*3.2. Chemical Modification of Enzymes, Directed Evolution*

More than 95% of all charged components are located on the surface of proteins, consisting mostly of hydrophilic moieties, while most of the hydrophobic ones are buried deep inside the core. As already discussed above, the physico-chemical microenvironment of the protein will be sensed intrinsically through those moieties, demonstrating the protein surface to be an attractive target for protein engineering to enhance protein stability. The efforts toward engineering enzyme catalysts with increased stability can be divided into two groups: (i) chemical protein modification and (ii) protein engineering, such as sitedirected mutagenesis and directed evolution. The combination of these two approaches is appealing for improving the catalytic properties of enzymes.
