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Remote Mutations and Active Site Dynamics Correlate with Catalytic Properties of Purine Nucleoside Phosphorylase

Suwipa Saen-Oon ^* , Mahmoud Ghanem , Vern L. Schramm  and Steven D. Schwartz ^* 

^* Department of Physiology and Biophysics and Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461

Correspondence: Address reprint requests to Steven D. Schwartz, Dept. of Physiology and Biophysics, Albert Einstein College of Medicine, Bronx, NY 10461. Tel.: 718-430-2139; E-mail: sschwartz{at}aecom.yu.edu.

It has been found that with mutation of two surface residues (Lys²² Glu and His¹⁰⁴ Arg) in human purine nucleoside phosphorylase (hPNP), there is an enhancement of catalytic activity in the chemical step. This is true although the mutations are quite remote from the active site, and there are no significant changes in crystallographic structure between the wild-type and mutant active sites. We propose that dynamic coupling from the remote residues to the catalytic site may play a role in catalysis, and it is this alteration in dynamics that causes an increase in the chemical step rate. Computational results indicate that the mutant exhibits stronger coupling between promotion of vibrations and the reaction coordinate than that found in native hPNP. Power spectra comparing native and mutant proteins show a correlation between the vibrations of Immucillin-G (ImmG):O5'ImmG:N4' and H257:NImmG:O5' consistent with a coupling of these motions. These modes are linked to the protein promoting vibrations. Stronger coupling of motions to the reaction coordinate increases the probability of reaching the transition state and thus lowers the activation free energy. This motion has been shown to contribute to catalysis. Coincident with the approach to the transition state, the sum of the distances of ImmG:O4'ImmG:O5'H257:N became smaller, stabilizing the oxacarbenium ion formed at the transition state. Combined results from crystallography, mutational analysis, chemical kinetics, and computational analysis are consistent with dynamic compression playing a significant role in forming the transition state. Stronger coupling of these pairs is observed in the catalytically enhanced mutant enzyme. That motion and catalysis are enhanced by mutations remote from the catalytic site implicates dynamic coupling through the protein architecture as a component of catalysis in hPNP.