Protein intermediate trapped by the simultaneous crystallization process. Crystal structure of an iron-saturated intermediate in the Fe3+ binding pathway of camel lactoferrin at 2.7 a resolution

A Peptide Bond from the Inter-lobe Segment in the Bilobal Lactoferrin Acts as a Preferred Site for Cleavage for Serine Proteases to Generate the Perfect C-lobe: Structure of the Pepsin Hydrolyzed Lactoferrin C-lobe at 2.28 Å Resolution

C-lobe represents the C-terminal half of lactoferrin which is a bilobal 80 kDa iron binding glycoprotein. The two lobes are designated as N-lobe (Ser1-Glu333) and C-lobe (Arg344-Arg689). The N- and C-lobes are connected by a 10-residue long α-helical peptide (Thr334-Thr343). Both lobes adopt similar conformations and have identical iron binding sites. The bilobal lactoferrin was hydrolyzed in a limited proteolysis using pepsin at pH 2.0. It produced a 40 kDa and fully functional C-lobe which was purified and crystallized at pH 8.0. The structure determination revealed that the structure contained residues from Tyr342 to Arg689 representing a fully functional monoferric C-lobe. It showed that pepsin cleaved lactoferrin at the peptide bond Arg341-Tyr342 which is part of the inter-lobe decapeptide. Interestingly, the two previously determined structures of the enzymatically produced C-lobe using trypsin and proteinase K also cleaved lactoferrin at the same peptide bond Arg341-Tyr342. This was a striking result as the three enzymes, pepsin, trypsin and proteinase K have different specificity requirements and yet they cleaved the bilobal lactoferrin at the same peptide bond and generated an identical and fully functional C-lobe. This shows that the observed cleavage site in lactoferrin adopts a highly favourable conformation for proteolysis. It is noteworthy that the three enzymes with different specificities cut the protein at the same peptide bond which may be of physiological significance because the antibacterial action of lactoferrin is extended further through the C-lobe.

Synergistic Killing of Pathogenic Escherichia coli Using Camel Lactoferrin from Different Saudi Camel Clans and Various Antibiotics

Current study aimed to analyze the synergistic killing of pathogenic Escherichia coli using camel lactoferrin from different Saudi camel clans and various antibiotics. Methods: using multiple microbiological and protein analysis techniques, the results were shown that the purified camel lactoferrins (cLfs) from different Saudi camel have strong antimicrobial potentials against two strains of E. coli. Although all cLfs were superior relative to human or bovine lactoferrins (hLf or bLf), there was no noticeable difference in the antimicrobial potentials of cLfs from different camel clans. The effects of antibiotics and cLfs were synergistic, indicating the superiority of using cLf-antibiotic combinations against E. coli growth. Since these combinations possessed distinguished synergy profiles, it is likely that they can be used to enhance the low efficacy of antibiotics, as well as to control the problems associated with bacterial resistance. Furthermore, these combinations can reduce the cost of cure of bacterial infections, especially in the developing countries. The analysis of the molecular mechanisms of lactoferrin action revealed that expression of several E. coli proteins was affected by the treatment with these antibacterial factors. Several proteins of different molecular weights interacting with cLf-biotin were found. Scanning and transmission electron microscopy analysis revealed the presence of noticeable morphological changes associated with the treatment of E. coli strains by antibiotic carbenicillin or cLf alone, and in combination. Camel lactoferrin has superior potential killing of E. coli over bovine and human lactoferrin, and this potential can be further synergistically enhanced of cLF is combined with antibiotics.

Identification and Characterization of a Metalloprotein Involved in Gallium Internalization in Pseudomonas aeruginosa

Gallium nitrate (Ganite) is a potential drug for the treatment of Pseudomonas aeruginosa infection. CRISPR/Cas9-based gene mutagenesis studies reveal that siderophore pyochelin-facilitated uptake and an ABC transporter are two major Ga³⁺ internalization pathways in Pseudomonas aeruginosa (P. aeruginosa). Crystal structures reveal that Ga³⁺ and Fe³⁺ occupy exactly the same metal site of HitA, a periplasmic iron-binding protein of the ABC transporter system. The study provides a molecular basis for Ga³⁺ internalization by P. aeruginosa and facilitates gallium-based antimicrobial drug development.

X-ray structures of transferrins and related proteins

BACKGROUND

Transferrins are a group of iron-binding proteins including serum transferrin, lactoferrin and ovotransferrin.

SCOPE OF REVIEW

The structures of transferrins are discussed.

GENERAL SIGNIFICANCE

The typical transferrin molecules are folded into two homologous lobes. X-ray crystallography revealed that each lobe is further divided into two similarly sized domains, and that an iron-binding site is contained within the inter-domain cleft. The six iron coordination sites are occupied by four residues and a bidentate carbonate anion.

MAJOR CONCLUSIONS

The structures of the apo- and holo-forms revealed that the transferrins undergo a large-scale conformational change upon the uptake and release of irons: domains rotate as rigid bodies around a screw axis passing through inter-domain contacts. The iron-release mechanism of transferrin N-lobe is also revealed by X-ray crystallography; two basic residues in two domains form an unusual hydrogen bond in neutral pH, and the bond should be broken and facilitate iron release at a low pH of the endosome. For ovotransferrin, the iron release kinetics of two lobes correspond well with the numbers of anion binding sites found in crystal structures. The structures of transferrins bound to other metals revealed that the flexibility of the transferrin structure allows the ability to bind to other metals. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Transferrin as a model system for method development to study structure, dynamics and interactions of metalloproteins using mass spectrometry

BACKGROUND

Transferrin (Tf) is a paradigmatic metalloprotein, which has been extensively studied in the past and still is a focal point of numerous investigation efforts owing to its unique role in iron homeostasis and enormous promise as a component of a wide range of therapies.

SCOPE OF REVIEW

Electrospray ionization mass spectrometry (ESI MS) is a potent analytical tool that has been used successfully to study various properties of Tf and Tf-based products, ranging from covalent structure and metal binding to conformation and interaction with their physiological partners.

MAJOR CONCLUSIONS

Various ESI MS-based techniques produce unique information on Tf properties and behavior that is highly complementary to information provided by other experimental techniques.

GENERAL SIGNIFICANCE

The experimental ESI MS-based techniques developed for Tf studies are not only useful for understanding of fundamental aspects of the iron-binding properties of this protein and optimizing Tf-based therapeutic products, but can also be applied to study a range of other metalloproteins. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

The role of vicinal tyrosine residues in the function of Haemophilus influenzae ferric-binding protein A

The periplasmic FbpA (ferric-binding protein A) from Haemophilus influenzae plays a critical role in acquiring iron from host transferrin, shuttling iron from the outer-membrane receptor complex to the inner-membrane transport complex responsible for transporting iron into the cytoplasm. In the present study, we report on the properties of a series of site-directed mutants of two adjacent tyrosine residues involved in iron co-ordination, and demonstrate that, in contrast with mutation of equivalent residues in the N-lobe of human transferrin, the mutant FbpAs retain significant iron-binding affinity regardless of the nature of the replacement amino acid. The Y195A and Y196A FbpAs are not only capable of binding iron, but are proficient in mediating periplasm-to-cytoplasm iron transport in a reconstituted FbpABC pathway in a specialized Escherichia coli reporter strain. This indicates that their inability to mediate iron acquisition from transferrin is due to their inability to compete for iron with receptor-bound transferrin. Wild-type iron-loaded FbpA could be crystalized in a closed or open state depending upon the crystallization conditions. The synergistic phosphate anion was not present in the iron-loaded open form, suggesting that initial anchoring of iron was mediated by the adjacent tyrosine residues and that alternate pathways for iron and anion binding and release may be considered. Collectively, these results demonstrate that the presence of a twin-tyrosine motif common to many periplasmic iron-binding proteins is critical for initially capturing the ferric ion released by the outer-membrane receptor complex.

The molecular mechanism for receptor-stimulated iron release from the plasma iron transport protein transferrin

Human transferrin receptor 1 (TfR) binds iron-loaded transferrin (Fe-Tf) and transports it to acidic endosomes where iron is released in a TfR-facilitated process. Consistent with our hypothesis that TfR binding stimulates iron release from Fe-Tf at acidic pH by stabilizing the apo-Tf conformation, a TfR mutant (W641A/F760A-TfR) that binds Fe-Tf, but not apo-Tf, cannot stimulate iron release from Fe-Tf, and less iron is released from Fe-Tf inside cells expressing W641A/F760A-TfR than cells expressing wild-type TfR (wtTfR). Electron paramagnetic resonance spectroscopy shows that binding at acidic pH to wtTfR, but not W641A/F760A-TfR, changes the Tf iron binding site > or =30 A from the TfR W641/F760 patch. Mutation of Tf histidine residues predicted to interact with the W641/F760 patch eliminates TfR-dependent acceleration of iron release. Identification of TfR and Tf residues critical for TfR-facilitated iron release, yet distant from a Tf iron binding site, demonstrates that TfR transmits long-range conformational changes and stabilizes the conformation of apo-Tf to accelerate iron release from Fe-Tf.

Indirect detection of protein-metal binding: interaction of serum transferrin with In3+ and Bi3+

Transferrins comprise a class of monomeric glycoproteins found in all vertebrates, whose function is iron sequestration and transport. In addition to iron, serum transferrin also binds a variety of other metals and is believed to provide a route for the in vivo delivery of such metals to cells. In the present study, ESI MS is used to investigate interactions between human serum transferrin and two nonferrous metals, indium (a commonly used imaging agent) and bismuth (a component of many antiulcer drugs). While the UV-Vis absorption spectroscopy measurements clearly indicate that both metals bind strongly to transferrin in solution, the metal-protein complex can be detected by ESI MS only for indium, but not for bismuth. Despite the apparently low stability of the transferrin-bismuth complex in the gas phase, presence of such complex in solution can be established by ESI MS indirectly. This is done by monitoring the evolution of charge state distributions of transferrin ions upon acid-induced protein unfolding in the presence and in the absence of the metal in solution. The anomalous instability of the transferrin-bismuth complex in the gas phase is rationalized in terms of conformational differences between this form of transferrin and the holo-forms of this protein produced by binding of metals with smaller ionic radii (e.g., Fe3+ and In3+). The large size of Bi3+ ion is likely to prevent formation of a closed conformation (canonical structure of the holo-protein), resulting in a non-native metal coordination. It is suggested that transferrin retains the open conformation (characteristic of the apo-form) upon binding Bi3+, with only two ligands in the metal coordination sphere provided by the protein itself. This suggestion is corroborated by the results of circular dichroism measurements in the near-UV range. Since the cellular consumption of metals in the transferrin cycle critically depends upon recognition of the holo-protein complex by the transferrin receptor, the noncanonical conformation of the transferrin-bismuth complex may explain very inefficient delivery of bismuth to cells even when a high dosage of bismuth-containing drugs is administered for prolonged periods of time.

A novel protein-mineral interface

Transferrins transport Fe3+ and other metal ions in mononuclear-binding sites. We present the first evidence that a member of the transferrin superfamily is able to recognize multi-nuclear oxo-metal clusters, small mineral fragments that are the most abundant forms of many metals in the environment. We show that the ferric ion-binding protein from Neisseria gonorrhoeae (nFbp) readily binds clusters of Fe3+, Ti4+, Zr4+ or Hf4+ in solution. The 1.7 A resolution crystal structure of Hf-nFbp reveals three distinct types of clusters in an open, positively charged cleft between two hinged protein domains. A di-tyrosyl cluster nucleation motif (Tyr195-Tyr196) is situated at the bottom of this cleft and binds either a trinuclear oxo-Hf cluster, which is capped by phosphate, or a pentanuclear cluster, which in turn can be capped with phosphate. This first high-resolution structure of a protein-mineral interface suggests a novel metal-uptake mechanism and provides a model for protein-mediated mineralization/dissimilation, which plays a critical role in geochemical processes.

Expression, purification, and characterization of equine lactoferrin in Pichia pastoris

Lactoferrin is an 80kDa iron-binding glycoprotein. It is secreted by exocrine glands. Many functions such as iron sequestering, anti-bacterial activity, regulation of gene expression, and immunomodulation are attributed to it. In the present study, we report the production of recombinant equine lactoferrin (ELF) in the methylotropic yeast Pichia pastoris using pPIC9K vector. The recombinant protein was purified by one-step affinity chromatography using heparin-Sepharose column. The purified protein has a molecular weight of 80kDa and reacted with antibody raised against the native equine lactoferrin. Its N-terminal sequence was identical to that of the native ELF. The iron-binding behavior and circular dichroism studies of the purified protein indicate that it has folded properly. The recombinant protein appears to be hyperglycosylated by the host strain, GS115. This is the first heterologous expression of equine lactoferrin and also the first report of intact lactoferrin expression using P. pastoris system. An yield of 40mg/l obtained in shake-flask cultures with this system, which is higher than the reported values for other systems.