W. KAUZMANN 21 



r 



trypsinogen is converted to chymotrypsin, [a]i, being -77° for the zymogen 

 and -66° for the enzyme (29). This change has recently been studied in 

 some detail by Rupley, Dreyer and Neurath (27), who find that the 

 change in optical rotation parallels the appearance of enzymatic activity. 

 Since no alpha amino end group is detectable in chymotrypsinogen, whereas 

 one is present in chymotrypsin, it is believed that the zymogen is a large 

 cyclic peptide and that one stage of the activation process entails the 

 opening of the polypeptide ring. It is natural to suspect that the presence of 

 the closed ring in the zymogen keeps the polypeptide chain from folding 

 as completely into a helix as it might otherwise if the ring were opened. 

 The rupture of one peptide link during the conversion to the enzyme would 

 then make it possible for more of the chain to fold into a helix, and since 

 the helix appears to be dextrorotatory, this would lead to the observed 

 decrease in the levorotation on activation of the enzyme. 



The large number of cystine disulfide crosslinkages in the serum albumin 

 molecule appears to interfere in a similar fashion with the development of 

 helices. Earlier work had shown (7), through the effect of reducing agents 

 on the viscosity of denatured serum albumin in urea, that the two halves 

 of many, if not all, of the cystine residues are not situated at adjacent 

 positions along the polypeptide chain, so that the molecule is rather 

 extensively cross-linked. Unless these cross links were especially favorably 

 located along the chain, they would be expected to interfere with the 

 normal development of a helical structure within the serum albumin 

 molecule. That such interferences actually exist is indicated by the rela- 

 tively large optical rotation of serum albumin ([a]u = —60°) and by 

 the great ease with which it may be denatured in urea. It is very interesting, 

 therefore, that Markus and Karush (22) have recently observed that 

 when the disulfide groups of serum albumin are reduced (detergent must 

 be present in order to make the disulfide groups sufficiently reactive), a 

 considerable decrease in the levorotation takes place. 



It is tempting to suppose that proteins can exist in two extreme states of 

 folding: an 'ultra-native' form which would have completely developed 

 helices and which would have a relatively small levorotation, or perhaps 

 even a small dextrorotation ; and a denatured form that contains no helices 

 at all and is rather strongly levorotatory at ordinary temperatures. In 

 native protein molecules as they exist in living systems, the polypeptide 

 chain exists in both states of folding, the relative amounts that are 

 present being different in different proteins. The magnitude of the levo- 

 rotation would then be a measure of the amount of the denatured type 

 of folding present in the molecule. Thus serum albumin ([a]i) = —60°) 

 would contain a relatively large amount of the denatured type of folding, 

 whereas ovalbumin {[a]u = —30°) and beta-lactoglobulin ([ajp = —26°) 



