Identification of the iron entry channels in apoferritin. Chemical modification and spectroscopic studies.

The knowledge of the route through which iron can enter and leave the apoferritin shell is a prerequisite for the understanding of ferritin's function. The involvement of the hydrophilic 3-fold channels in the iron uptake process has been studied by taking advantage of the reactivity of specific residues that line such channels, i.e., glutamic acid-127 and aspartic acid-130, the major Cd(II) binding sites, and cysteine-126. 113Cd NMR experiments have provided direct evidence for the competition between Fe(II) and Cd(II) binding to major Cd(II) binding sites on the protein and or a higher affinity of Fe(II) for these sites, in line with the well-known inhibitory effect of Cd(II) on iron uptake. Further evidence for the use of the 3-fold channels in the iron entry process has been obtained by means of chemical modification of Cys-126 with different mercurials. In particular, the introduction of the additional carboxylate carried by p-(chloromercuri)benzoate near Asp-127 and Glu-130 increases the initial rate of iron uptake and affects the coordination geometry of the metal in the Fe(III)-apoferritin complex as indicated by optical absorption and EPR data. The assignment of these effects to the carboxylate moiety of p-(chloromercuri)benzoate is brought out by the observation that the introduction in the 3-fold channel of the benzene ring only by means of phenylmercuric acetate has no effect on the initial iron uptake kinetics and on the spectroscopic properties of the Fe(III)-apoferritin complex.