The hpt-1 from the Bacillus anthracis str. 'Ames Ancestor' was co-crystallized with its product, GMP, and structure was solved by molecular replacement using the 3H83 structure as a model. Crystals of the GMP-complexed protein were obtained at the same conditions as of 3H83, which has cryo-protectant molecule (sucrose) bound in the vicinity of the active site. Due to the same packing, two structures favored association only of two molecules of the product or sucrose with the same two out of four protein molecules per asymmetric unit. These structures differ from the MES-bound hpt-1 structure (PDB ID 3HVU) only in the packing pattern of the N-terminal purification tag. Three polypeptides have very low rmsd values when superimposed in pairs and with residues V148 through V153 only undergoing structural perturbation upon binding of the ligands.