102 Martynas Ycas 



active region is approximately 1 5 A on a side. If the same is true of other biological functions, 

 a great deal of surface area in a protein is passive. At least it is passive for a given specific 

 function. Thus it is reasonable to inquire how much of a protein molecule you can whittle away 

 and keep a given biological property. 



There is a fairly convincing teleological explanation for this redundancy. In the early 

 history of living systems, the membranes containing the living material might have been rather 

 leaky. Thus to retain the small biologically-active components within the cell, they had to be 

 associated with a large but inactive structure which would not pass out through the large spaces. 

 In the evolutionary scheme, then, there remain many large units where really the functional 

 part is relatively small. So that when one amino acid is taken out and another put in, the sub- 

 stitution does not make much difference so long as it is not in the essential small functioning 

 unit of the protein molecule. 



YcAS : I am also of the opinion that mere size of an enzyme may be quite important for the 

 totality of its biological functions, even if it seems to make no difference to the catalytic function 

 as measured in a test tube. Which part of a protein is significant and which is not is a matter 

 of what function we are measuring. I doubt that at present we know all the functions of 

 a protein from the point of view of the organism itself. 



REFERENCES 



1. R. W. SwiCK and D. T. Handa: The distribution of fixed carbon in amino acids. /. 

 Biol. Chem. 218, 557 (1956). 



2. R. W. SwiCK, A. L. Koch, and D. T. Handa: The measurement of nucleic acid turnover 

 in rat liver. Arch. Biochem. Biophys. 63, 226-242 (1956). 



3. R. W. SwiCK and A. L. Koch: The measurement of nucleic acid phosphorus turnover 

 in rat liver by the constant exposure technique. Arch. Biochem. Biophys. 67, 59-73 (1957). 



4. A. L. Koch and H. R. Levy: Protein turnover in growing cultures of Escherichia coli. 

 J. Biol. Chem. 217, 947-951 (1955). 



5. S. Fox: Evolution of protein molecules and thermal synthesis of biochemical substances. 

 Amer. Sclent. 44, 347-359 (1956). 



