@[email protected] in which [email protected]@ iron is coordinated with cysteine sulfur and usually also with inorganic sulfur. Divided into three major categories: rubredoxins; 'simple iron-sulfur @[email protected]', containing only @[email protected]; and 'complex iron-sulfur @[email protected]', containing additional active redox centres such as @[email protected], molybdenum or @[email protected] In most iron-sulfur @[email protected], the clusters function as electron-transfer groups, but in others they have other functions, such as @[email protected] of hydratase/dehydratase reactions, maintenance of protein structure, or regulation of activity.