Ferritin subunit composition

Figure 6.1 Schematic representation of human isoferritins of different subunit composition. Each ferritin subunit is represented as a sausage and subunits are packed in a symmetrical shell. Twelve of the 24 subunits are visible, with H-chain subunits stippled and L-chain subunits plain. Homopolymers of H-chain and L-chain subunits are at the top and bottom of the figure respectively. The sources of various ferritins are listed in the right hand column. Reprinted from Harrison and Arosio, 1996. Copyright (1996), with permission from Elsevier Science.

Ferritin is composed by the arrangement of 24 protein subunits, which results in a hollow shell of 8 nm inner diameter and 13 nm outer diameter (Fig. 13). Ferritin from vertebrates have two types of subunits heavy (H) and light (L). The subunit composition of human ferritins depends on the origin of the protein H2L22 for liver ferritin, H20L4 for muscle ferritin, etc. Access channels are formed by the intersection of subunits. The 8 channels located at the intersection of three subunits are hydrophilic while the 6 channels located at the intersection of 4 subunits are hydrophobic. The empty protein is called apoferritin (30). 

For a more detailed discussion of ferritin biosynthesis and the possible physiological roles of isoferritins of different subunit compositions, the reader is referred to the recent literature (12, 13, 16, 18-22). In this paper the structures of the iron cores and protein coats of ferritins and the hemoferritins of bacteria are compared and the current state of knowledge concerning mineralization processes in these molecules is discussed in relation to this structural information. 

The crystallization of horse spleen apoferritin was in fact a fortuitous coincidence, because, as we mentioned earlier, attempts to crystallize horse-liver ferritin were not successful, whereas the iron-rich ferritin from horse spleen could be crystallized (Laufberger, 1937). This was certainly related to the relatively high content of H subunits (average composition L12H12) in horse liver (something that was only discovered 50 years later). It has generally proved very difficult if not impossible to crystallize heteropolymers, and the best results in crystallographic terms have been obtained with recombinant homopolymers. As will be discussed later in this chapter,... 

There are some unique examples of organic-inorganic hybrids in natural systems. Some kinds of protein possess inorganic components at their interior. Because such bio-originated hybrid structures have discrete size and composition, they can be used for preparation of fine mesoscopic structures. For example, ferritin, which is composed of 24 self-assembled peptide subunits and capable of including iron oxide, is a good candidate to fulfill this demand. Yamashita demonstrated use of a ferritin array for preparation of a mesoscopic nanodot array (Fig. 2.11 ).47 A Langmuir... 

Ferritin consists of a shell of protein subunits surrounding a core of ferric hydroxyphosphate. For some time the inorganic component of ferritin was thought to be attached to the surface of the protein (65). However, the demonstration that ferritin and apoferritin have the same electrophoretic mobilities and are precipitated equally well with horse ferritin antibody (66) together with electron microscopic evidence (67) clearly showed that the iron is concentrated in the middle of the apoferritin protein shell. The iron micelle has a diameter of 70—75 A whilst the protein shell has a diameter of the order of 120 A (67—70, 62). Somewhat lower values are found in dried preparations (71, 72). The micelle contains ferric iron, predominantly as (FeO.OH) but also with some 1—1.5% phosphate (4,50) and it seems that the iron phosphorus ratio is constant for ferritins of different iron content (52). The composition (FeO.OH) g. (FeO.OPOgHg) has been suggested (73, 74). The percentage of iron in the micelle on this basis is 57%. 

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