Title : Metal substitution in transferrins: the crystal structure of human copper-
lactoferrin at 2.1-A resolution
Abstract :
- The structural consequences of binding a metal other than iron to a transferrin have been examined by crystallographic analysis of human copper- lactoferrin , Cu2Lf
- X-ray diffraction data were collected from crystals of Cu2Lf, using a diffractometer, to 2.6-A resolution, and oscillation photography on a synchrotron source, to 2.1-A resolution
- The structure was refined crystallographically, by restrained least-squares methods, starting with a model based on the isomorphous diferric structure from which the ligands, metal ions, anions, and solvent molecules had been deleted
- The final model, comprising 5321 protein atoms (691 residues ), 2 Cu2+ ions, 2 (bi)carbonate ions, and 308 solvent molecules has good stereochemistry (rms deviation of bond lengths from standard values of 0.018 A) and gives a crystallographic R value of 0.196 for 43,525 reflections in the range 7.5-2.1-A resolution
- The copper coordination is different in the two binding sites
- In the N-terminal site , the geometry is square pyramidal, with equatorial bonds to Asp 60, Tyr 192, His 253 , and a monodentate anion and a longer apical bond to Tyr 92
- In the C-terminal site , the geometry is distorted octahedral, with bonds to Asp 395, Tyr 435, Tyr 528, and His 597 and an asymmetrically bidentate anion
- The protein structure is the same as for the diferric protein , Fe2Lf, demonstrating that the closure of the protein domains over the metal is the same in each case irrespective of whether Fe3+ or Cu2+ is bound and that copper could be transported and delivered to cells equally well as iron
- The differences in metal coordination are achieved by small movements of the metal ion and anion within each binding site , which do not affect the protein structure