<p>Adenylosuccinate synthetase (<db_xref db="EC" dbkey="6.3.4.4"/>) plays an important role in purine biosynthesis, by catalysing the GTP-dependent conversion of IMP and aspartic acid to AMP. Adenylosuccinate synthetase has been characterised from various sources ranging from <taxon tax_id="562">Escherichia coli</taxon> (gene purA) to vertebrate tissues. In vertebrates, two isozymes are present: one involved in purine biosynthesis and the other in the purine nucleotide cycle.</p><p>The crystal structure of adenylosuccinate synthetase from E. coli reveals that the dominant structural element of each monomer of the homodimer is a central beta-sheet of 10 strands. The first nine strands of the sheet are mutually parallel with right-handed crossover connections between the strands. The 10th strand is antiparallel with respect to the first nine strands. In addition, the enzyme has two antiparallel beta-sheets, comprised of two strands and three strands each, 11 alpha-helices and two short 3/10-helices. Further, it has been suggested that the similarities in the GTP-binding domains of the synthetase and the p21ras protein are an example of convergent evolution of two distinct families of GTP-binding proteins [<cite idref="PUB00006296"/>]. Structures of adenylosuccinate synthetase from <taxon tax_id="4565">Triticum aestivum</taxon> and <taxon tax_id="3702">Arabidopsis thaliana</taxon> when compared with the known structures from E. coli reveals that the overall fold is very similar to that of the E. coli protein [<cite idref="PUB00006475"/>].</p><p>This entry represents two conserved regions. The first one is a perfectly conserved octapeptide located in the N-terminal section and which is involved in GTP-binding, the second one includes a lysine residue known to be essential for the enzyme's activity [<cite idref="PUB00003352"/>].</p> Adenylosuccinate synthase, active site