Apoferritin symmetry

Horse-spleen apoferritin crystallizes in a face-centred, close-packed, cubic arrangement, in the space group F432, with molecules at the 432 symmetry points of the crystal lattice (Harrison, 1959). This publication was the logical extension of the DPhil thesis of the Oxford chemist Pauline M. Cowan (as she was before her marriage to Roy Harrison), and represented the first publication in what was to be a long and distinguished series of contributions on ferritin from the undisputed Iron Lady of iron metabolism. ... 

The arrangement of the 24 subunits of the apoferritin molecule in their 432 symmetry viewed down a fourfold axis is presented in Figure 6.3. Also included in Figure 6.3 is a labelling scheme of symmetry related subunits and a representation of the subunit as a ribbon diagram of the -carbon backbone. Of the 174 amino-acid residues of the L-chain 140 (80 %) are found in five a-helices. Each of the 24 subunits consists... 

Very low-angle X-ray data (26 A resolution) of horse spleen apoferritin fit approximately the Fourier transform of a uniform spherical shell with inner and outer diameters of 76 and 122 A (92, 95). Low-angle difference X-ray data for ferritin and apoferritin indicate iron cores of high scattering power that are approximately spherical (d = 78 A) (92). Cubic crystal point symmetry shows that ferritin molecules are composed of 24 structurally equivalent subunits related by 432 symmetry, there being one polypeptide chain per asymmetric unit. In mixed H and L chain copolymers, the apparent structural equivalence must be statistical, although very similar chain conformations are expected. 

The iron and the phosphate content as well as the exact amino acid composition, all appear to be to a degree species-dependant The term apoferritin is used to designate an empty protein shell which is devoid of iron and phosphate groups. The roughly spherical protein shell is built from 24 sub-units, arranged in octahedral symmetry and it contains eight channels which allow Fe atoms to pass in and out from the central core. The shell is stable in the pH range 2-10 and up to a temperature of about 70°C. 

The same concepts are present in nature that has evolved examples of natural supramolecular capsules deputed to the transport and/or the storage of important chemical species. Examples are the tobacco mosaic virus characterized by a rodlike appearance and composed of 2130 molecules of coat protein that surround one molecule of genomic RNA 6400 bases long. Another important representative is apoferritin, which is a capsular aggregate with octahedral symmetry formed by 24 identical peptides that present a cavity of over 230 that can store up to 4500 iron atoms as ferric hydrous oxides. 

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