Guanosine 5'-monophosphate synthetase (GMPS) is a widespread enzyme seen inall domains of life. GMPS is required for the final step of the de novosynthesis of guanine nucleotides, converting xanthosine 5'-monophosphate (XMP)into guanosine 5'-monophosphate (GMP), a precursor of DNA and RNA. GMPSconsists of two catalytic units, glutamine amidotransferase (GATase) and ATP pyrophosphatase (ATP-PPase). GATase hydrolyzes glutamineto yield glutamate and ammonia, while ATP-PPase utilises ammonia to convertadenyl xanthosine 5'-monophosphate (adenyl-XMP) into GMP. The two catalyticunits are either encoded by a single gene (two-domain type) in eucaryotes,bacteria, and some archaea, or encoded by two separate genes (two-subunittype) in other archaea. In two-domain-type GMPS, the GATase domain is locatedin the N-terminal half, and the ATP-PPase domain is located in the C-terminalhalf; in two-subunit-type GMPS, these two units exist as separatepolypeptides. ATP-PPase consists of two domains (N-domain and C-domain). TheN-domain contains an ATP-binding platform called P-loop,whereas the C-domain contains the XMP-binding site and also contributes tohomodimerisation [, , ].The GMP synthetase ATP-PPase ATP-binding domain is a twisted, five-strandedparallel β-sheet sandwiched between helical layers. Itcontains a glycine rich ATP-binding motif called the "P-loop motif"locatedafter the first β-strand [, ].
