Fig. 4.

Sacrificing an ancestral active site to engineer a new one. (A) The active site of T4 lysozyme (Left) provides an enzymatic function at the cost of reducing the stability of the protein structure (Right) The axes on the charts are purely qualitative to illustrate the stability-function trade-off. (B) Stability-restoring substitutions (such as E11F, D20N,T21C, and T142C) were engineered into the muramyl peptide site of lysozyme, eliminating the enzyme's activity as a hydrolase. (C) The restored stability of the lysozyme protein can now be traded for a new function, in this case Kemp eliminase activity with the introduction of the M102H mutation.