The specificity of trypsin is determined by on the enzyme. The specificity of elastase is determined by on the enzyme. Three amino acids: Asp, His and Ser, of form a triad to cleave peptide bonds of a substrate.

Another Asp in something like a deep pocket; a superficial amino acid binding pocket with either a slight neutral side, neither elastase nor trypsin.  

Explanation:

The homologs with chymotrypsin include trypsin but mostly elastase. As either a central amino acid, chymotrypsin encompasses serine.

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