RATIONALIZATION OF THE STRENGTH OF METAL-BINDING TO HUMAN SERUM TRANSFERRIN

A wide range of metal ions of natural, therapeutic, diagnostic and tox ic interest are transported by serum transferrin (80 kDa). It is there fore important to understand the factors that control the strength of metal binding. We show here that even though Sc3+ has only slightly la rger ionic radius than Fe3+ (0.075 nm versus 0.065 nm), it binds to th e C-lobe and N-lobe sites much more weakly: logK(1) (bicarbonate-inde pendent binding constant) 14.6 +/- 0.2, logK(2): 13.3 +/- 0.3, respec tively (10 mM Hepes, 5 mM bicarbonate, 310 K). Preferential binding to the C-lobe was established by H-1-NMR spectroscopy. We show that the strength of binding of divalent and trivalent metal ions to human seru m transferrin correlates with metal ion acidity [and therefore with th e strength of binding to hydroxide, K-1(OH)]. The correlations are of predictive Value for a range of other metal ions. The plot of logK(1) (human serum transferrin) Versus logK(1)(OH) has a negative intercept consistent with unfavorable entropy effects due to lobe closure of ap otransferrin on binding of metal ions. This interpretation was tested by comparison with similar correlations of the strength of metal bindi ng to the enzymes carbonic anhydrase and carboxypeptidase with that fo r the low-M(r), ligand imidazole. These plots have positive intercepts consistent with the preorganized (entatic) state of these metalloenzy mes (favorable entropy effects on metal binding).