Estimation of the ferrous-transferrin binding constants based on thermodynamic studies of nickel(II)-transferrin.
The equilibrium constants for the binding of Ni2+ to human serum transferrin in 0.01 M hepes containing 5 mM sodium bicarbonate at 25 degrees C and pH 7.4 have been measured. The effective binding constants are log K1 = 4.10 +/- 0.15 and log K2 = 3.23 +/- 0.31 for the reactions Ni2+ + apoTr (K1) in equilibrium Ni2+-Tr. Ni2+ + Ni2+-Tr (K2) in equilibrium Ni2+-Tr-Ni2+ where the explicit terms for bicarbonate and hydrogen ion have been incorporated into the effective binding constants. Titration of both forms of mono(ferric)transferrin indicates that unlike other metal ions, Ni2+ binds preferentially to the N-terminal binding site, but that the site preference is rather small. A linear-free-energy relationship (LFER) for the complexation of Ni2+ and Fe2+ has been prepared. This LFER has been used to estimate effective binding constants of log K1 = 3.2 and log K2 = 2.5 for the ferrous-transferrin complex. These ferrous constants have been combined with the literature binding constants for ferric-transferrin to estimate formal reduction potentials of -340 mV vs. NHE for the C-terminal site and -280 mV for the N-terminal site.
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D006801:Humans; D007501:Iron / Q000378:metabolism*; D007700:Kinetics; D009532:Nickel / Q000378:metabolism*; D011485:Protein Binding; D013056:Spectrophotometry, Ultraviolet; D013816:Thermodynamics; D014168:Transferrin / Q000378:metabolism*
D014168:Transferrin; C022838:nickel chloride; D009532:Nickel; D007501:Iron