Protein Structure and Information Content 113 



degree to which a particular type of residue is masked by a given treatment, 

 so that it is impossible to state exactly the fraction of surface residues which 

 are not critical. In a similar manner, it is possible to mask the lysine and arginine 

 residues on the surface of trypsin without destroying its activity (16). In fact, 

 acetyltrypsin is available commercially (17) and has the ideal feature that with 

 its lysine and arginine /^-groups masked, its ability to act as a substrate for 

 other molecules of trypsin is decreased. Haurowitz (18) has also pointed out 

 that some of the antigenic properties of proteins are in many cases not affected 

 by iodination or sulfonation of receptive surface groups. 



The work of Raacke (19) has shown that a certain amount of surface 

 heterogeneity (as demonstrated by electrophoretic behavior) is still compatible 

 with a fully active protein. Her results plus the uncertainty found in the analyses 

 of amino acid compositions indicate that an uncertainty of the order of 3 to 

 10 per cent can occur in the amino acid complement without loss in charac- 

 teristic function. The results of Roberts and Cowie (mentioned previously) 

 involving competition in the amino acid pool also indicate that about 3 to 20 

 per cent variabihty in amino acid incorporation can occur. However, it should 

 be borne in mind that each position in the polypeptide sequence may not have 

 a 3 to 10 per cent tolerance associated with it; rather, those residues which 

 participate in active sites likely have a zero tolerance. 



/j necessary: Kalnitsky and Rogers (20) have reported that approximately 

 15 per cent of the ribonuclease molecule can be digested off with carboxy- 

 peptidase before activity is lost. Work reported by Anfinsen (10, 21) indicates 

 that this estimate may be a little high. Rather, he reports that the carboxy- 

 tenninal three amino acids (valine, serine, alanine) can be removed with no 

 loss in activity; but, that digestion with pepsin which splits off these three 

 plus their neighbor, aspartic acid, and also ruptures a "strong" hydrogen 

 bond in the vicinity produces loss in activity. Partial digestion by subtilisin 

 (10, 22), which apparently digests central portions of the polypeptide chain, 

 leaves the activity of the RNase intact as long as the digested portion is not 

 oxidized. It is also known that fragments obtained either by hydrolysis or 

 partial enzymatic degradation from myosin (23-25), trypsin (26), chymotrypsin 

 (27, 28), lysozyme (29), papain (30) and pepsin (31, 32) retain their activity 

 in certain situations. The results with pepsin and papain are particularly 

 striking. Hill and Smith report no loss in the molar activity of papain (toward 

 a synthetic substrate) after an average of 120 of its 180 residues had been removed 

 by leucine-aminopeptidase (an N-terminal type enzyme). Perlmann has reported 

 that some of the dialyzable fragments (which represent 20 per cent of the 

 total original protein) resulting from pepsin auto-digestion retained 1 to 5 

 per cent of the original activity toward hemoglobin, but about 75 per cent 

 of the activity of the intact pepsin when tested against the synthetic substrate 

 acetyl 1-phenylalanyl diiodotyrosine. These latter results indicate strongly 

 that pepsin, at least, has more than one active site and the site specific for pep- 

 tide linkages adjacent to an aromatic amino acid depends upon the integrity 

 of only a small portion of the molecule. 



4 necessary: Of parallel interest to the above considerations is the question 

 of how much configurational infonnation, I„ is necessary for function? The 

 work of Anfinsen and others (10, 33) indicates that the configuration of RNase 



