The amino acid compositions of proteins are correlated with their molecular sizes.

Natural peptides and small proteins in general have amino acid compositions that diverge much more from the average composition of all proteins than do those of proteins. The effect is large and consistent enough to provide a rough check on the measured molecular mass of a protein and to indicate whether it is likely to have a significantly repetitive structure. For example, the alpha-chain of tropomyosin, a highly repetitive protein, has no amino acid composition that would be characteristic of a much smaller protein. The observation provides support for the suggestion [Taylor, Britton & van Heyningen (1983) Biochem. J. 209, 897-899] that tetanus toxin resembles a trimer of the light chain produced by proteolysis.