COMPLEXES OF ADENOSINE-DEAMINASE WITH 2 POTENT INHIBITORS - X-RAY STRUCTURES IN 4 INDEPENDENT MOLECULES AT PH OF MAXIMUM ACTIVITY

Adenosine deaminase, which catalyzes the irreversible hydrolytic deami nation of adenosine nucleosides to inosine nucleosides and ammonia, is a key enzyme in purine metabolism and lymphoid development. The X-ray structures of murine adenosine deaminase with bound potent inhibitors (K-i values approximate to 10(-13) M) (8R)-hydroxyl-2'-deoxycoformyci n (pentostatin), a transition state analogue, and (6S)-hydroxyl-1,6-di hydropurine riboside, a reaction coordinate analogue, have been determ ined and refined to resolutions of 2.6 and 1.95 Angstrom, respectively . Crystals of both complexes were obtained at pH 7, where the enzyme i s fully active. in an identical space group with the asymmetric unit c ontaining four molecules. In addition to the very high degree of simil arity between the four independent molecules in each complex structure , there is also considerable structural similarity of the complex with the dihydropurine riboside with that of an identical complex previous ly determined at pH 4.2 where the enzyme is 20% active. The interactio ns between the enzyme and the two analogues are extremely similar. The se include the coordination of the 8R- or 6S-hydroxyl group of the ana logues to the Zn2+ which mainly contributes to the strong potency and very high degree of stereospecificity of inhibition by these analogues . The interactions are further indicative of the structural and chemic al requirements of substrates. These structures and recent site-direct ed mutagenesis have further shed light on the catalytic mechanism of t he enzyme.