JBC, Vol. 250, Issue 15, 6026-6031, Aug, 1975
Iron binding to conalbumin. Calorimetric evidence for two distinct species with one bound iron atom
J. W. Donovan and K. D. Ross
When thermal denaturation of conalbumin solutions partially saturated with
Fe(III) is observed by differential scanning calorimetry, four endotherms
are observed between 40 and 100 degrees. The relative size of these four
endotherms is determined by the Fe(III) to conalbumin ration. At a heating
rate of 10 degrees/min, in Tris buffer at pH 7.5, observed endotherm
temperature maxima and enthalpies of denaturation are: conalbumin, 63
degrees, 320 kcal/mol; intermediate I, 68 degrees, intermediate I, 77
degrees; Fe2-conalbumin, 84 degrees, 630 kcal/mol. These four endotherms
are observed over a range of protein concentration from 7 to 100 mg/ml and
are unchanged when excess bicarbonate is present. Stoichiometric
calculations of both total protein and total iron indicate that each
intermediate endotherm results from denaturation of conalbumin molecules
containing only one ferric ion. These experimental results are thus
consistent with the presence of two different monomeric one-iron conalbumin
intermediates. They strongly suggest that the two iron binding sites of
conalbumin are not equivalent.
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