Iron in biology: a structural viewpoint

The element iron is essential for life and plays a number of key roles in biology. For example, iron is the most abundant metal in humans with healthy adults possessing some 3 to 4 g. The bulk of this iron is bound to the oxygen transporting haemoglobin in the red blood corpuscles, to the muscle oxygen storage protein myoglobin, or is stored by ferritin and haemosiderin. Less than 1% of this iron is bound to the various iron enzymes and redox proteins or is being transported through the blood by transferrin. In Nature, many redox reactions are dependent on iron-containing enzymes whereby electron transport is facilitated by changes in the oxidation state of the metal. Nitrogen fixation and photosynthesis are examples of processes in which iron-containing enzymes play vital roles. This review describes the three-dimensional molecular structures of some of the more important of these iron proteins and enzymes and attempts to explain their functional roles in terms of these structures. Much of the structural information has been obtained using macromolecular x-ray crystallographic methods. The three-dimensional molecular structures provide important templates which can be used to incorporate structural information obtained by complementary techniques (biochemical, molecular biological, biophysical and spectroscopic), to form a comprehensive model for interpreting structure - function relationships. The first part of the review briefly describes recent developments in macromolecular crystallography, including the vital role played by synchrotron x-radiation sources.