Two regions of the bifunctional protein aspartokinase I- homoserine dehydrogenase I are connected by a short hinge.

Four proteases differing in their specificity, i.e. subtilisin, trypsin, alpha-chymotrypsin and V8 staphylococcal protease, cleave the bifunctional protein Escherichia coli aspartokinase I-homoserine dehydrogenase I (composed of 820 residues) producing an active homoserine dehydrogenase fragment. This cleavage occurs within a short segment of the polypeptide chain extending from residue 293 to residue 300.