116 L. G. AUGENSTINE 



that the four or so amino acid residues which would occupy such a surface 

 area (13) may occur as neighbors on the same chain (30-32). Other results 

 mentioned previously (20, 21, 33) suggest that the critical amino acids do not 

 occur in sequence in a single polypeptide chain. This follows from the con- 

 sideration that digestion should be able to consume an average of about 50 

 per cent of the protein molecule before an active site composed of four or 

 five adjacent amino acids would be encountered ; whereas one of four or five 

 amino acids making up an active site should be encountered, on the average, 

 after about a 20 to 25 per cent digestion of the molecule if the amino acids are 

 distributed roughly at random. In addition, Kennedy and Koshland (39) has 

 found that phospho-glucomutase when placed in 6 M urea loses its activity but 

 recovers it upon dilution, which also indicates separated locations for the 

 critical amino acids. Therefore it may not be possible to state a general rule 

 concerning the relationship between the loci of critical amino acids within 

 polypeptide chains. 



It seems that the role of intramolecular bonds is to insure that the amino 

 acids which are critical for function are maintained in the proper spatial 

 relationship to each other so that function can occur. Here again it is impossible 

 to state a general rule as to how many of these intramolecular bonds can be 

 disrupted before loss of function occurs, since apparently all of the hydrogen 

 bonds can be broken in RNase without loss in function but not so in phospho- 

 glucomutase. However, the integrity of the more specific secondary bonds 

 (such as S — S) seems to be much more critical for the maintenance of function. 

 The digestion experiments with pepsin and papain indicate further that it is 

 important where in the molecule the bonds are destroyed. 



Other than ruling out redundancy as a possible reason for the discrepancy 

 between the large potential information and the measured performance, it 

 is difficult to choose among the other possibilities mentioned. The results 

 with pepsin and papain, which have been mentioned, suggest strongly that much 

 of the information content may be unnecessary for function, but has been 

 perpetuated along with the critical content. However, the results with pepsin 

 indicating that multiple sites do exist makes it impossible to assign a certain 

 fraction of the information content as 'garbage'. How much of the polypeptide 

 chain is involved in secondary features of information transmission and the 

 structural complexity necessary for transmitting one bit of information are 

 factors which are now being actively investigated by a number of workers. 



The various estimates of 7^, I^ and /<. are tallied in Table II. 



Table II 



I total ~ I sequence + 'configuration 



Maximum 

 Plausible 

 Minimum 



Necessary for performing 

 a single specific function 



— 4.32 — 



>4.5 3.5 >1.0 



1.0 15/A^ 1.0 



10-90% 25% 35-90% 



