Classification of Acid Denaturation of Proteins: Intermediates and Unfolded States

Abstract

A systematic investigation of the effect of acid on the denaturation of some 20 monomeric proteins indicates that several different types of conformational behavior occur, depending on the protein, the acid, the presence of salts or denaturant, and the temperature. Three major types 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 conformation, the A state, at lower pH. Two variations in this behavior were noted: some type I proteins, when titrated with HCl in the absence of salts, show only partial unfolding at pH 2 before the transition to the molten globule state; others of this class form an A state that is a less compact from 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, do not fully unfold but directly transform to the molten globule state, typically in the vicinity of pH 3. Type III proteins show no significant 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 given protein at low pH is a complex interplay between a variety of stabilizing and destabilizing forces, some of which are very sensitive to the environment. In particular, the protein conformation is quite sensitive to salts (anions) that affect the electrostatic interactions, denaturants, and temperature, which cause additional global destabilization.(ABSTRACT TRUNCATED AT 250 WORDS)

Thermodynamic insights into the binding of ANS with the salt induced molten globule states of cytochrome c

“…Salts mainly affect the electrostatic interactions in the protein molecules and induce a conformational transition at acidic or alkaline pH from a largely unfolded state to an intermediate conformational state. Such transitions have been reported for several globular and even multimeric proteins [11][12][13][14].…”

“…The A-state of cyt c has been reported in acidic solution containing high salt concentration [2,14,24] or through neutralization of charges by acetylation [25,26]. Solvent-dependent changes of charge repulsion have been suggested to be responsible for this change [13,14]. Anions may also affect the water structure, based on the Hofmeister series, which results in increasing or decreasing the hydrophobic interactions of proteins.…”

Acid and Chemical Induced Conformational Changes of Ervatamin B. Presence of Partially Structured Multiple Intermediates

“…In presence of a low-salt concentration, however, the enzyme switches from the native state to the A-state directly. This behavior is typical of the type IC proteins like papain, parvalbumin, and ribonuclease A (Fink et al, 1994). However, compared to other proteins of the type IC, the A-state is observed at very low pH for ervatamin B.…”

“…The spectra are similar to that of the unfolded protein in 6 M GuHCl. Such a loss of secondary, as well as tertiary structure, yields the acid-unfolded state (Fink et al, 1994). This indicates incomplete unfolding that is evidenced by the residual molar ellipticity in the far-UV region compared to that in 6M GuHCl.…”

Untitled

“…The extent of amyloidogenesis correlates with the concentration of amyloidogenic monomer, which is maximal at pH 4.4 9,10 . Upon further acidification (< pH 3.9), the structurally defined monomers adopt alternative conformations analogous to a molten globule-like aciddenatured state (A-state), which forms low molecular weight A-state aggregates but not amyloid fibrils 21 . These observations suggest that amyloidogenesis results from interactions between specific structural elements of alternatively folded TTR monomers.…”