Apotransferrin proton dissociation and interactions with hydrogencarbonate in neutral media

Spectroscopic Studies for Rhodium (III) Binding to Apo-Transferrin

Transferrin belongs to a class of monomeric glycoproteins, which sequestrate and transport iron inside the body. Apart from iron, transferrin binds with various other metal ions and is assumed to deliver medicinally important metal ions to cells. Hence, the investigation of binding characteristics may provide crucial information for new drug developments. To study the biological impacts of medicinally important metal ions, in this work, we explored the binding behavior of Rh(III) ion with serum apo-transferrin (ApoHST) using FT-IR and UV-Vis spectroscopy. In FT-IR, interaction of Rh(III) with ApoHST was studied at three concentrations (0.25, 0.5, and 1 mM) of metal ion at different time intervals (15, 30, and 60 min). The IR spectra of Rh(III)-ApoHST coordinates revealed a marked reduction in amide I and II band intensities with alterations in band positions. The α-helical part of protein secondary structure reduced considerably (from 53% to 49%, 42%, and 39%), followed by an increment in β-sheet and β-turn components with the increasing concentrations of metal ion. Saturation level reached at 1 mM concentration of Rh(III) ion. In the UV-Vis spectroscopic study, absorption of metal ion-protein coordinates successively raised as concentration of Rh(III) ion increased. The binding constant (K) was calculated as $1.16\times {10}^4{\text{M}}^{-1}$ , which showed a strong binding of the test metal ion with the protein. Upon coordination with a metal ion, the microenvironment of aromatic protein residues changed, which was detected by these spectroscopic techniques. The results revealed the existence of a significant interaction between Rh (III) ion and ApoHST. These research outcome may present new insight into the possible utilization of Rh(III) ion-based compounds in biomedicine. However, more investigations are needed to interpret the exact cellular mechanism.

Predicting long term cooperativity and specific modulators of receptor interactions in human transferrin from dynamics within a single microstate

PRS identifies regions contacting rapidly evolving residues that mechanically manipulate dissociation from the pathogen in the human transferrin–bacterial receptor complex.

Direct thermodynamic and kinetic measurements of Fe²⁺ and Zn²⁺ binding to human serum transferrin

Human serum transferrin (hTf) is a single-chain bilobal glycoprotein that efficiently delivers iron to mammalian cells by endocytosis via the transferrin/transferrin receptor system. While extensive studies have been directed towards the study of ferric ion binding to hTf, ferrous ion interactions with the protein have never been firmly investigated owing to the rapid oxidation of Fe(II) to Fe(III) and the difficulty in maintaining a fully anaerobic environment. Here, the binding of Fe(2+) and Zn(2+) ions to hTf has been studied under anaerobic and aerobic conditions, respectively, in the presence and absence of bicarbonate by means of isothermal titration calorimetry (ITC) and fluorescence spectroscopy. The ITC data indicate the presence of one class of strong binding sites with dissociation constants of 25.2 nM for Fe(2+) and 6.7 nM for Zn(2+) and maximum binding stoichiometries of 1 Zn(2+) (or 1 Fe(2+)) per hTf molecule. With either metal, the binding interaction was achieved by both favorable enthalpy and entropy changes (ΔH(0)~-12 kJ/mol and ΔS(0)~106 J/mol·K for Fe(2+) and ΔH(0)~-18 kJ/mol and ΔS(0)~97 J/mol·K for Zn(2+)). The large and positive entropy values are most likely due to the change in the hydration of the protein and the metal ions upon interaction. Rapid kinetics stopped-flow fluorescence spectroscopy revealed two different complexation mechanisms with different degrees of conformational changes upon metal ion binding. Our results are discussed in terms of a plausible scenario for iron dissociation from transferrin by which the highly stable Fe(3+)-hTf complex might be reduced to the more labile Fe(2+) ion before iron is released to the cytosol.

Uptake and release of metal ions by transferrin and interaction with receptor 1

BACKGROUND

For a metal to follow the iron acquisition pathway, four conditions are required: 1-complex formation with transferrin; 2-interaction with receptor 1; 3-metal release in the endosome; and 4-metal transport to cytosol.

SCOPE OF THE REVIEW

This review deals with the mechanisms of aluminum(III), cobalt(III), uranium(VI), gallium(III) and bismuth(III) uptake by transferrin and interaction with receptor 1.

MAJOR CONCLUSIONS

The interaction of the metal-loaded transferrin with receptor 1 takes place in one or two steps: a very fast first step (μs to ms) between the C-lobe and the helical domain of the receptor, and a second slow step (2-6h) between the N-lobe and the protease-like domain. In transferrin loaded with metals other than iron, the dissociation constants for the interaction of the C-lobe with TFR are in a comparable range of magnitudes 10 to 0.5μM, whereas those of the interaction of the N-lobe are several orders of magnitudes lower or not detected. Endocytosis occurs in minutes, which implies a possible internalization of the metal-loaded transferrin with only the C-lobe interacting with the receptor.

GENERAL SIGNIFICANCE

A competition with iron is possible and implies that metal internalization is more related to kinetics than thermodynamics. As for metal release in the endosome, it is faster than the recycling time of transferrin, which implies its possible liberation in the cell. This article is part of a Special Issue entitled Transferrins: Molecular mechanisms of iron transport and disorders.

Gallium uptake by transferrin and interaction with receptor 1

The kinetics and thermodynamics of Ga(III) exchange between gallium mononitrilotriacetate and human serum transferrin as well as those of the interaction between gallium-loaded transferrin and the transferrin receptor 1 were investigated in neutral media. Gallium is exchanged between the chelate and the C-site of human serum apotransferrin in interaction with bicarbonate in about 50 s to yield an intermediate complex with an equilibrium constant K (1) = (3.9 +/- 1.2) x 10(-2), a direct second-order rate constant k (1) = 425 +/- 50 M(-1) s(-1) and a reverse second-order rate constant k (-1) = (1.1 +/- 3) x 10(4) M(-1) s(-1). The intermediate complex loses a single proton with proton dissociation constant K (1a) = 80 +/- 40 nM to yield a first kinetic product. This product then undergoes a modification in its conformation which lasts about 500 s to produce a second kinetic intermediate, which in turn undergoes a final extremely slow (several hours) modification in its conformation to yield the gallium-saturated transferrin in its final state. The mechanism of gallium uptake differs from that of iron and does not involve the same transitions in conformation reported during iron uptake. The interaction of gallium-loaded transferrin with the transferrin receptor occurs in a single very fast kinetic step with a dissociation constant K (d) = 1.10 +/- 0.12 microM and a second-order rate constant k (d) = (1.15 +/- 0.3) x 10(10) M(-1) s(-1). This mechanism is different from that observed with the ferric holotransferrin and suggests that the interaction between the receptor and gallium-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of gallium incorporation by the transferrin receptor-mediated iron-acquisition pathway is discussed.

Mechanism of Formation of the Complex between Transferrin and Bismuth, and Interaction with Transferrin Receptor 1

The kinetics and thermodynamics of Bi(III) exchange between bismuth mononitrilotriacetate (BiL) and human serum transferrin as well as those of the interaction between bismuth-loaded transferrin and transferrin receptor 1 (TFR) were investigated at pH 7.4-8.9. Bismuth is rapidly exchanged between BiL and the C-site of human serum apotransferrin in interaction with bicarbonate to yield an intermediate complex with an effective equilibrium constant K(1) of 6 +/- 4, a direct second-order rate constant k(1) of (2.45 +/- 0.20) x 10(5) M(-1) s(-1), and a reverse second-order rate constant k(-1) of (1.5 +/- 0.5) x 10(6) M(-1) s(-1). The intermediate complex loses a single proton with a proton dissociation constant K(1a) of 2.4 +/- 1 nM to yield a first kinetic product. This product then undergoes a modification in its conformation followed by two proton losses with a first-order rate constant k(2) = 25 +/- 1.5 s(-1) to produce a second kinetic intermediate, which in turn undergoes a last modification in the conformation to yield the bismuth-saturated transferrin in its final state. This last process rate-controls Bi(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau(3)(-1) of (3 +/- 1) x 10(-2) s(-1). The mechanism of bismuth uptake differs from that of iron and probably does not involve the same transition in conformation from open to closed upon iron uptake. The interaction of bismuth-loaded transferrin with TFR occurs in a single very fast kinetic step with a dissociation constant K(d) of 4 +/- 0.4 microM, a second-order rate constant k(d) of (2.2 +/- 1.5) x 10(8) M(-1) s(-1), and a first-order rate constant k(-d) of 900 +/- 400 s(-1). This mechanism is different from that observed with the ferric holotransferrin and implies that the interaction between TFR and bismuth-loaded transferrin probably takes place on the helical domain of the receptor which is specific for the C-site of transferrin and HFE. The relevance of bismuth incorporation by the transferrin receptor-mediated iron acquisition pathway is discussed.

Transferrin's mechanism of interaction with receptor 1

The kinetics and thermodynamics of the interactions of transferrin receptor 1 with holotransferrin and apotransferrin in neutral and mildly acidic media are investigated at 37 degrees C in the presence of CHAPS micelles. Receptor 1 interacts with CHAPS in a very fast kinetic step (<1 micros). This is followed in neutral media by the interaction with holotransferrin which occurs in two steps after receptor deprotonation, with a proton dissociation constant (K(1a)) of 10.0 +/- 1.5 nM. The first step is detected by the T-jump technique and is associated with a molecular interaction between the receptor and holotransferrin. It occurs with a first-order rate constant (k(-1)) of (1.6 +/- 0.2) x 10(4) s(-1), a second-order rate constant (k(1)) of (3.20 +/- 0.2) x 10(10) M(-1) s(-1), and a dissociation constant (K(1)) of 0.50 +/- 0.07 microM. This step is followed by a slow change in the conformation with a relaxation time (tau(2)) of 3400 +/- 400 s and an equilibrium constant (K(2)) of (4.6 +/- 1.0) x 10(-3) with an overall affinity of the receptor for holotransferrin [(K'1)(-1)] of (4.35 +/- 0.60) x 10(8) M(-1). Apotransferrin does not interact with receptor 1 in neutral media, between pH 4.9 and 6, it interacts with the receptor in two steps after a receptor deprotonation (K(2a) = 2.30 +/- 0.3 microM). The first step occurs in the range of 1000-3000 s. It is ascribed to a slow change in the conformation which rate-controls a fast interaction between apotransferrin and receptor 1 with an overall affinity constant [(K(3))(-1)] of (2.80 +/- 0.30) x 10(7) M(-1). These results imply that receptor 1 probably exists in at least two forms, the neutral species which interacts with holotransferrin and not with apotransferrin and the acidic species which interacts with apotransferrin. At first, the interaction of the neutral receptor with holotransferrin is extremely fast. It is followed by the slow change in conformation, which leads to an important stabilization of the thermodynamic structure. In the acidic media of the endosome, the interaction of apotransferrin with the acidic receptor is sufficiently strong and rate-controlled by a very slow change in conformation which allows recycling back to the plasma membrane.

Aluminum exchange between citrate and human serum transferrin and interaction with transferrin receptor 1

The kinetics and thermodynamics of Al(III) exchange between aluminum citrate (AlL) and human serum transferrin were investigated in the 7.2-8.9 pH range. The C-site of human serum apotransferrin in interaction with bicarbonate removes Al(III) from Al citrate with an exchange equilibrium constant K1 = (2.0 +/- 0.6) x 10(-2); a direct second-order rate constant k1 = 45 +/- 3 M(-1) x s(-1); and a reverse second-order rate constant k(-1) = (2.3 +/- 0.5) x 10(3) M(-1) x s(-1). The newly formed aluminum-protein complex loses a single proton with proton dissociation constant K1a = (15 +/- 3) nM to yield a first kinetic intermediate. This intermediate then undergoes a modification in its conformation followed by two proton losses; first-order rate constant k2 = (4.20 +/- 0.02) x 10(-2) s(-1) to produce a second kinetic intermediate, which in turn undergoes a last slow modification in the conformation to yield the aluminum-loaded transferrin in its final state. This last process rate-controls Al(III) uptake by the N-site of the protein and is independent of the experimental parameters with a constant reciprocal relaxation time tau3(-1) = (6 +/- 1) x 10(-5) x s(-1). The affinities involved in aluminum uptake by serum transferrins are about 10 orders of magnitude lower than those involved in the uptake of iron. The interactions of iron-loaded transferrins with transferrin receptor 1 occur with average dissociation constants of 3 +/- 1 and 5 +/- 1 nM for the only C-site iron-loaded and of 6.0 +/- 0.6 and 7 +/- 0.5 nM for the iron-saturated ST in the absence or presence of CHAPS, respectively. No interaction is detected between receptor 1 and aluminum-saturated or mixed C-site iron-loaded/N-site aluminum-loaded transferrin under the same conditions. The fact that aluminum can be solubilized by serum transferrin in biological fluids does not necessarily imply that its transfer from the blood stream to cytoplasm follows the receptor-mediated pathway of iron transport by transferrins.

UV Raman evidence of a tyrosine in apo-human serum transferrin with a low pK(a) that is elevated upon binding of sulphate

The binding of sulphate to human serum apo-transferrin has been examined by ultraviolet absorption and ultraviolet resonance Raman difference spectroscopies between pH 6.0 and 9.0. The ultraviolet absorption data reveals a negative feature at 245 nm that increases in magnitude with pH, with an apparent pK(a) of 7.57, which the Raman difference data reveals to be due to tyrosine. The pK(a) of this tyrosine is unusually low and is measured at 7.84 by the Raman difference method and is elevated to greater than 9.0 upon addition of sulphate. Previous studies on the N-lobe imply that Tyr 188 is the tyrosine with a low pK(a) and also that Arg 124 is the primary binding site for the sulphate. The functional relevance may be that with sulphate bound, both carbonate binding and the deprotonation of Tyr will be disfavoured, and as a result so is iron binding.

Transferrins: iron release from lactoferrin

Iron loss in vitro by the iron scavenger bovine lactoferrin was investigated in acidic media in the presence of three different monoanions (NO(3)(-), Cl(-) and Br(-)) and one dianion (SO(4)(2-)). Holo and monoferric C-site lactoferrins lose iron in acidic media (pH< or =3.5) by a four-step mechanism. The first two steps describe modifications in the conformation affecting the whole protein, which occur also with apolactoferrin. These two processes are independent of iron load and are followed by a third step consisting of the gain of two protons. This third step is kinetically controlled by the interaction with two Cl(-), Br(-) and NO(3)(-) or one SO(4)(2-). In the fourth step, iron loss is under the kinetic control of a slow gain of two protons; third-order rate-constants k(2), 4.3(+/-0.2)x10(3), 3.4(+/-0.5)x10(3), 3.3(+/-0.5)x10(3) and 1.5(+/-0.5)x10(3) M(-2) s(-1) when the protein is in interaction with SO(4)(2-), NO(3)(-), Cl(-) or Br(-), respectively. This step is accompanied by the loss of the interaction with the anions; equilibrium constant K(2), 20+/-5 mM, 1.0(+/-0.2)x10(-1), 1.5(+/-0.5)x10(-1) and 1.0(+/-0.3)x10(-1) M(2), for SO(4)(-), NO(3)(-), Cl(-) and Br(-), respectively. This mechanism is very different from that determined in mildly acidic media at low ionic strength (micro<0.5) for the iron transport proteins, serum transferrin and ovotransferrin, with which no prior change in conformation or interaction with anions is required. These differences may result from the fact that in the transport proteins, the interdomain hydrogen bonds that consolidate the closed conformation of the iron-binding cleft occur between amino acid side-chain residues that can protonate in mildly acidic media. With bovine lactoferrin, most of the interdomain hydrogen bonds involved in the C-site and one of those involved in the N-site occur between amino acid side-chain residues that cannot protonate. The breaking of the interdomain H-bond upon protonation can trigger the opening of the iron cleft, facilitating iron loss in serum transferrin and ovotransferrin. This situation is, however, different in lactoferrin, where iron loss requires a prior change in conformation. This can explain why lactoferrin does not lose its iron load in acidic media and why it is not involved in iron transport in acidic endosomes.

Transferrin, is a mixed chelate-protein ternary complex involved in the mechanism of iron uptake by serum-transferrin in vitro?

Iron uptake by transferrin from triacetohydroxamatoFe(III) (Fe(AHA)3) in the presence of bicarbonate has been investigated between pH 7 and 8.2. The protein transits from the opened apo- to the closed holoform by several steps with the accumulation of at least three kinetic intermediates. All these steps are accompanied by proton losses, probably occurring from the protein ligands and the side-chains involved in the interdomain H-bonding nets. The minor bihydroxamatoFe(III) species Fe(AHA)2 exchanges its iron with the C-site of apotransferrin in interaction with bicarbonate without detectable formation of any intermediate protein-iron-ligand mixed complex; direct second-order rate constant k1=4.15(+/-0.05)x10(7) M(-1) s(-1). The kinetic product loses a single proton and undergoes a modification in its conformation followed by the loss of two or three protons; first-order rate constant k2=3.25(+/-0.15) s(-1). This induces a new modification in the conformation; first-order rate constant k3=5.90(+/-0.30)x10(-2) s(-1). This new modification in conformation rate controls iron uptake by the N-site of the protein and is followed by a single proton loss; K3a=6.80 nM. Finally, the holoprotein or the monoferric transferrin in its thermodynamic equilibrated state is produced by a last modification in the conformation occurring in about 4000 seconds. But for the Fe(AHA)3 dissociation and the involvement of Fe(AHA)2 in the first step of iron uptake, this mechanism is identical to that reported for iron uptake from FeNAc3. This implies that the exchange of iron between a chelate and serum-transferrin occurs by a single general mechanism. The nature of the iron-providing chelate is only important for the first kinetic step of the exchange, which can be slowed to such an extent that it rate limits the exchange of iron.

Transferrins, the mechanism of iron release by ovotransferrin

Iron release from ovotransferrin in acidic media (3 < pH < 6) occurs in at least six kinetic steps. The first is a very fast (</= 5 ms) decarbonation of the iron-loaded protein. Iron release from both sites of the protein is controlled by what appear to be slow proton transfers. The N-site loses its iron first in two steps, the first occurring in the tenth of a second range with second order rate constant k1 = (2.30 +/- 0.10) x 104 M-1.s-1, first order rate constant k-1 = (1.40 +/- 0.10) s-1 and equilibrium constant K1a = (60 +/- 6) microM. The second step occurs in the second range with a second order rate constant k2 = (5.2 +/- 0.15) x 103 M-1.s-1, first order rate constant k-2 = (0.2 +/- 0.02) s-1 and equilibrium constant K2a = (39 +/- 5) microM. Iron is afterward lost from the C-site of the protein by two different pathways, one in the presence of a strong Fe(III) ligand such as citrate and the other in the presence of weak ligands such as formate or acetate. The first step, common to both paths, is a slow proton uptake which occurs in the tens of second range with a second order rate constant k3 = (1.22 +/- 0.03) x 103 M-1.s-1 and equilibrium constant K3a = (1.0 +/- 0.1) mM. In the presence of citrate, this step is followed by formation of an intermediate complex with monoferric ovotransferrin; stability constant KLC = (0.435 +/- 0.015) mM. This last step is rate-controlled by slow proton gain which occurs in the hundred second range with a second order rate constant k4 = (1.05 +/- 0.05) x 104 M-1.s-1, first order rate constant k-4 = (1.0 +/- 0.1) x 10-2 s-1 and equilibrium constant K4a = (0.95 +/- 0.15) microM. In the presence of a weak iron(III) ligand such as acetate or formate, formation of an intermediate complex is not detected and iron release is controlled by two final slow proton uptakes. The first occurs in the hundred to thousand second range, second order rate constant k5 = (6.90 +/- 0.30) x 106 M-1.s-1. The last step occurs in the thousand second range. Iron release by ovotransferrin is similar but not identical to that of serum-transferrin. It is slower and occurs at lower pH values. However, as seen for serum-transferrin, it seems to involve the protonation of the amino acid side-chains involved in iron co-ordination and perhaps those implicated in interdomain H-bonds. The observed proton transfers are, then, probably controlled by the change in conformation of the binding lobes from closed when iron-loaded to open in the apo-form.

Transferrin--interactions of lactoferrin with hydrogen carbonate

The interaction of apolactoferrin with hydrogen carbonate (bicarbonate) has been investigated in the pH range 6.5-9.2. In the absence of bicarbonate apolactoferrin loses a single proton with pK1a of 8.10. This proton loss is independent of the interaction with the synergistic anion. The C-site of apolactoferrin interacts with bicarbonate with a very low affinity (K(-1)C = 3.2 M(-1)). This process is accompanied by a proton loss, which is probably provided by the bicarbonate in interaction with the protein. This proton loss can possibly be the result of a shift in the proton dissociation constant, pKa, of the bicarbonate/carbonate acid/base equilibrium, which would decrease from pKa 10.35 to pK2a 6.90 in the bicarbonate-lactoferrin adduct. The N-site of the protein interacts with bicarbonate with an extremely low affinity, which excludes the presence of the N-site-synergistic anion adduct in neutral physiological media. Contrary to serum transferrin, the concentration of the apolactoferrin in interaction with bicarbonate is pH dependent. Between pH 7.4 and pH 9 with [HCO3-] about 20 mM, the concentration of the serum transferrin-bicarbonate adduct is always about 30%, whereas that of the apolactoferrin-synergistic anion adduct varies from 25% at pH 7.5 to 90% at pH 9. This implies that, despite an affinity for bicarbonate two orders of magnitude lower than that of serum transferrin, lactoferrin interacts better with the synergistic anion. This can be explained by the possible interaction of lactoferrin with carbonate in neutral media, whereas transferrin only interacts with bicarbonate.