The method for producing peptides, peptide mimetics and proteins is characterized in that an enzyme is used as a biocatalyst together with a peptide mimetic. The native specificity of the enzyme is modified by chemical or genetic manipulation, the substrate mimetic is a carboxyl component with an ester leaving group whose specificity determinant is adapted to the recognition site of the enzyme. The invention also concerns the trypsin variant D189K,K60E and its use as a C-N ligase in segment condensations.