Enteropeptidase - Structure, function, and application in biotechnology

A preparative method for purification of enteropeptidase (enterokinase) (EC 3. 4. 21. 9) is developed. A highly purified form of this enzyme is stabil ized by calcium ions and does not contain any other proteolytic enzyme cont aminations. These enteropeptidase preparations were successfully used for c leavage of a variety of fusion proteins containing the tetraaspartyl-lysyl sequence in an arbitrary position on the polypeptide chain. A series of sub strates was methodically studied, which resulted in the suggestion that the peptide and fusion protein substrates (K-m = 200 mu M and 125 mu M, respec tively) were bound to the enzyme through the linker (Asp)(4)Lys at the bind ing site on the light chain of enteropeptidase. Much more efficient hydroly sis of the natural substrate trypsinogen (K-m = 2.4 mu M) testifies to a si gnificant contribution of other sites of the substrate and the enzyme in pr oductive binding. A variation in the enzyme's unique specificity was shown to be a result of the autolysis caused by the loss of calcium ions; the cle avage sites were determined. The truncated enzyme containing the C-terminal fragment 466-800 of its heavy chain and the intact light chain does not di stinguish the natural substrate trypsinogen, fusion protein, or peptide sub strates. These results suggest that the N-terminal fragment 118-465 of the enteropeptidase heavy chain contains a secondary substrate-binding site tha t interacts directly with trypsinogen.