The usage of oligopeptides in proteins correlates negatively with molecular weight

We scanned the Swissprot databank to study the distribution of oligopeptides in sequenced proteins. We observed that in 7839,000 amino acids in the bank, the frequency is strongly and negatively correlated with molecular weight. The correlation coefficient is -0.63 for single amino acids, -0.58 for dipeptides, -0.54 for tri- and -0.46 for tetrapeptides. In subsets of proteins of man, mouse, rat drosophila, yeast and E. coli, the correlations were very similar to those for the total database. The intensity of the correlation diminishes linearly with the length of the peptide. We attributed the loss of dominance of molecular weight in determining the frequency of oligopeptides of higher order, to the emergence of function in longer oligopeptides. In the case of dipeptides, we observed that those composed of the same amino acid are in great excess in comparison with their expectation. The WW dipeptide is singular, in the sense that is observed/expected ratio is 13.5 standard deviations above the average ratio.