CLASSIFICATION OF ACID DENATURATION OF PROTEINS - INTERMEDIATES AND UNFOLDED STATES

A systematic investigation of the effect of acid on the denaturation o f some 20 monomeric proteins indicates that several different types of conformational behavior occur, depending on the protein, the acid, th e presence of salts or denaturant, and the temperature. Three major ty pes of effects were observed. Type I proteins, when titrated with HCl in the absence of salts, show two transitions, initially unfolding in the vicinity of pH 3-4 and then refolding to a molten globule-like con formation, the A state, at lower pH. Two variations in this behavior w ere noted: some type I proteins, when titrated with HCl in the absence of salts, show only partial unfolding at pH 2 before the transition t o the molten globule state; others of this class form an A state that is a less compact form of the molten globule state. In the presence of salts, these proteins transform directly from the native state to the molten globule conformation. Type II proteins, upon acid titration, d o not fully unfold but directly transform to the molten globule state, typically in the vicinity of pH 3. Type III proteins show no signific ant unfolding to pH as low as 1, but may be caused to behave similarly to type I in the presence of urea. Thus, the exact behavior of a give n protein at low pH is a complex interplay between a variety of stabil izing and destabilizing forces, some of which are very sensitive to th e environment. In particular, the protein conformation is quite sensit ive to salts (anions) that affect the electrostatic interactions, dena turants, and temperature, which cause additional global destabilizatio n. The specific behavior of a particular system is determined by the u nderlying conformational phase diagram. A general model to account for these observations is proposed.