Iron has the capacity to accept and donate electrons readily, interconverting between ferric (Fe²⁺) and ferrous (Fe³⁺) forms. This capability makes it a useful component of cytochromes, oxygen-binding molecules (i.e., hemoglobin and myoglobin), and many enzymes. However, iron can also damage tissues by catalyzing the conversion of hydrogen peroxide to free-radical ions that attack cellular membranes, proteins, and DNA. Proteins sequester iron to reduce this threat. Iron ions circulate bound to plasma transferrin and accumulate within cells in the form of ferritin. Iron protoporphyrin (heme) and iron–sulfur clusters serve as enzyme cofactors. Under normal circumstances, only trace amounts . . .