In the de novo synthesis of purine nucleotides, IMP is the branch point metabolite at which point the pathway diverges to the synthesis of either guanine or adenine nucleotides. In the guanine nucleotide pathway, there are 2 enzymes involved in converting IMP to GMP, namely IMP dehydrogenase (IMPD1), which catalyzes the oxidation of IMP to XMP, and GMP synthetase, which catalyzes the amination of XMP to GMP.[6]
This enzyme belongs to the family of ligases, specifically those forming carbon-nitrogen bonds carbon-nitrogen ligases with glutamine as amido-N-donor. The systematic name of this enzyme class is xanthosine-5'-phosphate:L-glutamine amido-ligase (AMP-forming). This enzyme participates in purine metabolism and glutamate metabolism. At least one compound, Psicofuranin is known to inhibit this enzyme.
Structural studies
As of late 2007, 5 structures have been solved for this class of enzymes, with PDB accession codes 1GPM, 1WL8, 2A9V, 2D7J, and 2DPL.
Role in metabolism
Purine metabolism
GMP synthase is the second step in the generation of GMP from IMP; the first step occurs when IMP dehydrogenase generates XMP, and then GMP synthetase is able to react with glutamine and ATP to generate GMP. IMP may also be generated into AMP by adenylosuccinate synthetase and then adenylosuccinate lyase.[7]
Amino acid metabolism
GMP synthase is also involved in amino acid metabolism because it generates L-glutamate from L-glutamine.[7]
The amidotransferase domain is responsible for removal of the amide nitrogen from the glutamine substrate. The class I amidotransferase domain is made of the N terminal 206 residues of the enzyme, and consists of 12 beta strands and 5 alpha helices; the core of this domain is an open 7-stranded mixed beta sheet. Its catalytic triad includes Cys86, His181 and Glu183. His181 is a base and Glu183 is a Hydrogen bond acceptor from the Histidineimidazole ring. Cys86 is the catalytic residue and is conserved. It falls into a nucleophile elbow, where it is at the end of a beta strand and the beginning of an alpha helix, and has little flexibility in its phi and psi angles; thus, Gly84 and Gly88 are conserved and allow for the tight packing of amino acids surrounding the catalytic residue.[1]
Synthetase Domain: ATP Pyrophosphatase domain
The synthetase domain is responsible for the addition of the abstracted Nitrogen to the acceptor substrate. The ATP Pyrophosphatase domain consists of a beta sheet containing 5 parallel strands with several alpha helices on each side. The P loop is the nucleotide binding motif; residues 235-241 make up the P loop which specifically binds to pyrophosphate.[1]
The structure of this domain is what creates the specificity of this enzyme for ATP. The binding pocket forms hydrophobic interactions with the adenine ring, and the backbone of Val260 forms H bonds with multiple Nitrogens in the ring of AMP, which excludes substituents on the C2 purine ring. This creates extreme specificity for adenine and ATP binding.[1]
References
^ abcdefTesmer JJ, Klem TJ, Deras ML, Davisson VJ, Smith JL (January 1996). "The crystal structure of GMP synthetase reveals a novel catalytic triad and is a structural paradigm for two enzyme families". Nature Structural Biology. 3 (1): 74–86. doi:10.1038/nsb0196-74. PMID8548458. S2CID30864133.
Maruyama K, Sugano S (January 1994). "Oligo-capping: a simple method to replace the cap structure of eukaryotic mRNAs with oligoribonucleotides". Gene. 138 (1–2): 171–4. doi:10.1016/0378-1119(94)90802-8. PMID8125298.
Fedorova L, Kost-Alimova M, Gizatullin RZ, Alimov A, Zabarovska VI, Szeles A, et al. (1997). "Assignment and ordering of twenty-three unique NotI-linking clones containing expressed genes including the guanosine 5'-monophosphate synthetase gene to human chromosome 3". European Journal of Human Genetics. 5 (2): 110–6. doi:10.1159/000484744. PMID9195163.
Suzuki Y, Yoshitomo-Nakagawa K, Maruyama K, Suyama A, Sugano S (October 1997). "Construction and characterization of a full length-enriched and a 5'-end-enriched cDNA library". Gene. 200 (1–2): 149–56. doi:10.1016/S0378-1119(97)00411-3. PMID9373149.
Pegram LD, Megonigal MD, Lange BJ, Nowell PC, Rowley JD, Rappaport EF, Felix CA (December 2000). "t(3;11) translocation in treatment-related acute myeloid leukemia fuses MLL with the GMPS (GUANOSINE 5' MONOPHOSPHATE SYNTHETASE) gene". Blood. 96 (13): 4360–2. doi:10.1182/blood.V96.13.4360. PMID11110714.
Guo D, Han J, Adam BL, Colburn NH, Wang MH, Dong Z, et al. (December 2005). "Proteomic analysis of SUMO4 substrates in HEK293 cells under serum starvation-induced stress". Biochemical and Biophysical Research Communications. 337 (4): 1308–18. doi:10.1016/j.bbrc.2005.09.191. PMID16236267.
Abrams R, Bentley M (1959). "Biosynthesis of nucleic acid purines. III. Guanosine 5'-phosphate formation from xanthosine 5'-phosphate and L-glutamine". Arch. Biochem. Biophys. 79: 91–110. doi:10.1016/0003-9861(59)90383-2.
