There seems to be separate proofreading active site (s)on such synthetases In a few synthetases that activate amino acids having no close structural relatives. no such proofreading activities have yet identified(selected at the substrate binding level)• There seems to be separate proofreading active site (s) on such synthetases. • In a few synthetases that activate amino acids having no close structural relatives, no such proofreading activities have yet identified (selected at the substrate binding level)