Practice question

Iron necessity stems from essential cofactor roles in fundamental enzymes: ribonucleotide reductase small subunit R2 contains diferric tyrosyl radical centre essential for reducing ribonucleoside diphosphate to deoxyribonucleotide required for DNA replication and repair, cytochrome c oxidase complex IV contains heme a3 plus copper facilitating electron transport to oxygen, and aconitase iron-sulfur cluster catalyzing tricarboxylic acid cycle. Free ferric iron Fe3+ precipitates as insoluble Fe(OH)3 at neutral pH solubility product extremely low 10^-38, and participates in Fenton reaction Fe2+ plus H2O2 producing hydroxyl radical OH extremely reactive damaging DNA strand breaks and lipid peroxidation. Transferrin family 80 kDa bilobed glycoprotein synthesized by hepatocytes and secreted into plasma at 2 to 3 milligrams per ml, each lobe N and C lobe binds one Fe3+ ion plus synergistic carbonate anion coordinating octahedrally via two tyrosines, one histidine, one aspartate, forming complex with extremely low dissociation constant 10^-22 M keeping iron soluble and redox inert protected inside binding cleft.