Globular proteins denaturation, table

About 20% of milk protein is soluble in the aqueous phase of milk. These serum proteins are primarily a mixture of /3-lactoglobulin, a-lactalbumin, bovine serum albumin, and immunoglobulins. Each of these globular proteins has a unique set of characteristics as a result of its amino acid sequence (Swaisgood 1982). As a group, they are more heat sensitive and less calcium sensitive than caseins (Kinsella 1984). Some of these characteristics (Table 11.1) cause large differences in susceptibility to denaturation (de Wit and Klarenbeek 1984). 

The efficiencies of excitation energy transfer from tyrosine to tryptophan residues in globular proteins in native and denatured states has been measured by studying the wavelength dependence of the fluorescence quantum yield.The results are summarized in Table 25. Unlike findings from earlier work, energy transfer is almost completely absent in the denatured state. 

Enzymes are globular proteins with greatly differing particle sizes (cf. Table 1.26). As outhned in section 1.4.2, the protein structure is determined by its amino acid sequences and by its conformation, both secondary and tertiary, derived from this sequence. Larger enzyme molecules often consist of two or more peptide chains (subunits or protomers, cf. Table 1.26) arranged into a specified quaternary structure (cf. 1.4.2.3). Section 2.4.1 will show that the three dimensional shape of the enzyme molecule is actually responsible for its specificity and its effective role as a catalyst. On the other hand, the protein nature of the enzyme restricts its activity to a relatively narrow pH range (for pH optima, cf. 2.5.3) and heat treatment leads readily to loss of activity by denaturation (cf. 1.4.2.4 and 2.S.4.4). 

Globular proteins, such as albumen, soy protein and casein, partially denature while maintaining their globular shape. When they are heated or disulfide bonds are broken, these complexes form network structures [58]. Hard proteins, such as fibrin, elastin, and keratin, possess covalent crosslinks and are difiBcult to dissolve in water. However, they swell in acid or base solutions to some extent. Table 3 summarizes the examples of bio and natural polymers that gel via intermolecular physical bonding. 

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