<p>Nicotinate-nucleotide pyrophosphorylase (<db_xref db="EC" dbkey="2.4.2.19"/>), also known as quinolinate phosphoribosyltransferase (decarboxylating), catalyses the conversion of nicotinate D-ribonucleotide, pyrophosphate and carbon dioxide into pyridine-2,3-dicarboxylate and 5-phospho-alpha-D-ribose 1-diphosphate. This enzyme is a type II phosphoribosyltransferase which provides the <i>de novo</i> source of nicotinate mononucleotide (NAMN) for NAD biosynthesis in both prokaryotes and eukaryotes [<cite idref="PUB00017199"/>, <cite idref="PUB00017200"/>].</p><p>Structural studies have shown that the active form of this enzyme is a homodimer with an unsual fold [<cite idref="PUB00005289"/>, <cite idref="PUB00017201"/>, <cite idref="PUB00017204"/>]. The N-terminal forms a mixed alpha/beta domain, while the C-terminal region forms a multi-stranded, open alpha/beta barrel. The active site is found at the C-terminal ends of the beta strands of the alpha/beta barrel, and is bordered by the N-terminal domain of the second subunit of the dimer. It contains several conserved charged residues thought to be important determinants of substrate binding and catalysis.</p>