 The active site is located at a cleft between the two β-barrel domains, in which functionally important residues are contributed from each domain.Genetically engineered mutants of the chymotrypsin serine protease were shown to have some proteinase activity even though their active site residues were abolished and it has therefore been postulated that the duplication event enhanced the enzyme's activity.Modules frequently display different connectivity relationships, as illustrated by the kinesins and ABC transporters.