engleski

The effect of oxalate on the thermodynamic parameters for the binding of iron to human serum transferrin

Human transferrin is a serum glycoprotein that has ability to bind, transport and release ferric ions with two binding sites for metal ions, the C-site and N-site. Each bound metal ion is hexa- coordinated: four ligands to the metal are provided by the transferrin amino acid residues, while the remaining two are provided by an external anion, termed the synergistic anion. This synergistic anion is carbonate in vivo, however, other anions (such as oxalate, pyruvate, glycine, etc.) with similar structure can substitute for carbonate in vitro. In this study we determined the thermodynamic parameters for the binding of ferric ion to human serum transferrin in the presence of different synergistic anions (carbonate and oxalate) using isothermal titration calorimetry (ITC). Resulting ITC curves displayed two inflection points typical for a model of two binding sites. Data analysis of the curves was based on the model of two sets of independent sites and best-fit parameters were calculated using Marquardt algorithm. Different slope in the two binding regions on the titration curve can be attributed to different binding affinities of ferric ion to human serum transferrin in the presence of different synergistic anions, suggesting lower affinities in the presence of oxalate. Also, the observed differences in the heat released in the first part of titration suggest different binding enthalpy for the C-site of transferrin. Similar effect on the two binding sites having different metal binding properties in the presence of different synergistic anions is known.

human serum transferrin ; synergistic anion ; thermodynamic parameters ; binding constants ; binding enthalpy

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