The Coordination Chemistry of Iron in Biological Transport and Storage; Iron Removal in Vivo

Abstract

Serum transferrin is the iron transport agent of mammals. Its function and structure are increasingly well understood, particularly because of the relatively recent protein crystal structure information. Transferrin not only acts as a transport agent but also functions as an iron buffer, maintaining free ferric ion concentrations in the body at a very low but constant value. This role of transferrin, and our understanding of the mechanisms of iron binding and release by the protein, are important in four areas of medical science. First, iron storage and transport are critical to understanding the molecular basis of anemias. [Iron uptake by the gut involves the eventual complexation by transferrin.] Second, iron overload occurs in several disorders, the most common due to long-term transfusion therapy of íβ-thalassemia (Cooley’s anemia). [Iron is only regulated by uptake; there is no mechanism for spontaneous removal of iron.] Chelation therapy of iron overload has long used desferrioxamine B (Desferal®). There is a continuing search for new sequestering agents that would have improved properties, particularly oral activity. One issue thus raised is the thermodynamic and kinetic ability of such sequestering agents to remove iron from transferrin. Third, the release of iron, the reductive generation of Fe²⁺ and subsequent free radical generation from reaction with O₂ (Fenton chemistry) are now thought to be the major cause of tissue damage following myocardial infarction. The use of iron chelating agents may provide a way to block such damage by keeping free ferric ion at low levels - a function of transferrin in circulating serum that is disrupted by the heart attack. Finally, the fourth area of medical relevance for transferrin iron binding and release is related to bacterial infection. The role of iron in the pathogenicity of bacterial infections is now well established. Iron availability to the invading bacterium is known to be directly connected to the virulence of infections that cause infantile enteritis, leprosy, cholera, and tuberculosis, as major examples. Several siderophores of pathogenic microorganisms are capable of removing iron from serum transferrin. Hence the mechanism(s) by which iron is released from transferrin to such ligands is of medical, as well as general biochemical, significance.

Transferrin is a bilobal protein of molecular weight 78,000 that is apparently the result of the fusion of two units of an ancestral protein. Each of the two lobes contain one metal binding site that has a very high affinity for high-spin Fe³⁺. Single crystal structures of rabbit diferric and human monoferric transferrins and lactoferrin establish that the iron binding sites of these closely related proteins are essentially identical, composed of a bidentate carbonate, 2 phenolate groups (tyrosine), 1 nitrogen (histidine) and a carboxylate oxygen (aspartate). The stable form of metal-free, apotransferrin has an “open” conformation in which the iron binding site is in an open cleft near the protein surface and is accessible to the surrounding solution. In contrast, the stable form of the iron complex has the cleft closed so that the metal binding site is buried under the surface of the protein, thus making the metal inaccessible to competing ligands. The kinetic behavior of iron removal by several different types of ligands is examined with the view of new ligand design for ligands of use in human iron decorporation therapy.