desirable functional properties that differ from those of the native protein (eg, texturized plant proteins). Denaturation is generally reversible if the denatured protein (ie, unfolded polypeptide chain) is thermodynamically stabilized by the denaturant; removal of the denaturant allows the native conformation of the protein to be reestablished. In addition, small molecular weight proteins are more likely to renature than large molecular weight proteins. Denaturation is irreversible if the unfolded polypeptide chain is stabilized by interactions with other denatured proteins, as in acid or heat-induced protein aggregation. If disulfide bonds contribute to protein conformation and these are broken, denaturation is often irreversible.

Protein denaturation is generally a cooperative multistate process (4) with many more-or-less unfolded intermediates (Dj) between the N and D states:

These intermediate structures correspond to progressive stages in the alteration of the protein conformation. A protein does not have a rigid or static conformation but, rather, a dynamic conformation, undergoing rapid thermodynamic fluctuations about a mean conformation. Thus, each of the states in the N -ยป D; transition may be regarded as a population of structures of similar energy. Their existence or identification as discrete states requires that their populations not overlap.

Thermodynamic analysis of protein denaturation and/ or conformational stability according to the foregoing scheme is complicated. Although often an oversimplification, it is useful to consider the denaturation process in terms of a two-state equilibrium (Fig. 4) (4,6-8). Use of a two-state model is valid if the DSC enthalpy change (AH, Fig. 2) approximates the van't Hofif enthalpy change. The equilibrium or denaturation rate constant for a two-state equilibrium can be written

where kD is the equilibrium constant and fN is the fraction of protein molecules in the native state. Reversible and

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