All three sites are crucial for the enzyme's activity. In general, prokaryotic TrxRs reduce its substrate Trx in a catalytic cascade by an electron transport from the cofactors NADPH to FAD, then to the active disulfide of TrxR and finally to the active dithiol of Trx, so that Trx can then perform its essential function in redox homeostasis. In more detail, this process depends on a conformational change between an oxidised (FO) and a reduced (FR) conformation of the TrxR. In the FO conformation, FAD reduces the active disulfide. To allow the reduction of FAD by NADPH and the binding of thioredoxin with subsequent dithioldisulfide interaction, the NADPH-(Figure , grey) and FAD-binding domain (Figure , blue) rotate against each other to form the FR conformation. Also, the active disulfide of the TrxR becomes accessible in the FR conformation because it is presented on the surface while buried within the enzyme in the FO conformation. Suppose this rotation is inhibited due to an arrest in the FO conformation by a fragment located in the dimer interaction site. In that case, Trx may be unable to bind and interact with the active disulfide. If a fragment binds within the Trxbinding site, Trx binding might also be prevented. In the same manner, if the binding of NADPH is blocked, the enzymatic cascade and thus the activity of the enzyme is prevented.