sentences:  Often general acid or base catalysis is employed to activate nucleophile and/or electrophile groups, or to stabilizing leaving groups.Many amino acids with acidic or basic groups are this employed in the active site, such as the glutamic and aspartic acid, histidine, cystine, tyrosine, lysine and arginine, as well as serine and threonine.In addition, the peptide backbone, with carbonyl and amide N groups is often employed.