Biochemistry. If the RNA fits, use it

Mechanisms of cellular iron sensing, regulation of erythropoiesis and mitochondrial iron utilization

To maintain an adequate iron supply for hemoglobin synthesis and essential metabolic functions while counteracting iron toxicity, humans and other vertebrates have evolved effective mechanisms to conserve and finely regulate iron concentration, storage, and distribution to tissues. At the systemic level, the iron-regulatory hormone hepcidin is secreted by the liver in response to serum iron levels and inflammation. Hepcidin regulates the expression of the sole known mammalian iron exporter, ferroportin, to control dietary absorption, storage and tissue distribution of iron. At the cellular level, iron regulatory proteins 1 and 2 (IRP1 and IRP2) register cytosolic iron concentrations and post-transcriptionally regulate the expression of iron metabolism genes to optimize iron availability for essential cellular processes, including heme biosynthesis and iron-sulfur cluster biogenesis. Genetic malfunctions affecting the iron sensing mechanisms or the main pathways that utilize iron in the cell cause a broad range of human diseases, some of which are characterized by mitochondrial iron accumulation. This review will discuss the mechanisms of systemic and cellular iron sensing with a focus on the main iron utilization pathways in the cell, and on human conditions that arise from compromised function of the regulatory axes that control iron homeostasis.

The indispensable role of mammalian iron sulfur proteins in function and regulation of multiple diverse metabolic pathways

In recent years, iron sulfur (Fe–S) proteins have been identified as key players in mammalian metabolism, ranging from long-known roles in the respiratory complexes and the citric acid cycle, to more recently recognized roles in RNA and DNA metabolism. Fe–S cofactors have often been missed because of their intrinsic lability and oxygen sensitivity. More Fe–S proteins have now been identified owing to detection of their direct interactions with components of the Fe–S biogenesis machinery, and through use of informatics to detect a motif that binds the co-chaperone responsible for transferring nascent Fe–S clusters to domains of recipient proteins. Dissection of the molecular steps involved in Fe–S transfer to Fe–S proteins has revealed that direct and shielded transfer occurs through highly conserved pathways that operate in parallel in the mitochondrial matrix and in the cytosolic/nuclear compartments of eukaryotic cells. Because Fe–S clusters have the unusual ability to accept or donate single electrons in chemical reactions, their presence renders complex chemical reactions possible. In addition, Fe–S clusters may function as sensors that interconnect activity of metabolic pathways with cellular redox status. Presence in pathways that control growth and division may enable cells to regulate their growth according to sufficiency of energy stores represented by redox capacity, and oxidation of such proteins could diminish anabolic activities to give cells an opportunity to restore energy supplies. This review will discuss mechanisms of Fe–S biogenesis and delivery, and methods that will likely reveal important roles of Fe–S proteins in proteins not yet recognized as Fe–S proteins.