Amino Acid Sequence of Lima Bean Protease Inhibitor Component IV

Using ion exchange chromatography, gel filtration and paper electrophoresis eight peptides were isolated in pure form from a chymotryptic digest of lima bean inhibitor component IV. They account for 76 out of the 84 resiodues of the protein. Complete or partial sequnces of these peptides were determined using classical methods. This information, together with inforation obtained on the tryptic peptides and reported in the preceding paper allowed the deduction of the complete amino acid sequence of lima bean inhibitor IV. The results show that the original protein preparation was heterogeneous as reflected by the occurrence of conservative substitutions at positions 26, 37 and 39. A detailed examination of the sequence revealed the presence of a repetitive sequence with residues 50 through 61 clearly homologous to residues 23 through 34. Of added interest is the fact that the anti‐chymotrypsin site of lima bean inhibitor, as recently determined by Krahn and Stevns, was found to be located in one of the homologous regions. This has led us to speculated that the anti‐trypsin site could be located in the other homologous region and that this double‐headed inhibitor, with indpendent sites for trypsin and chymotrypsin, may have evolved by gene duplication.