Proteins are also known to contain [FeS] centres, which feature one iron less than the more common [FeS] cores. Three sulfide ions bridge two iron ions each, while the fourth sulfide bridges three iron ions. Their formal oxidation states may vary from [FeS] (all-Fe form) to [FeS] (all-Fe form). In a number of iron–sulfur proteins, the [FeS] cluster can be reversibly converted by oxidation and loss of one iron ion to a [FeS] cluster. E.g., the inactive form of aconitase possesses an [FeS] and is activated by addition of Fe and reductant.