Amino acid sequence and secondary structural analysis of the corn inhibitor of trypsin and activated Hageman Factor

The amino acid sequence of a corn inhibitor for trypsin and activated Hageman Factor (Factor XIIa) was determined by automated Edman degradation from the intact inhibitor and two fragments generated by specific cleavage of the inhibitor. The 112-residue sequence is unique at each position except 91, where both Ala and Glu were found. The structural heterogeneity suggests the occurrence of two genes (possibly allelic) for the inhibitor. Based on analysis of fragments produced by the interaction of the inhibitor with trypsin-agarose, the reactive site peptide bond is identified as Arg 36-Leu 37. There is no strong similarity between the sequence of the corn inhibitor and the sequences published for other serine protease inhibitors. Thus, the corn inhibitor represents a new family of protease inhibitors. Circular dichroism measurements and a theoretical prediction of secondary structure indicate that the inhibitor has helix and beta sheet contents of approximately 40 and 20%, respectively.