Protein prosthesis: a semisynthetic enzyme with a beta-peptide reverse turn.

beta-Amino acids are incorporated into an enzyme by using the method of expressed protein ligation. In the resulting semisynthetic enzyme, an R-nipecotic acid-S-nipecotic acid module replaces Asn113 and Pro114 of ribonuclease A. The semisynthetic enzyme not only retains full catalytic activity but also gains conformational stability. Thus, structural elements can be replaced with foldameric equivalents to endow proteins with more desirable properties.