Adenosine Deaminase Converts Purine Riboside into an Analogue of a Reactive Intermediate: A¹³C NMR and Kinetic Study

The ¹³C NMR spectra of [2-¹³C]- and [6-¹³C]purine ribosides have been obtained free in solution and bound to the active site of adenosine deaminase. The positions of the resonances of the bound ligand are shifted relative to those of the free ligand as follows: C-2, –3.7 ppm; C-6, –73.1 ppm. The binary complexes are in slow exchange with free purine riboside on the NMR time scale, and the dissociation rate constant is estimated to be 13.5 s^_1 from the slow exchange broadening of the free signal. In aqueous solution, protonation of purine riboside at N-l results in changes in ¹³C chemical shift relative to those of the free base as follows: C-2, –4.9 ppm; C-6, -7.9 ppm. The changes in chemical shift that occur when purine riboside binds to the enzyme indicate that the hybridization of C-6 changes from sp² to sp³ in the binary complex with formation of a new bond to oxygen or sulfur. A change in C-2 hybridization can be eliminated as can protonation at N-l as the sole cause of the chemical shift changes. The kinetic constants for the adenosine deaminase catalyzed hydrolysis of 6-chloro- and 6-fluoropurine riboside have been compared, and the reactivity order implies that carbon-halogen bond breaking does not occur in the rate-determining step. These observations support a mechanism for the enzyme in which formation of a tetrahedral intermediate is the most difficult chemical step. Enzymic stabilization of this intermediate may be an important catalytic strategy used by the enzyme to lower the standard free energy of the preceding transition state.