Non-conventional enzyme catalysis: application of proteases and zymogens in biotransformations.

One of the attractions of using enzymes for chemical syntheses is the control of stereochemistry: problems of racemization that attend chemical C-N ligation methods are completely avoided. Furthermore, the enzymatic approach has the advantage that only minimal protection-deprotection steps are involved. The Impetus to develop non-conventional catalysis procedures has sprung from the lack of usable native enzymes that normally catalyze the formation of peptide bonds for biotransformation. In peptide syntheses that make use of the 'reverse hydrolysis potential' of proteases several problems need to be considered, especially the necessity of minimizing competing hydrolysis of weakly activated acyl donor esters and the need to circumvent undesired product cleavage. Some approaches to suppress competitive reactions have been developed in our group, namely leaving group manipulations at the acyl donor in kinetically controlled reactions, enzymatic synthesis in organic solvent-free micro-aqueous systems, cryoenzymatic peptide synthesis, and biotransformations in frozen aqueous systems. Finally, for the first time, zymogens, which are known as catalytically inactive precursors of proteases, could be used as biocatalysts for practically irreversible peptide bond formation.