In spite of the enormous progress in the synthesis of peptides and proteins using commercial custom peptide synthesis and the immense technological possibilities of recombinant DNA technology, a C.sbd.N ligase is an indispensable tool for the racemization-free fragment condensation of peptides. Since activation of the C-terminal .alpha.-carboxyl groups of a peptide segment could cause partial racemization, chemical condensations of peptide fragments are prone to racemization.
For the synthesis of the huge number of peptides and proteins, however, nature has only developed the ribosomal peptidyltransferase, which exhibits its full catalytic function independent of the side-chain functions of the amino acids being coupled. However, its function requires coordination with numerous other ribosomal factors. Besides the limited possibilities of using multienzyme complexes of bacterial custom peptide synthesis, the only alternatives to peptidyltransferase are proteases, which, based on their in vivo function as hydrolases, cannot act as ideal ligases. However, by exploiting the intrinsic reversibility of hydrolytic reactions and by adjusting appropriate physicochemical reaction parameters, the protease activity can be used in the direction of ligation.
Undoubtedly, the course of kinetically controlled, serine and cysteine protease-catalyzed reactions can be more efficiently influenced than the equilibrium-controlled protease-catalyzed synthesis. This article describes the influence of the enzyme specifically on the efficiency of kinetically controlled synthesis and points the way toward a broad exploitation of serine and cysteine protease for the catalysis of C.sbd.N bond formation.