Amino acid sequences involving the histidine residues of porcine trypsin.

is identical with that found in the region of the active center of bovine trypsin, which includes histidine-29 and histidine-46, except that a serine residue (asterisk) replaces a tyrosine residue in the bovine enzyme. The sequence of one of the two other histidine-containing peptides bears some similarity to the sequence surrounding histidine-79 of bovine trypsin. The lack of structural homology in this region of the molecule between bovine trypsin and bovine chymotrypsin would suggest, however, that this histidine residue is of little functional importance. The 4th histidine residue of porcine trypsin (for which there is no equivalent in the bovine enzyme) appears to lill in a deletion which had been postulated as being necessary in order to demonstrate the structural homology between bovine trypsin and bovine chymotrypsin.