OPRTase, as a member of type I PRTases, has a prominent loop next to its active site. It is flexible in its open state and can hardly be seen in electronic density maps for some OPRTases. For catalysis to occur, a dimer must exist in which a loop from one subunit covers the active site from the other one. In "Salmonella typhimurium", a new pair of antiparallel β-sheets is created and five new interatomic contacts are formed in the loop, between the loop and the rest of the protein and between the loop and the ligands.