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The structure of human lactoferrin has been refined crystallographically at 2.8 A (1 A = 0.1 nm) resolution using restrained least squares methods. The starting model was derived from a 3.2 A map phased by multiple isomorphous replacement with solvent flattening. Rebuilding during refinement made extensive use of these experimental phases, in combination with phases calculated from the partial model. The present model, which includes 681 of the 691 amino acid residues, two Fe3+, and two CO3(2-), gives an R factor of 0.206 for 17,266 observed reflections between 10 and 2.8 A resolution, with a root-mean-square deviation from standard bond lengths of 0.03 A. As a result of the refinement, two single-residue insertions and one 13-residue deletion have been made in the amino acid sequence, and details of the secondary structure and tertiary interactions have been clarified. The two lobes of the molecule, representing the N-terminal and C-terminal halves, have very similar folding, with a root-mean-square deviation, after superposition, of 1.32 A for 285 out of 330 C alpha atoms; the only major differences being in surface loops. Each lobe is subdivided into two dissimilar alpha/beta domains, one based on a six-stranded mixed beta-sheet, the other on a five-stranded mixed beta-sheet, with the iron site in the interdomain cleft. The two iron sites appear identical at the present resolution. Each iron atom is coordinated to four protein ligands, 2 Tyr, 1 Asp, 1 His, and the specific Co3(2-), which appears to bind to iron in a bidentate mode. The anion occupies a pocket between the iron and two positively charged groups on the protein, an arginine side-chain and the N terminus of helix 5, and may serve to neutralize this positive charge prior to iron binding. A large internal cavity, beyond the Arg side-chain, may account for the binding of larger anions as substitutes for CO3(2-). Residues on the other side of the iron site, near the interdomain crossover strands could provide secondary anion binding sites, and may explain the greater acid-stability of iron binding by lactoferrin, compared with serum transferrin. Interdomain and interlobe interactions, the roles of charged side-chains, heavy-atom binding sites, and the construction of the metal site in relation to the binding of different metals are also discussed.  相似文献
2.
Structure, function and flexibility of human lactoferrin   总被引:2,自引:0,他引:2  
X-ray structure analyses of four different forms of human lactoferrin (diferric, dicupric, an oxalate-substituted dicupric, and apo-lactoferrin), and of bovine diferric lactoferrin, have revealed various ways in which the protein structure adapts to different structural and functional states. Comparison of diferric and dicupric lactoferrins has shown that different metals can, through slight variations in the metal position, have different stereochemistries and anion coordination without any significant change in the protein structure. Substitution of oxalate for carbonate, as seen in the structure of a hybrid dicupric complex with oxalate in one site and carbonate in the other, shows that larger anions can be accommodated by small side-chain movements in the binding site. The multidomain nature of lactoferrin also allows rigid body movements. Comparison of human and bovine lactoferrins, and of these with rabbit serum transferrin, shows that the relative orientations of the two lobes in each molecule can vary; these variations may contribute to differences in their binding properties. The structure of apo-lactoferrin demonstrates the importance of large-scale domain movements for metal binding and release and suggests that in solution an equilibrium exists between open and closed forms, with the open form being the active binding species. These structural forms are shown to be similar to those seen for bacterial periplasmic binding proteins, and lead to a common model for the various steps in the binding process.  相似文献
3.
The amino terminal half of human lactoferrin (LfN) produced from transfected baby hamster kidney cells has been crystallized in its iron-saturated and iron-free forms. The crystals of glycosylated LfN and deglycosylated LfN are monoclinic, space group C2, with cell dimensions a = 133.0 A, b = 58.3 A, c = 58.3 A, alpha = 90.0 degrees, beta = 114.7 degrees, gamma = 90.0 degrees, and one molecule per asymmetric unit. Crystals of apo LfN have also been prepared using deglycosylated protein. These crystals are tetragonal, space group P4(1)2(1)2 (or P4(3)2(1)2), with cell dimensions of a = b = 58.4 A and c = 217.2 A and one molecule per asymmetric unit. Both the iron-saturated and the iron-free crystals are suitable for high resolution X-ray analysis.  相似文献
4.
Human apo-lactoferrin in both native and deglycosylated forms has been purified, and crystals obtained by dialysis against low ionic strength buffer solutions. The crystals of native apo-lactoferrin are orthorhombic, space group P2(1)2(1)2(1) with cell dimensions a = 222.0 A, b = 115.6 A, c = 77.8 A and have two protein molecules per asymmetric unit. Two crystal forms of deglycosylated apo-lactoferrin have been obtained. One is orthorhombic, space group P2(1)2(1)2(1), with cell dimensions a = 152.1 A, b = 94.6 A, c = 55.8 A. The second is tetragonal, space group I4, with cell dimensions a = b = 189.4 A, c = 55.1 A. Both of the latter have only one molecule per asymmetric unit, and are suitable for high-resolution X-ray structure analysis.  相似文献
5.
Spectroscopic and electrochemical studies, incorporating electronic spectra, electron paramagnetic resonance (EPR) spectra, resonance Raman (RR) spectra, and measurements of the redox potential, have been carried out on the blue copper protein azurin, from Alcaligenes denitrificans. These data are correlated with the refined crystal structure of this azurin and with corresponding data for other blue copper proteins. The electronic spectrum, characterized by an intense (epsilon = 5100 M-1 cm-1) charge-transfer band at 619 nm, the EPR spectral parameters (g perpendicular = 2.059, g parallel of = 2.255, A parallel of = 60 X 10(-4) cm-1), and the resonance Raman spectrum are similar to those obtained from other azurins and from plastocyanins. Both the electronic spectrum and the EPR spectrum are unchanged over the pH range 4-10.5, but major changes occur above pH 12 and below pH 3.5. A small reversible change occurs at pH approximately 11.4. In the RR spectrum the Cu-S stretching mode is shown to contribute to all of the five principal RR peaks. Deuterium substitution produces shifts in at least seven of the peaks; these shifts may be attributable, at least in part, to the NH...S hydrogen bond to the copper-ligated Cys-112. Measurements of the redox potential, using spectroelectrochemical methods, over the temperature range 4.8-40.0 degrees C, give values for delta H0' and delta S0' of -55.6 kJ mol-1 and -97.0 J K-1 mol-1, respectively. The redox potential of A. denitrificans azurin at pH 7.0, Eo', is 276 mV. These data are interpreted in terms of a copper site, in azurin, comprising three strong bonds, in an approximately trigonal plane, from Cys-112, His-46, and His-117 and much longer axial approaches from Met-121 and the peptide carbonyl oxygen of Gly-45. Spectral differences within the azurin family and between azurin and plastocyanin are attributed to differences in the strengths of these axial interactions. Likewise, the distinctly lower Eo values for azurins, as compared with plastocyanins, are related to the more copper(II)-like site in azurin [with a weaker Cu-S(Met) interaction and a Cu-O interaction not found in plastocyanin]. On the other hand, the relative constancy of the EPR parameters between azurin and plastocyanin suggests they are not strongly influenced by weakly interacting axial groups.  相似文献
6.
The purification of bovine lactoferrin, its crystallization at low ionic strength, and preliminary X-ray crystallographic data are reported. The crystals, which grow from a two-phase system, are radiation-stable and suitable for a medium-resolution X-ray analysis. They are orthorhombic, space group P2(1)2(1)2(1), with cell dimensions a = 138.4 A, b = 87.1 A, c = 73.6 A, and one protein molecule in the asymmetric unit.  相似文献
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