Biological Chemistry - Berezov, T. T., & Korovkin, B. F. 1998

Protein Chemistry
Physicochemical Properties of Proteins
Protein Denaturation

Natural protein bodies possess a specific, strictly defined spatial configuration and exhibit a range of characteristic physicochemical and biological properties at physiological temperatures and medium pH. Under METABOLISM/18.html">The Influence of various Physical and Chemical factors, Proteins undergo aggregation and precipitation, losing their native properties. Thus, Denaturation is defined as the disruption of the overall pattern of the unique native protein molecule Structure—predominantly its tertiary structure—leading to the loss of its characteristic properties (such as solubility, electrophoretic mobility, biological activity, etc.). Most proteins denature when their solutions are heated above 50—60°С.

The outward manifestations of denaturation include the loss of solubility, especially at the isoelectric point, an increase in the Viscosity of Protein solutions, an increase in the number of free functional SH groups, and changes in X-ray scattering patterns. The most characteristic hallmark of denaturation is a sharp decrease or complete loss of the protein's biological activity (catalytic, antigenic, or hormonal). During Protein denaturation induced by 8M urea or other agents, non-covalent bonds (specifically, hydrophobic interactions and Hydrogen Bonds) are primarily disrupted. Disulfide Bonds are cleaved in the presence of the reducing agent mercaptoethanol, whereas the peptide bonds of the polypeptide backbone itself remain intact. Under these conditions, the globules of native protein molecules unfold to form random, disordered structures (Fig. 1.12).

With brief exposure and rapid removal of Denaturing Agents, Protein renaturation is possible, resulting in the complete restoration of the original three-dimensional structure and native Properties of the molecule (Fig. 1.13), including biological activity. Thus, upon denaturation, a protein molecule completely loses its biological properties, thereby demonstrating the close relationship between Structure and function. For practical purposes, denaturation under mild conditions is sometimes employed—for instance, when isolating Enzymes or other biologically active protein preparations at low temperatures in the presence of salts and at an appropriate pH*. During protein lyophilization (vacuum drying by sublimation of moisture from the frozen state), chemical additives (such as simple sugars, glycerol, and organic anions) are often used to prevent denaturation.

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Fig. 1.12. Denaturation of a protein molecule (schematic diagram).

a - initial state; b - early reversible disruption of the molecular structure; c - irreversible unfolding of the polypeptide chain.

Fig. 1.13. Denaturation and renaturation of Ribonuclease (after Anfinsen).

a - unfolding (urea + mercaptoethanol); b - refolding.

* The denaturation process is utilized in medical practice: in cases of poisoning with mercuric chloride or other heavy metal salts, the patient is given milk or an egg white solution as an antidote.