Structural domains comprising this superfamily share the structure of the peptidases in family G1, a subset of what were formerly termed 'pepstatin-insensitive carboxyl proteinases'. The unusual active sites of the 'pepstatin-insensitive carboxyl peptidases' were difficult to characterise, but now it is established that the enzymes from bacteria are acid-acting serine peptidases in family S53 (clan SB), whereas the fungal enzymes are in family G1, formerly A4 PMID:17069854. The latter includes scytalidoglutamic peptidase and aspergilloglutamic peptidase, both of which are involved in the cleavage of the Tyr26-Thr27 bond in the B chain of oxidized insulin; a bond not cleaved by other acid-acting endopeptidases PMID:14993599.