The catalytic site structural gate of adenosine deaminase allosterically modulates ligand binding to adenosine receptors

The enzyme adenosine deaminase (ADA) is a multifunctional protein that can both degrade adenosine and binds extracellularly to adenosine receptors, acting as an allosteric modulator regulating the hormonal effects of adenosine. The molecular regions of ADA responsible for the latter are unknown. In this paper, alanine scanning mutagenesis of various ADA amino acid stretches, selected through in silico docking experiments, allowed to identify regions of the enzyme responsible for modulating both its catalytic activity and its ability to modulate agonist binding to A1 and A2A adenosine receptors (A1R and A2AR). The combination of computational and in vitro experiments show that the structural gate to the catalytic site, i.e the α-1 helix containing residues L58-I72 and the loop containing residues A184-I188 of ADA were important to maintain both the catalytic efficiency of the enzyme and its action as an allosteric modulator of the adenosine receptors. These data are consistent with a predicted supramolecular assembly, in which ADA bridges A2AR and CD26 and are in line with the notion that the interaction of ADA with adenosine receptors has an important role in the immunosynapse. We propose that it is the ADA open form, but not the closed one, that is responsible for the functional interaction with A1R and A2AR.