Iron-sulfur clusters: Formation, perturbation, and physiological functions

Iron-sulfur proteins occur in all life forms. Ferredoxins and Rieske proteins each contain a (2Fe2S) cluster whereas photosystem I (PSI) contains three (4Fe4S) clusters. Essential enzymes such as sulfite reductase, nitrite reductase, nitrogenase, glutamate synthase, aconitase, succinate dehydrogenase, ferredoxin/thioredoxin reductase, as well as many other vital proteins, each contain a (4Fe4S) cluster. Iron-sulfur clusters are formed enzymatically from cysteinyl-sulfur and ferritin-sequestered iron. Many iron-sulfur clusters are inactivated by O2 and/or reactive oxygen species (ROS) such as O2•−. Perhaps 0.1 % of the electrons passing through either the mitochondrial electron transport chain or PSI result in the formation of O2•−. Many plant stresses increase ROS formation, which subsequently may perturb iron-sulfur clusters. Plants have evolved three different superoxide dismutases (SODs) to control the internal concentrations of harmful ROS. Possible roles of functional and non-functional iron-sulfur clusters in the coordination of metabolic activities of stressed and non-stressed plants are discussed.