Protein Denaturation

Protein denaturation has traditionally been defined as any modification of conformation that is not accompanied by cleavage of peptide bonds involved in primary structure. This definition is interpreted differently by various researchers and disciplines owing to a general inability to recognize the phenomenon when it occurs (5,6,10). Many problems in recognizing denaturation of food proteins are due to the fact that they are rarely pure entities, often precluding direct measurement of changes in their conformation; rather, they are usually a heterogeneous mixture in isolates and in food. In addition, a conformational change may not be sufficient to effect a detectable or significant change in functional properties of a food protein. However, a significant change in functional properties is usually the result of structural alteration. Thus, from a food science perspective, protein denaturation may best be defined operationally as any modification of conformation not accompanied by alteration of primary structure that results in a change in one or more of the functional properties of the protein. Certainly, direct measurement of changes in protein conformation is preferable; however, measurement of functional properties (Table 1) as a function of denaturing conditions may provide a meaningful assessment of food protein denaturation. Denaturation may also be assessed by measuring other properties of proteins (Table 3). Because the transition from the native (N) to denatured (D) state of a protein is accompanied by a change in energy, manifested by absorption or liberation of heat (ie, enthalpy), one of the most useful and direct methods of measuring parameters associated with food protein denaturation is differential scanning calorimetry (DSC) (6). A comprehensive review of thermal analysis of proteins is given by Harwalker and Ma (39). The primary advantage of DSC is that from thermograms generated, the following parameters can be obtained directly: heat capacity (Cp) of N and D states, change in heat capacity (ACp) associated with the N ->• D transition, enthalpy change (AH) associated with the transition, and the temperature of denaturation (Td) (Fig. 2).

Denaturation may ultimately lead to a completely unfolded polypeptide structure (ie, random coil), to a more compact globular structure, or to any of a number of intermediary and often short-lived conformations (eg, molten globule). Despite the fact that the native conformation of various (purified) proteins have been characterized by X-

Table 3. Properties of Proteins to Assess Denaturation

Functional Thermodynamic Denaturation temperature Enthalphy Heat capacity Hydrodynamic

Sedimentation behavior Electrophoretic

Surface charge Spectroscopic Ultraviolet, visible absorption Intrinsic fluorescence Extrinsic fluorescence Circular dichroism Optical rotary dispersion Light scattering Infrared

Nuclear magnetic resonance Electron microscopy Biological

Catalytic/enzymatic Immunological Digestibility by proteases Chemical reactivity of functional groups o

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