Crystallization and preliminary X-ray analysis of the open form of human ecto-5'-nucleotidase (CD73)

Eukaryotic ecto-5'-nucleotidase (e5NT) catalyses the hydrolysis of extracellular AMP to adenosine and plays a pivotal role in switching on adenosine signalling via the P1 receptors of the purinergic signalling pathway. With such an important regulatory role, e5NT has become an appealing new drug target, with potential applications in the treatment of inflammation, chronic pain, hypoxia and cancer. In order to gain insight into the structure and function of the eukaryotic e5NT enzymes and to assist in structure-based drug design, the crystal structure of human e5NT has been solved. Recombinant human e5NT comprising four asparagine-to-aspartate surface mutations targeting potential glycosylation sites was refolded from bacterial inclusion bodies. Refolded and purified human e5NT crystallized in space group P4₃32 and a data set to 1.85 Å resolution was obtained. The structure could be solved by molecular replacement using a polyalanine model generated from Thermus thermophilus 5'-nucleotidase (5NT). An anomalous data set revealed the presence of a metal-ion binding site, as well as calcium and chloride ion-binding sites. Structural comparisons with bacterial 5NT homologues showed that the human e5NT crystal structure has an open conformation in which the metal- and substrate-binding sites are distant from each other. Here, the crystallization and preliminary X-ray crystallo­graphic analysis of an open structural conformation of human e5NT are described.