 However, it is important to note that the accessory clusters are not as effective at extremely low potentials.The reaction occurs in the [2Fe] moiety of the active H cluster site, which also includes a [4Fe4S] cluster covalently bonded via a cysteinyl thiolate.An additional accessory site known as the F-domain, which contains four Fe-S clusters, is known as the main gate for electron transfer to and from the catalytic site.