Helicobacter pylori is pathogenic bacterium which colonizes the gastric epithelia.[1] It is categorized as number one carcinogen inducer for gastric carcinoma.[2] Since H. pylori lacks the genes for de novo synthesis of purines, to maintain its viability, the pathogen uses purines from recycling pathway.[3] Adenylosuccinate synthetase (AdSS) is an essential enzyme in the purine synthesis and it operates in both purine de novo synthesis and purine recycling pathway. AdSS utilizes L-aspartate and IMP along with GTP and magnesium ions to form S-AMP which can be converted into AMP by adenylosuccinate lyase in the further step. As one of the steps on the path to decipher and describe the catalytic mechanism of this enzyme, we obtained the crystals and solved the structure of AdSS complexed with substrates (IMP and GTP-Mg) and inhibitor hadacidin. Analysis of the active site showed the presence of stable intermediate 6’-phosphoryl-IMP. Molecular dynamics’ simulations showed differences in conformation of the GTP binding loop depending on the presence of IMP in the active site. [1] S. Nishiumi, M. Yoshida, T. Azuma, Microb. Pathog. 2017, 109, 78–85. [2] U: IARC Monogr. Eval. Carcinog. Risks Hum.: 1994. [3] K. M. Sanders, T. A. Stern, P. T. O’gara, T. S. Field, S. L. Rauch, R. E. Lipson, K. A. Eagle, Psychosomatics 1992, 33, 35–44.