Uteroferrin phosphate complex

Figure 3. First shell (1.1-2.3A) EXAFS spectra of methemerythrin azide, semimethemerythrin azide, ribonucleotide reductase B2 subunit, oxidized uteroferrin-phosphate complex, and reduced uteroferrin.

Figure 6. Mossbauer spectra of the reduced uteroferrin phosphate complex at 55K (a) and 4.2K (b) and the oxidized uteroferrin-phosphate complex at 4.2K (c). (Reproduced with permission from ref. 90. Copyright 1986 American Society for Biological Chemists.)...

Thus, with respect to color and EPR activity, the initial uteroferrin-phosphate complex resembles the oxidized uteroferrin-phosphate complex, but the two complexes differ significantly in catalytic activity and phosphate affinity. Whereas the oxidized uteroferrin-phosphate complex is irreversibly formed (203), the initial uteroferrin-phosphate complex can be dissociated by gel filtration to regenerate native enzyme (185). An appropriate scheme to rationalize the above observations involving two distinct phosphate complexes is shown below. 

The initial complex is reversibly formed with a in the millimolar range as determined by inhibition kinetics this species would be the reduced uteroferrin-phosphate complex shown by Mossbauer studies to contain a mixed-valence diiron unit (45). This complex oxidizes to the diferric state upon standing in air, forming the oxidized uteroferrin phosphate... 

Figure 23. EPR spectra of (a) reduced uteroferrin and (b) its phosphate complex at 8 K and 10 G modulation amplitude and (c) the phosphate complex at 2 K and 40 G modulation amplitude. Reprinted with permission from Ref. 54. American Society of Biochemistry and Molecular Biology.

Day EP, David SS, Peterson J, Dunham WR, Bonvoisin JJ, Sands RH, Que Jr L. 1988. Magnetization and electron paramagnetic resonance studies of reduced uteroferrin and its "EPR-silent" phosphate complex. J Biol Chem 263 15561-15567. 

Figure 36. ENDOR spectra of uteroferrin in D2O buffer and in complex with molybdate, arsenate, and phosphate (top panel showing broad sweep and bottom panel showing narrow sweep). Reprinted with permission from [560]. Copyright 2002, American Chemical Society.

Recently, a pink high molecular weight (Mr 80,000) form of uteroferrin has been isolated from the nterine secretions of pigs This form appears to be a heterodimer comprised of the usual species of uteroferrin (Mr 36,000) noncovalently complexed to an antigenically unrelated polypeptide of Mr 50,000. Its optical, EPR, and enzymatic properties resemble closely those of its low molecular weight counterpart, except that the protein remains pink in the presence of inorganic phosphate. 

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