10 INFLUENCE OF TEMPERATURE ON BIOLOGICAL SYSTEMS 



protein may possess) may also be brought about by treatment with numer- 

 ous other relatively mild chemical and physical agents. 



It is natural to ask what changes in the structure of the protein molecule 

 are responsible for the process of denaturation that has just been described. 

 We know that proteins consist of long polypeptide chains that are coiled 

 up in a reproducible fashion so that the surface of the molecule presents 

 definite patterns of amino acid residues and peptide linkages to its environ- 

 ment. These patterns are believed to determine the characteristic proper- 

 ties of proteins. When the patterns are changed, the protein may lose 

 these properties. Of course, one can always change the surface pattern of 

 a protein by chemical alteration of its amino acid residues — e.g. by acetyl- 

 ating the free amino groups on the lysine residues, by iodinating the tyro- 

 sine rings, by letting the sulfhydryl groups react with silver or mercury 

 or by changing the state of ionization of a carboxyl or amino group. Chemi- 

 cal changes of this kind are not, however, usually considered to be examples 

 of denaturation, even though they may lead to a loss of activity. The kinds 

 of structural changes that might be considered to lead to denaturation 

 include the following: 1) association of two or more protein molecules 

 into larger units within which essential portions of the surface pattern 

 are buried; 2} dissociation of the protein molecule into smaller units, thus 

 fragmenting the essential patterns; 3} changes in the way in which the 

 polypeptide chain is folded, without any change in the molecular weight. 



There is much evidence that the process of denaturation described in 

 the first paragraph of this paper frequently results from the third type of 

 change. It is therefore natural to identify the process of denaturation 

 with changes resulting from an alteration in the manner in which the poly- 

 peptide chain is folded. In some instances, however, it is possible that loss 

 of biological activity may result from dissociation into sub-units without 

 any changes in the manner of folding of the polypeptide chains within 

 the sub-units. This may be the case, for instance, in the inactivation of 

 tobacco mosaic virus by urea (19). There is little evidence, however, that 

 association without accompanying changes in the state of folding ever 

 results in a loss of activity. 



In the paragraphs immediately preceding there are to be found the 

 roots of two entirely different possible definitions of the word 'denatura- 

 tion.' The definition that would result from the first paragraph of this 

 section of the paper is based on directly observable experimental criteria 

 of solubility and biological activity. The definition derived from the second 

 imragraph of this section would be phrased in terms of changes in the 

 structure of the protein molecule that may be deduced only rather in- 

 directly from experimental observations. Definitions derived from each 

 point of view have been widely used by protein chemists. Each kind of 



