PROTEINS 145 



rise to definite changes in chemical, physical, or biological properties." 

 Modifications of structure by addition of a group such as acetylation, 

 which obviously changes the properties of the protein, are not included 

 in the term. 



Denaturation, or more properly coagulation, of protein has been ob- 

 served by man ages before he knew about proteins. It is a phenomenon 

 that must have come to his attention soon after he began to prepare his 

 food by heating it. Despite its apparent nature, a clear understanding 

 of coagulation is still lacking. 



A most striking example of coagulation is the conversion of egg white 

 from a liquid to a solid when an egg is boiled. This change appears 

 first to involve denaturation of the egg white followed by aggregation 

 of the denatured protein into floes and then into a solid coagulum. The 

 evidence for these stages is the fact that if a solution of egg white is 

 heated in a salt-free medium below or above the isoelectric point of 

 the protein, about pH 4.7, the solution remains clear. However, the 

 increases in viscosity and SH groups, e.g., in cysteine, show that the 

 egg white has been denatured. If the pH of the clear solution is adjusted 

 to 4.7, coagulation takes place without any further heating. 



A decrease in solubility is only one of the changes that occur when a 

 protein is denatured. A greater susceptibility to the action of enzymes 

 is another effect. The digestibility of egg white, for example, is much 

 increased by heating. A third effect is a partial or complete loss of 

 biological activity, if the native protein possesses such a property. En- 

 zymes, antibodies, and viruses lose their potency when completely de- 

 natured. Loss of crystallizability, changes in viscosity, and an increase 

 in the number of reactive groups {e.g., sulfhydryl (— SH), disulfide, 

 phenol, and indole) are other changes brought about by denaturation. 

 Not all of these changes are equally apparent and, if measured quantita- 

 tively, do not run parallel to one another. These differences are prob- 

 ably an indication that different structural arrangements in the molecule 

 are affected to a varying degree by the denaturing agent. It is apparent 

 that no single criterion is adequate as an index of denaturation. If the 

 denaturation has not gone too far, the process may be reversed under 

 suitable conditions, and much of the protein recovered in the original 

 state. 



Denaturation may be brought about not only by heat but also by 

 freezing, irradiation with ultraviolet light, acid, alkali, alcohol, urea, 

 guanidine salts, and some of the new type of detergents (p. 87). The 

 last three compounds are the reagents ordinarily used in experimental 

 work to produce denaturation. 



The measurement most commonly used to detect denaturation is titra- 

 tion of the SH and S — S groups with a mild oxidizing agent. The S— S 

 groups are first reduced with cyanide to SH and then titrated. The 



