Ferri-porphyrin proteins

Cytochromes have been reported at low concentrations in the milk fat globule membrane (Gregory et al., 1976 Jarasch et al., 1977). The powerful pro-oxidant properties of ferri-porphyrin proteins, together with their juxtaposition with lipids in the milk fat globule membrane, suggest that cytochromes may play a role in oxidation. Gregory et al. (1976) concluded that they do exert a role in milk lipid oxidation. 

In view of the Greek origin of the word cytochrome, it seems to us inadvisable to substitute cytichrome for ferricytochrome ferri- and ferro- will therefore be used as prefixes to cytochrome. Hemoproteins will be used for all iron porphyrin proteins, irrespective of whether they contain ferrous or ferric iron. 

Fig. 12. Schematic views of bis-histidyl ferri-, ferro-, and CO-ferro-heme-hemopexin. Unlike myoglobin with one open distal site, heme bound to hemopexin is coordinated to two strong field ligands, either of which a priori may be displaced by CO. This may well produce coupled changes in protein conformation like the Perutz mechanism for 02-binding by hemoglobin (143). The environment of heme bound to hemopexin and to the N-domain may be influenced by changes in the interactions of porphyrin-ring orbitals with those of aromatic residues in the heme binding site upon reduction and subsequent CO binding.

The reactions of NO with the high-spin ferri-hemes (Equation 7.3), either in model porphyrin complexes or in proteins (with H20 being eventually replaced by other ligands such as histidine or a thiolate),5 show some similarities with the classical complexes discussed in Section 7.3.1 (cf. Equation 7.1). 

It is a beautiful red, iron-porphyrin-containing protein which functions as a link in the chain of the cell-respiration enzymes, the iron atom now taking up and now giving off an electron, and the iron thus alternating valency between the 3-valent ferri and the 2-valent ferro stages. It is a very pleasant substance to work with, not merely because it is lovely to look at, but also because it is uncommonly stable and durable. From 100 kg heart-meat of horse one can produce 3-4 g of pure cytochrome c. The molecule weighs about 12,000 and contains one mol iron porphyrin per mol. 

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