Ferroxidases identification

This feature of the process may be clarified by the use of spectroscopic methods. EPR (28), EXAFS 115), and Mbssbauer 12,155) spectroscopies have been applied to the study of the early stages of iron uptake and core formation and various types of iron clusters have been identified. For example, Chasteen et al. 28) have shown by EPR that a spin-coupled Fe -Fe dimer is produced at low iron protein ratios. Allied to the mutagenesis-crystallographic approach, spectroscopic examinations should aid the identification of ferroxidase and nuclea-tion sites. 

Fe(II) must then move from the 12 A long channel, and traverse a further distance of about 8 A along a hydrophilic pathway from the inner side of the three-fold channel to the ferroxidase site, and a putative pathway for Fe(II) is shown in Figure 19.6. The diiron ferroxidase centre is located in the central region of the four-helix subunit bundle and the coordination geometry of the ferroxidase centre of human H-chain ferritin is shown in Figure 19.7. Detailed analysis of the ferroxidase reaction in H-chain ferritin has allowed the identification of a number of intermediates, which are illustrated in Figure 19.8. 

Several important points have emerged from the structural analyses performed on single crystals of ferritins. The identification of the ferroxidase centers is one, the similarities and differences between ferritins from different parts of the same organism another, and the differences between, for example, mammalian and bacterioferritins a third. Another relevant consideration is the fact that very little is really known about the iron sites in vivo, especially the construction of the core. Most of what has been suggested about the coordination environments of the iron centers has been inferred from a combination of mutagenesis studies, comparison with better-characterized systems, e.g., diiron protein sites to compare with the ferroxidase center or iron oxyhydroxide minerals for the core, and some model compound studies. 

A second relevant aspect here is the consideration of models for the various dimetal species that could be important in the reactions at the ferroxidase centers. The area of di-iron metallosites has received a great deal of attention from the early 1980s since the identification of such sites in hemerythrin and the production of the first coordination chemistry model compounds for these (Chapters 8.12 and 8.13). Subsequently several other systems with this motif have been studied and the ones of most relevance to the ferroxidase centers would appear to be those involved in oxygen activation rather than transport. ... 

Lebrun NE, Andrews SC, Guest JR, Harrison PM, Moore GR, Thomson AJ. 1995. Identification of the ferroxidase center of Escherichia coli bacterioferritin. Biochem J 312 385-392. 

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