosphates can supply units for chain extension. The reversible reaction itself contains no complementary template, so synthetic activity alone does not establish the sequence-copying mechanism of an RNA polymerase.^[ref]

A 2000 X-ray crystallography study revealed a three-subunit assembly with a central channel. The structure contained a phosphate analogue at catalytic sites but no bound RNA. The proposed connection between the channel, RNA handling and processivity was therefore a structural model, rather than a direct movie of a chain moving through the enzyme.^[ref]

Xrn1 is an exoribonuclease that degrades RNA from its $5^{\prime }$ end toward its $3^{\prime }$ end. It hydrolyzes successive phosphodiester bonds, releasing nucleoside monophosphates. Xrn1 participates in cytoplasmic messenger RNA turnover, and terminal chemistry helps determine which substrates it can attack.^[ref]

A short sequence makes the direction concrete. For $5^{\prime }$-ACGU-$3^{\prime }$, initial removal of A leaves CGU. By contrast, polynucleotide phosphorylase acting at the other end first removes U. An internal cut and sequential end removal therefore leave different intermediate patterns even if both eventually destroy the same original molecule.

A substrate-complex study identified a pocket recognizing the terminal $5^{\prime }$ monophosphate. The crystallized complex used a truncated, catalytically impaired protein with a short DNA oligonucleotide; biochemical assays separately tested RNA degradation. That distinction matters when connecting a static structure to the behavior of an active enzyme.^[ref]

The same study tested defined RNA–DNA hybrids with different single-stranded RNA overhangs. Degradation through the paired region occurred with five- and eight-nucleotide $5^{\prime }$ extensions but failed with a two-nucleotide extension under the tested conditions. This supports coupling of hydrolysis to strand separation after productive end engagement, without claiming that every structured RNA is equally accessible.^[ref]

End chemistry, structure and processivity thus describe separate constraints. A terminal monophosphate can favor recognition while an inaccessible end or an obstructing structure still limits degradation.