Calorimetric studies of melanotransferrin (p97) and its interaction with iron

Preparation and characterization of iron(III) nitrilotriacetate complex in aqueous solutions for quantitative protein binding experiments

Various preparations of iron(III) nitrilotriacetate (FeNTA) solution reported in the literature lack a comprehensive method for accurate determination of FeNTA concentration and often result in unstable solutions. A detailed procedure for the preparation of FeNTA solution is presented that includes the standardization of both components of the chelate. The standardization of the components allowed the accurate determination of the molar absorption coefficients for the calculation of the FeNTA concentration in two different buffers at pH 5.6 and 7.4. The variation of pH in this range or ionic strength in the range from 0 M to 3 M (KCl) has little effect on the value of the molar absorption coefficient. The precise concentrations of all species involved in the equilibria between Fe and NTA were determined in the pH range 2-12 using the Jenkins-Traub algorithm to solve the 5th-order polynomial in Microsoft Excel. In view of the experimental observations and the calculated distribution of species, the stability of FeNTA solutions may be affected by the Fe : NTA ratio and the total concentrations, with dilute solutions and those with an excess of NTA over Fe showing higher stability.

Biomimetic generation of the strongest known biomaterial found in limpet tooth

The biomaterial with the highest known tensile strength is a unique composite of chitin and goethite (α-FeO(OH)) present in teeth from the Common Limpet (Patella vulgata). A biomimetic based on limpet tooth, with corresponding high-performance mechanical properties is highly desirable. Here we report on the replication of limpet tooth developmental processes ex vivo, where isolated limpet tissue and cells in culture generate new biomimetic structures. Transcriptomic analysis of each developmental stage of the radula, the organ from which limpet teeth originate, identifies sequential changes in expression of genes related to chitin and iron processing. We quantify iron and chitin metabolic processes in the radula and grow isolated radula cells in vitro. Bioinspired material can be developed with electrospun chitin mineralised by conditioned media from cultured radula cells. Our results inform molecular processes behind the generation of limpet tooth and establish a platform for development of a novel biomimetic with comparable properties.

The highest tensile strength biomaterial known exists in limpet teeth and replicating this material is of interest. Here, the authors report on the ex vivo growth of teeth and use of isolated limpet tissue and cells providing foundations for the development of this high-tensile biomaterial.

Discovery of a Highly Conserved Peptide in the Iron Transporter Melanotransferrin that Traverses an Intact Blood Brain Barrier and Localizes in Neural Cells

The blood-brain barrier (BBB) hinders the distribution of therapeutics intended for treatment of diseases of the brain. Our previous studies demonstrated that that a soluble form of melanotransferrin (MTf; Uniprot P08582; also known as p97, MFI2, and CD228), a mammalian iron-transport protein, is an effective carrier for delivery of drug conjugates across the BBB into the brain and was the first BBB targeting delivery system to demonstrate therapeutic efficacy within the brain. Here, we performed a screen to identify peptides from MTf capable of traversing the BBB. We identified a highly conserved 12-amino acid peptide, termed MTfp, that retains the ability to cross the intact BBB intact, distributes throughout the parenchyma, and enter endosomes and lysosomes within neurons, astrocytes and microglia in the brain. This peptide may provide a platform for the transport of therapeutics to the CNS, and thereby offers new avenues for potential treatments of neuropathologies that are currently refractory to existing therapies.

Four cellulose-active lytic polysaccharide monooxygenases from Cellulomonas species

BACKGROUND

The discovery of lytic polysaccharide monooxygenases (LPMOs) has fundamentally changed our understanding of microbial lignocellulose degradation. Cellulomonas bacteria have a rich history of study due to their ability to degrade recalcitrant cellulose, yet little is known about the predicted LPMOs that they encode from Auxiliary Activity Family 10 (AA10).

RESULTS

Here, we present the comprehensive biochemical characterization of three AA10 LPMOs from Cellulomonas flavigena (CflaLPMO10A, CflaLPMO10B, and CflaLPMO10C) and one LPMO from Cellulomonas fimi (CfiLPMO10). We demonstrate that these four enzymes oxidize insoluble cellulose with C1 regioselectivity and show a preference for substrates with high surface area. In addition, CflaLPMO10B, CflaLPMO10C, and CfiLPMO10 exhibit limited capacity to perform mixed C1/C4 regioselective oxidative cleavage. Thermostability analysis indicates that these LPMOs can refold spontaneously following denaturation dependent on the presence of copper coordination. Scanning and transmission electron microscopy revealed substrate-specific surface and structural morphological changes following LPMO action on Avicel and phosphoric acid-swollen cellulose (PASC). Further, we demonstrate that the LPMOs encoded by Cellulomonas flavigena exhibit synergy in cellulose degradation, which is due in part to decreased autoinactivation.

CONCLUSIONS

Together, these results advance understanding of the cellulose utilization machinery of historically important Cellulomonas species beyond hydrolytic enzymes to include lytic cleavage. This work also contributes to the broader mapping of enzyme activity in Auxiliary Activity Family 10 and provides new biocatalysts for potential applications in biomass modification.

Complex of human Melanotransferrin and SC57.32 Fab fragment reveals novel interdomain arrangement with ferric N-lobe and open C-lobe

Melanotransferrin (MTf) is an iron-binding member of the transferrin superfamily that can be membrane-anchored or secreted in serum. On cells, it can mediate transferrin-independent iron uptake and promote proliferation. In serum, it is a transcytotic iron transporter across the blood-brain barrier. MTf has been exploited as a drug delivery carrier to the brain and as an antibody-drug conjugate (ADC) target due to its oncogenic role in melanoma and its elevated expression in triple-negative breast cancer (TNBC). For treatment of TNBC, an MTf-targeting ADC completed a phase I clinical trial (NCT03316794). The structure of its murine, unconjugated Fab fragment (SC57.32) is revealed here in complex with MTf. The MTf N-lobe is in an active and iron-bound, closed conformation while the C-lobe is in an open conformation incompatible with iron binding. This combination of active and inactive domains displays a novel inter-domain arrangement in which the C2 subdomain angles away from the N-lobe. The C2 subdomain also contains the SC57.32 glyco-epitope, which comprises ten protein residues and two N-acetylglucosamines. Our report reveals novel features of MTf and provides a point of reference for MTf-targeting, structure-guided drug design.

The membrane-bound and soluble form of melanotransferrin function independently in the diagnosis and targeted therapy of lung cancer

Melanotransferrin (MFI2) is a newly identified tumor-associated protein, which consists of two forms of proteins, membrane-bound (mMFI2) and secretory (sMFI2). However, little is known about the expression pattern and their relevance in lung cancer. Here, we found that both two forms of MFI2 are highly expressed in lung cancer. The expression of MFI2 in lung cancer was detected by using the public database and qRT-PCR. Overexpression and knockdown cell lines and recombinant sMFI2 protein were used to study the function of mMFI2 and sMFI2. RNA-seq, protein chip, ChIP assay, Immunoprecipitation, ELISA, and immunofluorescence were used to study the molecular biological mechanism of mMFI2 and sMFI2. We found that mMFI2 promoted the expression of EMT’s common marker N-cadherin by downregulating the transcription factor KLI4, which in turn promoted tumor metastasis; sMFI2 could promote the metastasis of autologous tumor cells in an autocrine manner but the mechanism is different from that of mMFI2. In addition, sMFI2 was proved could inhibit the migration of vascular endothelial cells and subsequently enhance angiogenic responses in a paracrine manner. We propose that the expressions and functions of the two forms of MFI2 in lung cancer are relatively independent. Specifically, mMFI2 was a potential lung cancer therapeutic target, while sMFI2 was highly enriched in advanced lung cancer, and could be used as a tumor staging index.

Functional characterisation of two ferric-ion coordination modes of TtFbpA, the periplasmic subunit of an ABC-type iron transporter from Thermus thermophilus HB8

The ferric ion binding protein A of Thermus thermophilus HB8 (TtFbpA) is the periplasmic subunit of an ABC-type iron transporter.

Iron Binding and Iron Removal Efficiency of Desferrioxamine Based Polymeric Iron Chelators: Influence of Molecular Size and Chelator Density

Desferrioxamine (DFO) is a clinically approved, high affinity iron chelator used for the treatment of iron overload. Due to its short half-life and toxicity, DFO is administered for 8-12 h per day, 5-7 d per week. In this manuscript, the influence of molecular properties of hyperbranched polyglycerol (HPG)-DFO conjugates on their iron binding by isothermal titration calorimetry, iron removal efficiency from ferritin in presence and absence of a low molecular weight (MW) iron chelator, and protection against iron mediated oxidation of proteins is reported. The iron binding properties of HPG-DFO are slightly altered with size and DFO density of conjugates. The lower MW conjugate shows greater iron removal efficiency at room temperature, however, the efficacy of high MW conjugates increases at physiological temperature. The iron removal from ferritin by HPG-DFO conjugates increases significantly in presence of a low MW chelator, suggesting the potential of combination therapy. The molecular properties of the polymer scaffold also have influence on the prevention of iron mediated oxidation of proteins by the conjugates. The results therefore help to define the iron binding thermodynamics of HPG-DFO and their dependence on MW, and can be extended to improve the general understanding of polymeric chelator-iron interactions in situ.

The thermodynamic and binding properties of the transferrins as studied by isothermal titration calorimetry

BACKGROUND

In mammals, serum-transferrins transport iron from the neutral environment of the blood to the cytoplasm by receptor-mediated endocytosis. Extensive in-vitro studies have focused on the thermodynamics and kinetics of Fe(3+) binding to a number of transferrins. However, little attention has been given to the thermodynamic characterization of the interaction of transferrin with its receptor.

SCOPE OF REVIEW

Iron-loaded transferrin (Tf) binds with high affinity to the specific transferrin receptor (TfR) on the cell surface. The Tf-TfR complex is then internalized via receptor mediated endocytosis into an endosome where iron is released. Here, we provide an overview of recent studies that have used ITC to quantify the interaction of various metal ions with transferrin and highlight our current understanding of the thermodynamics of the transferrin-transferrin receptor system at physiological pH.

GENERAL SIGNIFICANCE

The interaction of the iron-loaded transferrin with the transferrin receptor is a key cellular process that occurs during the normal course of iron metabolism. Understanding the thermodynamics of this interaction is important for iron homeostasis since the physiological requirement of iron must be appropriately maintained to avoid iron-related diseases.

MAJOR CONCLUSIONS

The thermodynamic data revealed stoichiometric binding of all tested metal ions to transferrin with very high affinities ranging between 10(17) and 10(22)M(-1). Iron-loaded transferrin (monoferric or diferric) is shown to bind avidly (K~10(7)-10(8)M(-1)) to the receptor at neutral pH with a stoichiometry of one Tf molecule per TfR monomer. Significantly, both the N- and the C-lobe contribute to the binding interaction which is shown to be both enthalpically and entropically driven. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Calorimetric studies of the interaction between the insulin-enhancing drug candidate bis(maltolato)oxovanadium(IV) (BMOV) and human serum apo-transferrin

Bis(maltolato)oxovanadium(IV) (BMOV), and its ethylmaltol analog, bis(ethylmaltolato)oxovanadium(IV) (BEOV), are candidate insulin-enhancing agents for the treatment of type 2 diabetes mellitus; in mid-2008, BEOV advanced to phase II clinical testing. The interactions of BMOV and its inorganic congener, vanadyl sulfate (VOSO(4)), with human serum apo-transferrin (hTf) were investigated using differential scanning calorimetry (DSC). Addition of BMOV or VOSO(4) to apo-hTf resulted in an increase in thermal stability of both the C- and N-lobes of transferrin as a result of binding to either vanadyl compound. A series of DSC thermograms of hTf solutions containing different molar ratios of BMOV and VOSO(4) were used to determine binding constants; at 25 degrees C the binding constants of BMOV to the C- and N-lobes of apo-hTf were found to be 3 (+/-1)x10(5) and 1.8 (+/-0.7)x10(5)M(-1), respectively. The corresponding values for VOSO(4) were 1.7 (+/-0.3)x10(5) and 7 (+/-2)x10(4)M(-1). The results show that the vanadium species initially presented as either BMOV or VOSO(4) had similar affinities for human serum transferrin due to oxidation of solvated vanadyl(IV) prior to complexation to transferrin. Binding of metavanadate (VO(3)(-)) was confirmed by DSC and isothermal titration calorimetry (ITC) experiments of the interaction between sodium metavanadate (NaVO(3)) and hTf.

Brain iron metabolism: neurobiology and neurochemistry

New findings obtained during the past years, especially the discovery of mutations in the genes associated with brain iron metabolism, have provided key insights into the homeostatic mechanisms of brain iron metabolism and the pathological mechanisms responsible for neurodegenerative diseases. The accumulated evidence demonstrates that misregulation in brain iron metabolism is one of the initial causes for neuronal death in some neurodegenerative disorders. The errors in brain iron metabolism found in these disorders have a multifactorial pathogenesis, including genetic and nongenetic factors. The disturbances of iron metabolism might occur at multiple levels, including iron uptake and release, storage, intracellular metabolism and regulation. It is the increased brain iron that triggers a cascade of deleterious events, leading to neuronal death in these diseases. In the article, the recent advances in studies on neurochemistry and neuropathophysiology of brain iron metabolism were reviewed.

Calorimetric, spectroscopic, and model studies provide insight into the transport of Ti(IV) by human serum transferrin

Evidence suggests that transferrin can bind Ti(IV) in an unhydrolyzed form (without bound hydroxide or oxide) or in a hydrolyzed form. Ti(IV) coordination by N,N'-di(o-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid (HBED) at different pH values models the two forms of Ti(IV)-loaded transferrin spectrally and structurally. 13C NMR and stopped-flow kinetic experiments reveal that when the metal is delivered to the protein using an unhydrolyzed source, Ti(IV) can coordinate in the typical distorted octahedral environment with a bound synergistic anion. The crystal structure of TiHBED obtained at low pH models this type of coordination. The solution structure of the complex compares favorably with the solid state from pH 3.0 to 4.0, and the complex can be reduced with E1/2 = -641 mV vs NHE. Kinetic and thermodynamic competition studies at pH 3.0 reveal that Ti(citrate)3 reacts with HBED via a dissociative mechanism and that the stability of TiHBED (log beta = 34.024) is weaker than that of the Fe(III) complex. pH stability studies show that Ti(IV) hydrolyzes ligand waters at higher pH but still remains bound to HBED until pH 9.5. Similarly, at a pH greater than 8.0 the synergistic anion that binds Ti(IV) in transferrin is readily displaced by irreversible metal hydrolysis although the metal remains bound to the protein until pH 9.5. Thermal denaturation studies conducted optically and by differential scanning calorimetry reveal that Ti(IV)-bound transferrin experiences only minimal enhanced thermal stability unlike when Fe(III) is bound. The C- and N-lobe transition Tm values shift to a few degrees higher. The stability, competition, and redox studies performed provide insight into the possible mechanism of Ti2-Tf transport in cells.

Survey of the year 2005: literature on applications of isothermal titration calorimetry

Isothermal titration calorimetry (ITC) can provide a full thermodynamic characterization of an interaction. Its usage does not suffer from constraints of molecular size, shape or chemical constitution. Neither is there any need for chemical modification or attachment to solid support. This ease of use has made it an invaluable instrumental resource and led to its appearance in many laboratories. Despite this, the value of the thermodynamic parameterization has, only quite recently, become widely appreciated. Although our understanding of the correlation between thermodynamic data and structural details continues to be somewhat naïve, a large number of publications have begun to improve the situation. In this overview of the literature for 2005, we have attempted to highlight works of interest and novelty. Furthermore, we draw attention to those works which we feel have provided a route to better analysis and increased our ability to understand the meaning of thermodynamic change on binding.

Strategy for selecting and characterizing linker peptides for CBM9-tagged fusion proteins expressed inEscherichia coli

The influence of linker design on fusion protein production and performance was evaluated when a family 9 carbohydrate-binding module (CBM9) serves as the affinity tag for recombinant proteins expressed in Escherichia coli. Two bioinformatic strategies for linker design were applied: the first identifies naturally occurring linkers within the proteome of the host organism, the second involves screening peptidases and their known specificities using the bioinformatics software MEROPS to design an artificial linker resistant to proteolysis within the host. Linkers designed using these strategies were compared against traditional poly-glycine linkers. Although widely used, glycine-rich linkers were found by tandem MS data to be susceptible to hydrolysis by E. coli peptidases. The natural (PT)(x)P and MEROPS-designed S(3)N(10) linkers were significantly more stable, indicating both strategies provide a useful approach to linker design. Factor X(a) processing of the fusion proteins depended strongly on linker chemistry, with poly(G) and S(3)N(10) linkers showing the fastest cleavage rates. Luminescence resonance energy transfer studies, used to measure average distance of separation between GFP and Tb(III) bound to a strong calcium-binding site of CBM9, revealed that, for a given linker chemistry, the separation distance increases with increasing linker length. This increase was particularly large for poly(G) linkers, suggesting that this linker chemistry adopts a hydrated, extended configuration that makes it particularly susceptible to proteolysis. Differential scanning calorimetry studies on the PT linker series showed that fusion of CBM9 to GFP did not alter the T(m) of GFP but did result in a destabilization, as seen by both a decrease in T(m) and DeltaH(cal), of CBM9. The degree of destabilization increased with decreasing length of the (PT)(x)P linker such that DeltaT(m) = -8.4 degrees C for the single P linker.

Molecular and clinical aspects of iron homeostasis: From anemia to hemochromatosis

The discovery in recent years of a plethora of new genes whose products are implicated in iron homeostasis has led to rapid expansion of our knowledge in the field of iron metabolism and its underlying complex regulation in both health and disease. Abnormalities of iron metabolism are among the most common disorders encountered in practical medicine and may have significant negative impact on physical condition and life expectancy. Basic insights into the principles of iron homeostasis and the pathophysiological and clinical consequences of iron overload, iron deficiency and misdistribution are thus of crucial importance in modern medicine. This review summarizes our current understanding of human iron metabolism and focuses on the clinically relevant features of hereditary and secondary hemochromatosis, iron deficiency anemia, anemia of chronic disease and anemia of critical illness. The interconnections between iron metabolism and immunity are also addressed, in as much as they may affect the risk and course of infections and malignancies.