Figure 2.

(a) Enzyme evolution via weak-link metabolism. The overall pathway flux on the left is dependent upon the rate of reaction B, which acts as the bottleneck for the whole pathway and, in a hypothetical single-celled organism, sets the rate for cell growth and division. Selection acts to improve the activity of enzyme B and increase the flux through this step. Step C becomes the bottleneck of the pathway and the new target of selection. (b) A hypothetical trajectory for enzyme evolution over time.