The effects of salts and amino group modification on the iron binding domains of transferrin

The origins of the effects of salts on the properties of the iron binding sites of transferrin have been investigated. The chaotropically distinct salts NaCl and NaClO4 each induce characteristic changes in the EPR lineshapes of the N- and C-terminal Fe3+ binding domains, respectively. To a good approximation the perturbed EPR spectrum of diferric transferrin in the presence of salts is the sum of the EPR spectra of the N- and C-terminal monoferric proteins. Acetylation of amino groups causes spectral and kinetic changes in the protein similar to those induced by NaClO4. Thus, both acetylation and NaClO4 cause a loss of structure in the g' = 4.3 EPR signal of the N-terminal domain, and both retard iron removal from this domain. In contrast, iron removal from the C-terminal domain is accelerated by acetylation or the presence of NaClO4. These observations are ascribed to charge effects of lysine residues which are probably in the vicinity of the iron binding sites.