Adenosine deaminase prefers a distinct sugar ring conformation for bindingand catalysis: Kinetic and structural studies

Several recent X-ray crystal structures of adenosine deaminase (ADA) in com plex with various adenosine surrogates have illustrated the preferred mode of substrate binding for this enzyme. To define more specific structural de tails of substrate preferences for binding and catalysis, we have studied t he ADA binding efficiencies and deamination kinetics of several synthetic a denosine analogues in which the furanosyl ring is biased toward a particula r conformation. NMR solution studies and pseudorotational analyses were use d to ascertain the preferred furanose ring puckers (P, nu(MAX)) and rotamer distributions (chi and gamma) of the nucleoside analogues. It was shown th at derivatives which are biased toward a "Northern" (3'-endo, N) sugar ring pucker were deaminated up to 65-fold faster and bound more tightly to the enzyme than those that preferred a "Southern" (2'-endo, S) conformation. Th is behavior, however, could be modulated by other structural factors. Simil arly, purine riboside inhibitors of ADA that prefer the N hemisphere were m ore potent inhibitors than S analogues. These binding propensities were cor roborated by detailed molecular modeling studies. Docking of both N- and S- type analogues into the ADA crystal structure coordinates showed that N-typ e substrates formed a stable complex with ADA, whereas for S-type substrate s, it was necessary for the sugar pucker to adjust to a 3'-endo (N-type) co nformation to remain in the ADA substrate binding site. These data outline the intricate structural details for optimum binding in the catalytic cleft of ADA.