Dynamic structure of human serum transferrin from transient electric birefringence experiments

In order to investigate the secondary, tertiary, and dynamic structure of the iron-free (apo) and iron-saturated (holo) forms of human serum transferrin and its amino (N)-terminal lobe at the physiologically relevant pHs 7.4 and 5.0, we have combined ultraviolet circular dichroism (CD) spectroscopy with transient-electric birefringence (TEB) measurements. No significant changes are found in the protein's secondary structure under the different conditions studied. The tertiary structure as monitored by near-UV CD is affected by iron binding, but does not change upon decrease in pH. In contrast, TEB results indicate dramatic changes in the dynamic structure of transferrin both upon binding of iron and decrease of pH. In apotransferrin freedom of movement is found for the lobes with respect to each other, and for the domains within the lobes. The interlobal flexibility is considerably enhanced at the lower pH. Holotransferrin is found to behave as a rigid molecule.