Biochemical Engineering Fundamentals, Part 1 - Bailey J., Ollis D. 1989

Kinetics of Enzyme-Catalyzed Reactions
Enzyme Inactivation
Mechanisms of Protein Denaturation and Associated Effects

As we already know, Cell/13.html">Protein Structure is stabilized by weak interactions, which provide functionally essential molecular mobility. On the other hand, the weakness of these stabilizing forces implies that, from an energetic standpoint, Proteins can relatively easily adopt several alternative, biologically less active Conformations. This property of proteins is illustrated by the thermodynamic characteristics of various states of α-lactalbumin shown in Fig. 3.26. It is easy to see that the difference in Free energy between the native and fully denatured states of this protein is only 9.0 kcal/mol (although transitioning from one conformation to another may require overcoming more significant energy barriers of transition states). Therefore, it must be admitted that the Stability of the native protein structure is extremely low.

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FIG. 3.26. Schematic representation of the stepwise unfolding of the α-lactalbumin protein molecule and the corresponding thermodynamic data. [Reproduced with permission from: Kuwajima K., A Folding Model of a-Lactalbumin Deduced from the Three-state Denaturation Mechanism; J. Molec. Biol., 114, 241 (1977).]

Designations: N — native conformation; A* — critically activated state; ID — partially disordered conformation; RC — random coil (fully denatured state).

In this regard, it is not surprising that disruptions of the natural geometric and Chemical Structure of proteins, accompanied by a decrease in activity, can be caused by a variety of Physical and Chemical factors (Table 3.9). While identifying individual factors that cause Protein Denaturation, it should also be kept in mind that the Rate of protein denaturation is determined not by each of these factors individually, but by their combination. For example, the susceptibility of a protein to denaturation at elevated temperatures can vary widely depending on the pH of the solution, and the combined effect of a specific pH and Temperature on a protein depends heavily on its nature.

A number of different but often interrelated protein properties can be identified that change under the Influence of the physicochemical factors listed in Table 3.9. External manifestations of protein denaturation include changes in properties such as solubility, the tendency to form gels or crystallize, as well as the loss of biological activity (catalytic activity, antibody-binding capacity) of protein solutions. Just as the combined action of various factors is paramount in denaturation, the combination of effects accompanying denaturation plays a key role, rather than each individual effect on its own. For example, if an enzyme is isolated by precipitation, it is essential that the precipitation conditions do not cause irreversible loss of enzyme activity.