Neighboring group participation in the transition state of human purine nucleoside phosphorylase

The X-ray crystal structures of human purine nucleoside phosphorylase (PNP) with bound inosine or transition-state analogues show His²⁵⁷ within hydrogen bonding distance of the 5′-hydroxyl. The mutants His257Phe, His257Gly, and His257Asp exhibited greatly decreased affinity for Immucillin-H (ImmH), binding this mimic of an early transition state as much as 370-fold (K_m/K_i) less tightly than native PNP. In contrast, these mutants bound DADMe-ImmH, a mimic of a late transition state, nearly as well as the native enzyme. These results indicate that His²⁵⁷ serves an important role in the early stages of transition-state formation. Whereas mutation of His²⁵⁷ resulted in little variation in the PNP·DADMe-ImmH·SO₄ structures, His257Phe· ImmH·PO₄ showed distortion at the 5′-hydroxyl, indicating the importance of H-bonding in positioning this group during progression to the transition state. Binding isotope effect (BIE) and kinetic isotope effect (KIE) studies of the remote 5′-³H for the arsenolysis of inosine with native PNP revealed a BIE of 1.5% and an unexpectedly large intrinsic KIE of 4.6%. This result is interpreted as a moderate electronic distortion toward the transition state in the Michaelis complex with continued development of a similar distortion at the transition state. The mutants His257Phe, His257Gly, and His257Asp altered the 5′-³H intrinsic KIE to -3, -14, and 7%, respectively, while the BIEs contributed 2, 2, and -2%, respectively. These surprising results establish that forces in the Michaelis complex, reported by the BIEs, can be reversed or enhanced at the transition state.