Evaluation of compositional nonrandomness in proteins

The finite sampling component of Q, a measure of nonrandomness in the amino acid composition of proteins, is quantitatively estimated by using the natural abundances of amino acids rather than the genetic code table frequencies, and the revised value of Q implies that the value of Q(c), a measure of selective effects above and beyond those imposed by the average natural abundance of the amino acids, should be 9.7 rather than its previous value of 24.3. Individual Q(c) values are given for 81 protein families. The standard deviation of the population of Q(c) values is 12.5 but the Q(c) value differs significantly from the value of zero expected were the natural abundances of the amino acids the only selective constraint. The small value of Q(c) indicates that quantitatively minimal adjustments away from the average protein composition are necessary to maintain many different biological functions.