Protein stability extrinsic determinants

Because protein-ba sed foams depend upon the intrinsic molecular properties (extent and nature of protein-protein interactions) of the protein, foaming properties (formation and stabilization) can vary immensely between different proteins. The intrinsic properties of the protein together with extrinsic factors (temperature, pH, salts, and viscosity of the continuous phase) determine the physical stability of the film. Films with enhanced mechanical strength (greater protein-protein interactions), and better rheological and viscoelastic properties (flexible residual tertiary structure) are more stable (12,15), and this is reflected in more stable foams/emulsions (14,33). Such films have better viscoelastic properties (dilatational modulus) ( ) and can adapt to physical perturbations without rupture. This is illustrated by -lactoglobulin which forms strong viscous films while casein films show limited viscosity due to diminished protein-protein (electrostatic) interactions and lack of bulky structure (steric effects) which apparently improves interactions at the interface (7,13 19). 

A number of factors may affect enzyme stabihty in vivo. Certain enzymes from thermophiles have been shown to be less stable in the pure state or in cell-free extracts than in vivo, integrity being enhanced by factors in the cellular environment [29], such as high protein concentration, substrates, prosthetic groups and other molecules [30-34]. The fact remains, however, that thermophUic enzymes as a class are inherently more thermostable than their mesophilic counterparts, and that this stability is primarily intrinsic rather than being determined by extrinsic factors. 

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