The Protein Text 



85 



aggregation of smaller, chemically identical units. Furthermore, from the 

 evidence presented in Table III, the several peptide chains constituting some 

 proteins may not be identical, but are nevertheless quite similar. The statistically 

 relevant size of hemoglobin would then be somewhere between 600, the approxi- 

 mate number of residues in the whole molecule, and 150, the average size of 

 the four subunits. 



Disregarding this difficulty, 1 have plotted the variance of each amino acid, 

 calculated from Table V, against pq (Fig. 4). All points (except glx) fall within 



O.IO 



0.05 



• GLX 



• PRO 



gly^^'leu 



CYS» /ARG, 



0.05 



0.10 



pq 



Fig. 4. Plot of variances of amino acids against/?^, where/? = mean frequency 

 of occurrence of amino acid, ^ = 1 — /?. Line n = 100 calculated variance for 

 sample size (protein) of 100 residues, (glx) is plot with the values from tropo- 



mysin and y casein omitted. 



(or very close to) one standard error of the line for n — 100. The fact that the 

 sizes of the proteins are not identical tends to scatter the points, making agree- 

 ment with the hypothesis somewhat more significant. The large deviation of glx 

 is due to its abundance in two proteins, y casein and tropomyosin. If these are 

 omitted the agreement is good. 



The evidence therefore permits (but of course does not prove) the hypothesis 

 that the composition of proteins is mainly determined not by selection, but 

 rather approximates to a 'random grab' from a single universe of amino acids. 



There is of course no question that selection can produce proteins of very 



