Human H chains

Table 6.1 Amino-acid sequence alignment of four mammalian ferritins (Horse L chain, HoL Human L chain, HuL Human H chain, HuH Rat H, RaH) and of one of the ferritins, FTN, and the bacterioferritin, BFR of...

The detailed high-resolution structure of the non-haem-containing ferritin of E. coli EcFTNA has been recently published (Stillman et al, 2000) and shows considerable structural similarity to human H-chain ferritin (r.m.s. deviation of main chain... 

Figure 19.1 (a) View of the 24-subunit structure of human H-chain ferritin (rHuHF) viewed down the four-fold symmetry axis, (b) The subunit of rHuHF, with the short E-helix at the top of the four-helix bundle, (c) rHuHF viewed down the three-fold symmetry axis. (From Lewin et al., 2005. Copyright with permission from The Royal Society of Chemistry, 2005.)... 

Souroujoun, M.C., Rubinstein, D.B., Schwartz, R.S., et al. (1989). Polymorphisms in human H chain V region genes from the VHIII gene family. J. Immunol., 143, 706-711. 

Sanz, I. (1991). Multiple mechanisms participate in the generation of diversity of human H chain CDR3 regions. J. Immunol. 147,1720-1729. 

Scheme 2.12 Representations of the structure of human H chain ferritin viewed down the four- and three-fold symmetry axes (left and right respectively). The interactions about the fourfold axis are at 90° and are based on four subunit tetramers. Protein trimers arranged at 60 degrees to one another form the three-fold axis. (Reproduced with permission from Reference 20).

Each ferritin H-chain subunit possesses a conserved dinuclear iron site known as the ferroxidase center, which catalyzes iron oxidation. Residues E27, E61, E62, H65, and E107 act as metal hgands while residues Y34 and Q144 form hydrogen bonds (the numbering refers to the human H-chain sequence) (see Figure 6). 

Oxidation of iron by EcFtnA produces a blue-colored intermediate with an absorption maximum at 650 nm and a shoulder at 370mn. This intermediate in human H-chain ferritin corresponds to a diferric peroxo species and is likely to be the same for EcFtnA, as mutation of Y24 has indicated that it caimot be an iron-tyrosinate. ... 

The next example shows how to choose a kinetic model. The unusual kinetic curves observed by stopped-flow spectrophotometry have been successfully simulated for all levels of iron loading of human H-chain ferritin. The presence of an additional intermediate, postulated to be a hydroperoxo diFe(III) complex, was suggested by a detailed analysis of the kinetic data of 48Fe/protein samples. A mathematical model for catalysis was developed that explains the observed kinetics. The model consists of two sequential mechanisms. ... 

Schistosoma mansoni sequences 1 and 2 (76) 9, human liver L chain (74) 10, rat liver L chain (77) 11, horse spleen L chain (78). Sequences use human H chain numbering. Secondary structure elements are indicated. 

Glu 27, Glu 62, and His 65. In human and horse L ferritin these residues are replaced, respectively, by Tyr, Lys and Gly and in rat L ferritin, by His, Lys, and Gly. Examination of the sequences of ferritins given in Table VI shows that residues Glu 27, Glu 62, His 65, and the nearby Glu 107 and Gin 141 that make hydrogen bonds to a metal-coordinated water are conserved in H chains of human, rat, and chicken. They are also found in tadpole H and M subunits and the two sequences of Schistosoma ferritin (76) (which were designated H on the basis of their greater similarity to H than to L chains). Thus the ferroxidase center seems to be a property of primitive H chains as well as of mammalian H ferritins. The tadpole L chain (18) does not have these residues conserved, although it has 61% identity with human H chain and only 49% with human L and is also more similar to rat H chains (63%) than L chain (49%). It has residues Lys 27, Gin 107, and Ser 141 in place of Glu 27, Glu 107, and Gin 141 and hence would not be expected to show activity. 

Toussaint, L., Cuypers, M. G., Bertrand, L., Hue, L., Romao, C. V., Saraiva, L. M., et al. (2009). Comparative Fe and Zn K-edge X-ray absorption spectroscopic study of the ferroxidase centres of human H-chain ferritin and bacterioferritin from desulfovibrio desulfuricans. Journal of Biological Inorganic Chemistry, 14, 35—49. 

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. 

Figure 6 Schematic model of the human H-chain ferroxidase center based on the X-ray structure of Tb(111)-containing ferritin ...

In this chapter we will discuss sequences of human H chains, with some reference to H chains of the rabbit. 

Comparison of Amino Acid Sequences in the Constant Regions of Rabbit and Human H Chains... 

In contrast to the C region of human H chains, sequences in the V regions are not class-specific. This is illustrated in Fig. 4.14, which shows the complete amino acid sequences of the Vh regions of a number... 

Suran and Papermaster made the remarkable observation that in five of these positions the amino acids present are also commonly found in human k chains. These studies were extended by Klaus et al. (52) who showed, first, that there are some blocked (by pyrrolidonecar-boxylic acid) as well as unblocked H and L chains in leopard shark immunoglobulins. Most of their sequence data were, however, obtained with unblocked H chains or L chains. Sequences up to position 10 showed homology of shark L chains with human k and X chains and impressive homology of shark and human H chains. Of the first 10 positions, eight residues present in the highest proportion in shark H chains are also found at the same position in some human H chains. 

Shark H chains similarly show significant homology with human H chains. A comparison with a human Vhh sequence is shown in Table 7.3. (The H chains of the shark antistreptococcal antibodies are unblocked at the N-terminus.) In comparing the shark H chain sequence with prototypes for human Vhi, Vhh, and Vhih, the number of differences observed in the first 28 positions are 17, 13, and 14, respectively (53). 

Since the shark H chains are more closely related to mammalian H chains than to shark L chains, Sledge et al. concluded that the and Vh genes must have been in existence before the divergence of elas-mobranchs from the path leading to present-day mammals (—400 x 10 years ago). Although the shark H chains are unblocked, they bear no more resemblance to the unblocked human Vhih subgroup than to the other human Vh subgroups. Another point of interest is that shark and human H chains differ about as much as shark and human L chains. 

---