Ovotransferrin half-molecules

Complementing the structural studies of the intact transferrins, a number of fragments have also been crystallized, including proteolytic N-terminal half-molecules of rabbit serum transferrin (69) and chicken ovotransferrin (70), recombinant N-terminal half-molecules of human lactoferrin (71) and human serum transferrin (72), and a quarter-molecule fragment of duck ovotransferrin (73). All of these have now led to high-resolution structures (74-77). 

The bilobal structure of transferrins means that half-molecules, representing either the N-terminal or C-terminal lobe, can be relatively easily prepared, either by limited proteolysis or by recombinant DNA methods (Section III.A). Relatively high-resolution crystal structures have been determined for three such half-molecules, the proteolytic N-lobes of rabbit transferrin (74) and chicken ovotransferrin (77) at 2.3 A and the recombinant N-lobe of human lactoferrin at 2.0-A resolution (75). These show that both the protein structure and the metal and anion binding sites are the same as in the intact parent structures. In fact comparison of the metal and anion sites of the lactoferrin and transferrin half-molecules with each other and with the N-lobe of lactoferrin shows very close correspondence 92 atoms from the nine residues, plus metal and anion, making up the immediate binding site can be superimposed with an rms deviation of only 0.4 A (75). 

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