Ferritin: A Model System for Iron Biomineralization

Abstract

Iron plays an essential role in many metabolic functions such as electron transfer and oxygen transport. Although it can sometimes be replaced for a specific function, e.g. the Cu-containing haemocyanins carry oxygen in some arthropods and molluscs, there are very few organisms, possibly only some lactobacilli for which it is not an essential element. Iron biology must reflect its chemical properties. At physiological pH free unco-ordinated Fe(III) is highly insoluble (10-17M at pH7) readily hydrolysing and precipitating as oxyhydroxide and this can act as a powerful driving force for the oxidation of Fe(II). These properties have been exploited by the ferritin molecule, which is designed to enable Fe(II) oxidation and hydrolysis to take place within the confined interior of a protein shell1,2. The ferric oxyhydroxide so formed is a crystalline mineral known as fer rihydrite. In ferritin the particle size of the mineral is limited to less than 8.0nm diameter, the size of the cavity in iron-free coat protein (apoferritin). Since the three-dimensional structure of apoferritin is known, since it can be altered by site-directed mutagenesis and since the formation of its ferrihydrite “iron-core” can be investigated by a variety of spectroscopic methods and by electron microscopy, ferritin provides a good model system for studies of iron biomineralization. Nevertheless, it is not a simple system.