Iron phosvitin

Phosvitin is a highly phosphorylated, iron-containing protein found in egg yolk and serves as an important source of dietary iron.1138 It could be viewed as a source of phosphate, as the protein contains about 130 residues of phosphoserine which appear to serve as ligand groups for iron. 

Isolated phosvitin usually has two to three atoms of iron, with Fe/phosphate ratio about 1 50. Considerably larger amounts of Ca2+ and Mg2+ are bound [(Ca2+ + Mg2+)/phosphate =0.7]. It appears to have no known enzymatic or regulatory role. 

It has been proposed that a low molecular weight phosphoprotein of phosvitin nature (47) which binds iron with high affinity may be involved in the cytosolic transport of iron in liver cells (48, 49), but no energized uptake of iron has been demonstrated such as in isolated rat liver mitochondria using a Fe(III)-phosvitin from avian egg yolk (mol wt 40,000) as the substrate (50). The biochemical significance of the cytosolic iron binding phosphoprotein is therefore still uncertain. 

Not all organic compounds that contain both phosphorus and iron are good sources of iron. Numerous investigators have demonstrated that the iron in egg yolk was less bioavailable than the iron in reference salts to both men (16, 17) and animals (18-22) This effect of egg yolk on the utilization of iron by animals is due to phosvitin, a glyco-protein in egg yolk (23, 24). This protein contains about 135 phosphoserine groups per molecule. 

Hegenauer, J., Saltman, P., and George, N. (1979). Iron(in)-phosphoprotein chelates stoichiometric equilibrium constant for interaction of iron(lll) and phosphorylserine residues of phosvitin and casein. Biochemistry. 18, 3865-3879. 

Another iron (III) protein where [Fe(III)On] coordination is hkely is iron(III) phosvitin. The fact that 46 Fe(III) ions are bound by one molecule of phosvitin has recently been estabhshed by Saltman and Multani (iO). Electronic absorption spectra of the iron (III) protein are shown in Figure 7 (Ji). Three weak LF bands are resolvable in the 150 K spectrum, at 447, 426, and 400 nm. The relatively high energies of the first two LF bands suggest [Fe(III)04]tet coordination, and in fact a very satisfactory fit of the spectrum is obtained with the LF parameters Atet = 5270 cm S B = 495 cm" and C/B = 8. Notably, the spectrum is apparently devoid of peaks in the 800-1000-nm region where [Fe(III)06]oct invariably appears. Thus, we suggest that iron (III) phosvitin provides a biological example of the fairly rare [Fe(III)04]tet coordination. 

More than a hundred different phosphoproteins have now been recognised. The best known of these include milk casein, the egg proteins phosvitin and ovalbumin, and the iron storage protein ferritin. In addition, nearly all known enzymes are proteins, and enzyme action is usually associated with phosphorylation-dephosphorylation reactions (Chapter 11.4). 

In addition to the iron present in hemoglobin, there are three important iron storage proteins Phosvitin, Ferritin and Hemosiderin. Sub-varieties of these exist. 

Two varieties, a and p, have been identified by chromatographic techniques. Phosvitin is stable for several hours at 100°C and will tightly bind iron. The estimated molecular weight is 36,000-40,000. 

The other amino add that is usually modified is L-serine. It can be esterified with phosphoric add. This ester, O-phosphoserine (2-9), occurs in many proteins, such as glycophosphoprotein phosvitin (also known as phosphovitin) from egg yolk (see Section 2.4.S.3.2). Phosvitins are one of the most phosphorylated (10%) proteins in nature, and are important for sequestering cations of calcium, iron and other metals for the developing embryo. Serine in phosvitin represents nearly 50% of the amino adds and there... 

The predominating form of iron in animal tissues is haem (particularly myoglobin and haemoglobin). In egg white, iron is bound in conalbumin, and in the yolk to phosphoprotein phosvitin. Milk contains the iron metaUoprotein lactoferrin and part of the iron is bound to casein. Conalbumin and lactoferrin are structurally similar to serum transferrin. In plants, iron is bound in various complexes, especially with phytic acid, aUphatic hydroxycarboxyhc acids, aminocarboxyhc acids, thiols, phenolic substances, nucleotides, peptides and proteins. The iron content in selected foods is shown in Table 6.8. Foods rich in iron are offal dishes, meat, eggs, pulses, tea and cocoa. Moderate amounts of iron are found in fish, poultry, cereals, spinach, parsley and nuts. Low levels of iron are present in milk, dairy products, fats and oils, potatoes and most fruit. 

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