Lactoferrin-metal interactions: first crystal structure of a complex of lactoferrin with a lanthanide ion (Sm3+) at 3.4 A resolution

Reconstituted Super Paramagnetic Protein "Magnetotransferrin" for Brain Targeting to Attenuate Parkinsonism

Transferrin is an iron transporting protein consisting of bilobal protein shells (apotransferrin) with dual domains in each lobe, holding an interdomain iron binding cleft. This cleft is useful in synthesizing an iron oxide core inside the transferrin shell. In vitro reconstitution chemistry provides a nano-dimensional synthesis of the mineral core inside the protein shell. The present study demonstrates the synthesis of magnetotransferrin with reconstitution of apotransferrin to form iron oxide nanoparticles within the transferrin. Transmission electron microscopy investigations along with analysis of electronic diffraction patterns and magnetometry studies indicate entrapment of superparamagnetic iron (III) oxide nanoparticles. In vivo/ex vivo imaging of the brain and immunogold staining of brain sections further validate the brain targeting potential of "magnetotransferrin". The in vivo therapeutic potential of magneto transferrin has been demonstrated by induction of TRPV1 magnetic stimuli protein, having an important regulatory role in Parkinsonism management. In an exploration of neuroprotective mechanisms, deacetylation of H3K27 of synuclein has been revealed through the TRPV1-mediated HDAC3 activation in the treatment of Parkinsonism. Thus, this magnetic protein could be a potent candidate for brain targeting, bio-imaging, and therapy of neurological infirmities.

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.

Exploring Serum Transferrin Regulation of Nonferric Metal Therapeutic Function and Toxicity

Serum transferrin (sTf) plays a pivotal role in regulating iron biodistribution and homeostasis within the body. The molecular details of sTf Fe(III) binding blood transport, and cellular delivery through transferrin receptor-mediated endocytosis are generally well-understood. Emerging interest exists in exploring sTf complexation of nonferric metals as it facilitates the therapeutic potential and toxicity of several of them. This review explores recent X-ray structural and physiologically relevant metal speciation studies to understand how sTf partakes in the bioactivity of key non-redox active hard Lewis acidic metals. It challenges preconceived notions of sTf structure function correlations that were based exclusively on the Fe(III) model by revealing distinct coordination modalities that nonferric metal ions can adopt and different modes of binding to metal-free and Fe(III)-bound sTf that can directly influence how they enter into cells and, ultimately, how they may impact human health. This knowledge informs on biomedical strategies to engineer sTf as a delivery vehicle for metal-based diagnostic and therapeutic agents in the cancer field. It is the intention of this work to open new avenues for characterizing the functionality and medical utility of nonferric-bound sTf and to expand the significance of this protein in the context of bioinorganic chemistry.

Computational approaches for deciphering the equilibrium and kinetic properties of iron transport proteins

With the advances in three-dimensional structure determination techniques, high quality structures of the iron transport proteins transferrin and the bacterial ferric binding protein (FbpA) have been deposited in the past decade. These are proteins of relatively large size, and developments in hardware and software have only recently made it possible to study their dynamics using standard computational resources. We review computational techniques towards understanding the equilibrium and kinetic properties of iron transport proteins under different environmental conditions. At the level of detail that requires quantum chemical treatments, the octahedral geometry around iron has been scrutinized and it has been established that the iron coordinating tyrosines are in an unusual deprotonated state. At the atomistic level, both the N-lobe and the full bilobal structure of transferrin have been studied under varying conditions of pH, ionic strength and binding of other metal ions by molecular dynamics (MD) simulations. These studies have allowed questions to be answered, among others, on the function of second shell residues in iron release, the role of synergistic anions in preparing the active site for iron binding, and the differences between the kinetics of the N- and the C-lobe. MD simulations on FbpA have led to the detailed observation of the binding kinetics of phosphate to the apo form, and to the conformational preferences of the holo form under conditions mimicking the environmental niches provided by the periplasmic space. To study the dynamics of these proteins with their receptors, one must resort to coarse-grained methodologies, since these systems are prohibitively large for atomistic simulations. A study of the complex of human transferrin (hTf) with its pathogenic receptor by such methods has revealed a potential mechanistic explanation for the defense mechanism that arises in evolutionary warfare. Meanwhile, the motions in the transferrin receptor bound hTf have been shown to disfavor apo hTf dissociation, explaining why the two proteins remain in complex during the recycling process from the endosome to the cell surface. Open problems and possible technological applications related to metal ion binding-release in iron transport proteins that may be handled by hybrid use of quantum mechanical, MD and coarse-grained approaches are discussed.

“Anion clamp” allows flexible protein to impose coordination geometry on metal ions

X-ray crystal structures of human serum transferrin (77 kDa) with Yb^III or Fe^III bound to the C-lobe and malonate as the synergistic anion show that the large Yb^III ion causes the expansion of the metal binding pocket while octahedral metal coordination geometry is preserved, an unusual geometry for a lanthanide ion.

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.

Structural and functional characterization of recombinant human serum transferrin secreted from Pichia pastoris

Serum transferrin is an iron-binding glycoprotein with a bilobal structure. It binds iron ions in the blood serum and delivers them into target cells via transferrin receptor. We identified structural and functional characteristics of recombinant human transferrin which is produced in the yeast Pichia pastoris. Using the signal sequence of the alpha factor of the yeast Saccharomyces cerevisiae, high-level secretion was obtained, up to 30 mg/l of culture medium. Correct processing at designed sites was confirmed by N-terminal sequence analysis. Carbohydrate modification was determined by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS) analysis after digestion with endo-beta-N-acetylglucosaminidase H. Reflecting the secondary structure, the circular dichroism spectrum of the recombinant protein was indistinguishable from that of serum transferrin. Consequently, the recombinant product had an iron binding function just as the serum specimen has: two Fe(3+) sites existed in a recombinant transferrin molecule, as estimated by titration analysis using visible absorption, fluorescence spectra, and electrophoretic behavior in urea denaturing polyacrylamide gel electrophoresis (PAGE).

A novel localized amyloidosis associated with lactoferrin in the cornea

We report a novel localized amyloidosis associated with lactoferrin. To elucidate the precursor protein of corneal amyloidosis associated with trichiasis, we analyzed amyloid deposits from three patients by histopathology and biochemistry. Amyloid deposits showed immunoreactivity, confirmed by electron microscopy, for only anti-human lactoferrin antibody. Electrophoresis of amyloid fibrils revealed lactoferrin with and without sugar chains; N-terminal sequence analysis revealed full-length lactoferrin and a truncated tripeptide of N-terminal amino acids, Gly-Arg-Arg. Carboxymethylated wild-type lactoferrin formed amyloid fibrils in vitro. Lactoferrin gene analysis in the three patients revealed a Glu561Asp mutation in all of the patients and a compound heterozygote of Ala11Thr and Glu561Asp mutations in one patient. A heterozygotic Glu561Asp mutation appeared in 44.8% of healthy Japanese volunteers, suggesting that the mutation may not be an essential mutation for amyloid formation (p = 0.104). Results thus suggest that lactoferrin is this precursor protein.