Native Protein Structure And Stability

The functional properties of food proteins ultimately arise from their unique molecular conformations, which derive from all four levels of structural hierarchy (ie, primary, secondary, tertiary, quaternary) (11). During synthesis on the ribosome, the polypeptide chain, pending subsequent post-translational processing (see section on protein folding, molten globules), adopts a unique molecular conformation traditionally referred to as the native conformation. This particular structure is dictated by the nature and sequence of amino acids in the polypeptide chain and is greatly influenced by solvent effects (12). The folding and resultant (native) conformation of a protein is largely governed by (equilibrium) thermodynamics. The third law of thermodynamics states that

where AG is the change in Gibbs free energy, AH is the change in enthalpy, T is absolute temperature (Kelvin), and AS is the change in entropy. To obey this law, the polypeptide chain must fold into a conformation such that the least amount of free energy is expended in maintaining it. In terms of free energy, hydrophobic effects contribute most to protein folding and stabilization. Hydrophobic interactions are nonspecific and result from the strong hydrogen-bonding properties of water. Nonpolar amino acid residues generally cannot participate in hydrogen-bonding; thus, water molecules surrounding nonpolar residues hydrogen bond with each other to form a highly ordered icelike structure that is associated with unfavorable (ie, low) entropy. The native polypeptide chain, therefore, tends to bury its hydrophobic residues within the interior of the molecule with exclusion of water from this core and to orient its hydrophilic amino acids toward the protein exterior. Exclusion of water from the protein interior causes a large increase in the entropy of the previously structured surrounding water, while the close packing of residues in the protein interior generally leads to decreased enthalpy. The folding process may thus be regarded as an entropy-driven transition from a state of higher free energy (eg, random coil) to that of lower free energy (eg, native conformation). Although this transition is largely governed by hydrophobic effects, the particular folding patterns and final protein conformation(s) are governed by formation of (specific) hydrogen bonds, disulfide linkages, and electrostatic and van der Waals interactions within the protein. Maximization of these strongly interacting driving forces leads to the native conformation of proteins. The adopted conformation may not correspond to the global minimum of free energy because this structure may represent a metastable state (ie, local minimum). Therefore, in addition to thermodynamic constraints, protein folding and conformational stability are also governed by kinetic constraints.

The thermodynamic stability of the native protein is marginal, generally not exceeding 60 kJ/mol, or the equivalent strength of only 3 to 4 hydrogen bonds or a single electrostatic interaction (Table 2) (7,8). This lability suggests that such environmental and processing factors as pH, temperature, pressure, and solvent effects may readily alter protein conformation and, consequently, functional properties and product quality to varying degrees. The ability to predict and/or estimate protein stability and conformational potential ("the capacity of biopolymers to form intermolecular junction zones that generate the desired structural rheological and other physico-chemical properties of a given food system," [13]) is, therefore, of vital importance in protein isolation and in processing of protein-containing foodstuffs (9). For example, during isolation of food proteins (especially enzymes), it is generally desirable to avoid or to at least minimize disruption of native structure because native proteins possess higher conformational potential and, as a consequence, superior functional properties. During processing, knowledge of protein stability and of the extent of conformational (and functional) change resulting from manipulation of processing factors is advantageous and often necessary if high-quality protein-containing food products are to be produced and maintained throughout subsequent storage.

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