Iron and bismuth bound human serum transferrin reveals a partially-opened conformation in the N-lobe

Human serum transferrin (hTF) binds Fe(III) tightly but reversibly, and delivers it to cells via a receptor-mediated endocytosis process. The metal-binding and release result in significant conformational changes of the protein. Here, we report the crystal structures of diferric-hTF (Fe N Fe C-hTF) and bismuth-bound hTF (Bi N Fe C-hTF) at 2.8 and 2.4 Å resolutions respectively. Notably, the N-lobes of both structures exhibit unique 'partially-opened' conformations between those of the apo-hTF and holo-hTF. Fe(III) and Bi(III) in the N-lobe coordinate to, besides anions, only two (Tyr95 and Tyr188) and one (Tyr188) tyrosine residues, respectively, in contrast to four residues in the holo-hTF. The C-lobe of both structures are fully closed with iron coordinating to four residues and a carbonate. The structures of hTF observed here represent key conformers captured in the dynamic nature of the transferrin family proteins and provide a structural basis for understanding the mechanism of metal uptake and release in transferrin families. © 2012 Macmillan Publishers Limited. All rights reserved.published_or_final_versio

Surface-barrier and polarization effects in the photoemission from GaAs(110)

We have developed a theory of photoemission from III-V compound semiconductors within the one-step model and discuss the effect of transition-matrix elements, final states, and the surface barrier on the energy distribution of photoelectrons. As a prototype the (110) surface of GaAs is studied, and theoretical spectra are compared with experiment in normal as well as in non-normal emission. The specific position of the interface vacuum crystal, appearing in the calculation of the transition-matrix elements, influences the shape of the spectra considerably. The energy-distribution curves strongly depend on the escape angles of the photoelectrons as well as the incidence angles of the radiation leading to significant intensity variations especially of the dangling-bond surface state. Band-mapping methods prove to be misleading, in the worst cases by an error of about 100 meV in the estimation of the valence-band energies. The corrugated surface barrier is also investigated; our analysis favors a smooth saturated image potential barrier