Aminoacyl tRNA synthetases require one active site to link tRNA to
amino acid, and another site with esterase activity to cleave off
mis-attached amino acids.
In pancreatic RNAse, two histidine residues act as a charge-relay
pair to catalyse the acid-hydrolysis of the phosphate ester bonds of
RNA. In carbonic anhydrase, three histidine residues bind a zinc ion,
on which a hydroxide ion is bound.