<p>The ATP-grasp superfamily currently includes 17 groups of enzymes, catalyzing ATP-dependent ligation of a carboxylate containing molecule to an amino or thiol group-containing molecule [<cite idref="PUB00020972"/>]. They contribute predominantly to macromolecular synthesis. ATP-hydrolysis is used to activate a substrate. For example, DD-ligase transfers phosphate from ATP to D-alanine on the first step of catalysis. On the second step the resulting acylphosphate is attacked by a second D-alanine to produce a DD dipeptide following phosphate elimination [<cite idref="PUB00015340"/>].</p><p>The ATP-grasp domain contains three conserved motifs, corresponding to the phosphate binding loop and the Mg(2+) binding site [<cite idref="PUB00015341"/>]. The fold is characterised by two alpha-beta subdomains that grasp the ATP molecule between them. Each subdomain provides a variable loop that formspart of the active site, with regions from other domains also contributing to the active site, even though these other domains are not conserved between the various ATP-grasp enzymes [<cite idref="PUB00015342"/>].</p>