Native and Unfolded States of Pepsinogen. Ii. the Kinetics of the Structural Transition Induced by Urea.

In the preceding communication, the molecular properties of pepsinogen in water and water-urea solutions were evaluated (1). In this report the rates of unfolding of pepsinogen as a function of pH and temperature in urea solutions are presented. A comparison can then be made between the influence of these variables on pepsinogen denaturation and the results of a similar study reported several years ago for pepsin, the enzymatically active form of this protein (2-4). Since only about 20% of the molecule is cleaved off in the conversion of zymogen to enzyme, it should be possible to assess its influence on the kinetics of transformation of pepsinogen. It is instructive to preface an account of the kinetic properties of pepsinogen in solution with a brief summary of the relatively well studied behavior of its active derivative, pepsin. The first order rate constant of pepsin inactivation increases very rapidly in the neutral pH range, varying inversely with the 3.4th power of the hydrogen ion activity (3). The transformation from the active to the inactive molecule results in the acquisition of hydrodynamic properties, including a high and ionic strength-dependent viscosity, which are suggestive of a relatively loose and flexible configuration (2). A number of experimental observations, including the high pH dependence of inactivation rates in the neutral pH zone, the dependence of the temperature coefficient on pH in this zone, and the effects of various metallic cations such as Cu*, Pb* and Cd* in accelerating the rate, suggest that the “trigger groups ” of denaturation are the carboxyl groups (3, 4). The