This element is sandwiched between two cyclopentadienyl ligands in a compound which is the prototypical example of a metallocene.
In a protein that regulates the transport of this element, it is bound by an aspartate, a histidine, and two tyrosine residues inside an N-lobe and a C-lobe. Along with two phosphorus atoms, this heteroatom is found in the center of a Josiphos ligand. A high binding affinity for this element is possessed by enterobactin, which is an example of a siderophore. Several proteins which regulate the metabolism of this element contain a specific stem-loop motif in their mRNA transcripts known as this element's namesake "response element." This element is found in the center of cytochrome c, and it is surrounded by a tetrapyrrole group in a certain cofactor. This element is sandwiched between two cyclopentadienyl ligands in a compound which is the prototypical example of a metallocene. For 10 points, name this element which forms namesake clusters with sulfur in several proteins and is found in the center of hemoglobin.
In a protein that regulates the transport of this element, it is bound by an aspartate, a histidine, and two tyrosine residues inside an N-lobe and a C-lobe.
iron [or Fe]