Preparation and properties of a single-sited fragment from the C-terminal domain of human transferrin.

A single-sited iron-binding fragment of human transferrin has been obtained by thermolysin cleavage of the protein, selectively loaded with iron in the C-terminal binding site, in a urea-containing buffer. The fragment contains carbohydrate, and hence derives from the C-terminal half of transferrin. Its metal-binding site accepts Fe3+ and Cu2+ with bicarbonate as accompanying anion, but only Fe3+ with oxalate as anion. EPR spectroscopic properties of the fragment are similar to those of the corresponding site in the intact protein. However, iron-binding by the fragment is weaker than by the C-terminal site of the intact protein, particularly at low pH, suggesting that overall as well as local protein conformation influences the metal-binding functions of the site.