What is the role of glu 270 and arg 145 in catalysis?

Glu 270 as a GBC (general base catalyst) in step one and as a GAC (general acid catalyst) in step two. A rg 145 p rovides specific ion-ion interactions with the C-terminal carboxylate of the substrate which confers specificity for cleavage of the C-terminal residue from the peptide substrate