Transferrin prosthetic group

FIGURE 5.8 Complex hyperfine patterns due to axes noncolinearity in a low-symmetry prosthetic group. The X-band spectrum is from 65Cu(II)-bicarbonate in human serum transferrin (a,b) experimental spectrum (c,e) simulation assuming axial symmetry (d, f) simulation assuming triclinic symmetry with the A-axes rotated with respect to the g-axes over 15° about the gz-axis and then 60° about the new y -axis. Traces b, e, and f are 5x blow-ups of traces a, c, d, respectively (Hagen 2006). (Reproduced by permisson of The Royal Society of Chemistry.)... 

The apparent absence in the transferrins of any prosthetic group or chemical structure uncommon to proteins might suggest that the transferrins contain some unusual sequences of amino acids or some unusual juxtaposition of residues in the tertiary structure. Unfortunately, there is no sequence data available and very little is known about tertiary structure so the information presently available is essentially nothing more than sterile listings of amino acid compositions. 

The relationships among the transferrins in the different fluids from the same individual are of obvious interest. Unfortunately, only limited data are available on this subject. A close immunological relationship of chicken serum transferrin and chicken ovotransferrin has been reported many times (20, 65, 79, 92, 136). Williams (136) in a detailed study by immunological methods, peptide mapping, and amino acid analyses reported that chicken serum transferrin and chicken ovotransferrin differ only in their carbohydrate prosthetic groups. 

It is known that superoxide reacts very slowly with all amino-acids since all rate constants are below 100 mol l 1 s (89). Hence its reactivity with proteins without prosthetic group is low (89). One exception seems to be collagen, in which proline residues are oxidized into hydroxyproline (90). On the other hand, superoxide reacts efficiently with free radicals such as tryptophanyl radical (91). Reaction is fast with metalloproteins. It proceeds mostly by oxidizing or reducing the metal center. Some characteristics and rate constants of reactions with metalloproteins are given in table 7. It is obvious that products are often unknown and that the mechanism is sometimes unclear. It seems that there is no reaction with transferrin (92) and horseradish and lacto-peroxidase compounds II (93). The reason is unknown. 

Chromoproteins proteins which contain a colored prosthetic group bound covalently or noncova-lently. The group includes heme proteins and iron prophyrin enzymes, flavoproteins, chlorophyll-protein complexes, and the non-porphyrin iron and copper proteins in the blood of vertebrates (e. g. transferrin and ceruloplasmin), and invertebrates (e.g. hemery-thrin and hemocyanin). 

Chemists have a variety of different interests in the transferrins. One interest concerns the chemistry of the binding of metal ions. No prosthetic or unusual chemical groupings are present in the proteins, and the binding is rather a property of the unique folding and juxtaposition of the normal chemical groupings in protein molecules (49). Another interest is in the mechanism of the release of iron by the serum transferrin to the tissues. The iron is so tightly bound that some investigators have attempted to invoke a special enzymatic mechanism for the dissociation in the tissues, but no evidence for this has been found. 

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