112 L. G. AUGENSTINE 



error in which he postulated that an organism (or for instance a protein- 

 controlled system) will commit as many errors as are consistent with normal 

 function, but that the inherent error rate, which is probably quite high for such 

 reactions, is maintained at a tolerable level by the use of redundancy. Resorting 

 again to the language analogy — a protein corresponds to a paragraph in 

 complexity and its function may correspond to the thought which is conveyed 

 by a paragraph. 



Does the difference in information content between the two contexts mean 

 that in the process of evolution the organisms found that particular polypeptide 

 configurations contained structures which could perform useful functions, 

 but that these polypeptide permutations contained a large amount of excess 

 and useless infomiation which has been perpetuated along with the small 

 amount of information associated with the necessary structure ? 



Does it indicate that much of the protein structure is involved in secondary 

 features of information transmission (e.g. the acquisition, concentration, 

 and transport of energy) and only a small part of the total information content 

 of the protein is intimately engaged in the process of information transmission ? 



Or does it indicate that each enzyme or p'rotein is capable of mediating 

 many reactions and our experimental ingenuity has not been able to determine 

 more than just a few of them ? (This is analogous to attempting to measure 

 the information transmitted by a source wliich is transmitting through many 

 channels, by monitoring only a single channel.) 



The discussion which follows will attempt to throw some hght on these 

 questions. However, two important considerations must always be borne in 

 mind when one is deahng with proteins. They are first and foremost colloidal 

 in nature and therefore much of their activity falls in the realm of surface 

 reactions. In the globular proteins it is quite likely that much of the total 

 structural information content is in the interior of the molecules and therefore 

 is unavailable to participate in information transfer occurring at their surface 

 and can only participate in secondary operations similar to those mentioned 

 above. The second consideration involves the question, just what is required 

 for the transmission of one bit of information by a protein system? It seems 

 very likely that one bit of potential structural information will not always 

 transmit the same amount of information; rather, the efficiency of transmission 

 will depend upon the context within which the performance is measured. 

 For example, it is probably much simpler to attach either a hydroxyl or methyl 

 group to a benzene molecule (which would involve one bit of determination) 

 than it is to construct either a 3.7 or 5.1 helix (which also involves one bit 

 of determination). This is somewhat analogous to the relative difficulties of 

 determining whether a symbol is or 1, or to determining whether one should 

 get married or not ! 



Ig necessary: It appears in some cases that a fairly large fraction of the 

 potential surface information due to the amino acids present is superfluous. 

 For instance, it has been found in insulin that a large fraction of the residues 

 cannot be critical for function. lodination, sulfonation and chelation, each 

 of which can mask surface i?-groups, have been found not to affect insulin 

 activity. Those residues which are species-specific can also be ruled out as 

 being critical for function. Unfortunately, it is difficult to determine the exact 



