Figure 2.

The relationship between enzyme connectivity in the yeast metabolic network and evolutionary constraint quantified by the K_a/K_s ratio. Spearman's rank correlation r = -0.20, P = 1.1 × 10^-4; Pearson's correlation r = -0.18, P = 7 × 10^-4. The connectivity of a metabolic enzyme is equal to the total number of other network enzymes producing or consuming the enzyme's reactants and products. K_a is the fraction of amino acid replacement substitutions per amino acid replacement site on DNA; K_s is the fraction of silent substitutions per silent site on DNA. The inset shows the histogram of binned enzyme connectivity versus median evolutionary constraint K_a/K_s (using the same data as in the main figure). The standard errors in each bin are also shown.