Ann. Rev. Biophys. Bioeng. 1979. 8:69-97 Copyright © 1979 by Annual Reviews Inc. All rights reserved ENZYME DYNAMICS: THE 09127 STATISTICAL PHYSICS APPROACH G. Careri Istituto de Fisica, Universita di Roma, Rome, Italy P. Fasella Istituto di Chimica Biologica, Universita di Roma, Rome, Italy E. Gratton Department of Physics, University of Illinois at Urbana-Champaign, Urbana, Illi- nois 61801 1 INTRODUCTION In spite of the fact that for a long time enzymes have been thought of as “floppy bodies,” namely as systems capable of lowering the free energy of the transition state by a convenient set of conformational fluctuations, the experimental evidence for such fluctuations and the significance of their role in catalysis are comparatively recent acquisitions. The first experimental evidence for the presence of rapid (in the nano- second range) structural fluctuations in a large number of globular proteins and enzymes comes from experiments on fluorescence quenching carried out in 1973 by Lakowicz & Weber (39). Almost contemporarily it was suggested (9) that the ability to correlate in time the fluctuations of some relevant conformational variables could be an essential kinetic property of the enzyme macromolecule. Since then, the notion of the enzyme as a fluctuating unit has gained credit and its experimental and theoretical basis has been progressively strengthened. We (10) have already offered a physical assessment of the observed statistical time events reported in the vast enzymologic literature, have discussed their identification at a molecular level, and have pointed out their possible significance for catalysis. 69 OO84——6589/ 79/ 0615—0069$0 1.00