Histidine sequences in the active centres of some ‘serine’ proteinases

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Abstract
A comparison of the diagonal "maps" of chymotrypsin A and "tosylphenylalanyl chloromethyl ketone"-inhibited chymotrypsin A showed that His-57 is alkylated specifically by this substrate analogue. From peptic digests of chymotrypsinogen A and B, trypsin and elastase it was demonstrated by the diagonal elec-trophoretic technique that a common di-histidine cystine-bridged structure is present in all 4 enzymes. The sequences of these pep-tides were determined and show that the positions of the 2 histidine residues relative to the dlsulfide bond are a common feature. Thus His-40 of chymotrypsin A is only 2 residues removed from CyS-42, and His-57 is adjacent to the other half of this bridge, CyS-58. Considerable variation In sequence occurs about His-40, where the aromatic residues 39 and 41 of the chymotrypsins and trypsin are replaced by alanine and threonine in elastase. There is a remarkable similarity in sequence following CyS-42 and preceding CyS-58 in all 4 enzymes.