Mass Modulation of Protein Dynamics Associated with Barrier Crossing in Purine Nucleoside Phosphorylase.
The role of protein dynamics on different time scales in enzyme catalysis remains an area of active debate. The connection between enzyme dynamics on the femtosecond time scale and transition state formation has been demonstrated in human purine nucleoside phosphorylase (PNP) through the study of a mass-altered enzyme. Isotopic substitution in human PNP (heavy PNP) decreased the rate of on-enzyme chemistry but did not alter either the transition state structure or steady-state kinetic parameters. Here we investigate the underlying atomic motions associated with altered barrier crossing probability for heavy PNP. Transition path sampling was employed to illuminate the molecular differences between barrier crossing in light and heavy enzymes. The mass effect is apparent in promoting vibrations that polarize the N-ribosidic bond, and that promote the stability of the purine leaving group. These motions facilitate barrier crossing.
Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Blvd., Tucson, AZ 85721.
Dimitri
D
Antoniou
Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Ave, Bronx, NY 10461.
Xiaoxia
X
Ge
Department of Biochemistry, Albert Einstein College of Medicine, 1300 Morris Park Ave, Bronx, NY 10461.
Vern L
VL
Schramm
Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Blvd., Tucson, AZ 85721.
Steven D
SD
Schwartz
barrier crossing; heavy enzyme; kinetic isotope effect; promoting vibration; purine nucleoside phosphorylase; transition path sampling