The mechanism of iron release from transferrin. Slow-proton-transfer-induced loss of nitrilotriacetatoiron(III) complex in acidic media

Modified Fluoroquinolones as Antimicrobial Compounds Targeting Chlamydia trachomatis

Chlamydia trachomatis causes the most common sexually transmitted bacterial infection and trachoma, an eye infection. Untreated infections can lead to sequelae, such as infertility and ectopic pregnancy in women and blindness. We previously enhanced the antichlamydial activity of the fluoroquinolone ciprofloxacin by grafting a metal chelating moiety onto it. In the present study, we pursued this pharmacomodulation and obtained nanomolar active molecules (EC₅₀) against this pathogen. This gain in activity prompted us to evaluate the antibacterial activity of this family of molecules against other pathogenic bacteria, such as Neisseria gonorrhoeae and bacteria from the ESKAPE group. The results show that the novel molecules have selectively improved activity against C. trachomatis and demonstrate how the antichlamydial effect of fluoroquinolones can be enhanced.

Mechanisms of iron and copper-frataxin interactions

Frataxin is a mitochondrial protein whose deficiency is the cause of Friedreich's ataxia, a hereditary neurodegenerative disease. This protein plays a role in iron-sulfur cluster biosynthesis, protection against oxidative stress and iron metabolism. In an attempt to provide a better understanding of the role played by metals in its metabolic functions, the mechanisms of mitochondrial metal binding to frataxin in vitro have been investigated. A purified recombinant yeast frataxin homolog Yfh1 binds two Cu(ii) ions with a Kd1(CuII) of 1.3 × 10-7 M and a Kd2(CuII) of 3.1 × 10-4 M and a single Cu(i) ion with a higher affinity than for Cu(ii) (Kd(CuI) = 3.2 × 10-8 M). Mn(ii) forms two complexes with Yfh1 (Kd1(MnII) = 4.0 × 10-8 M; Kd2(MnII) = 4.0 × 10-7 M). Cu and Mn bind Yfh1 with higher affinities than Fe(ii). It is established for the first time that the mechanisms of the interaction of iron and copper with frataxin are comparable and involve three kinetic steps. The first step occurs in the 50-500 ms range and corresponds to a first metal uptake. This is followed by two other kinetic processes that are related to a second metal uptake and/or to a change in the conformation leading to thermodynamic equilibrium. Frataxin deficient Δyfh1 yeast cells exhibited a marked growth defect in the presence of exogenous Cu or Mn. Mitochondria from Δyfh1 strains also accumulated higher amounts of copper, suggesting a functional role of frataxin in vivo in copper homeostasis.

The crystal structure of iron-free human serum transferrin provides insight into inter-lobe communication and receptor binding

Serum transferrin reversibly binds iron in each of two lobes and delivers it to cells by a receptor-mediated, pH-dependent process. The binding and release of iron result in a large conformational change in which two subdomains in each lobe close or open with a rigid twisting motion around a hinge. We report the structure of human serum transferrin (hTF) lacking iron (apo-hTF), which was independently determined by two methods: 1) the crystal structure of recombinant non-glycosylated apo-hTF was solved at 2.7-A resolution using a multiple wavelength anomalous dispersion phasing strategy, by substituting the nine methionines in hTF with selenomethionine and 2) the structure of glycosylated apo-hTF (isolated from serum) was determined to a resolution of 2.7A by molecular replacement using the human apo-N-lobe and the rabbit holo-C1-subdomain as search models. These two crystal structures are essentially identical. They represent the first published model for full-length human transferrin and reveal that, in contrast to family members (human lactoferrin and hen ovotransferrin), both lobes are almost equally open: 59.4 degrees and 49.5 degrees rotations are required to open the N- and C-lobes, respectively (compared with closed pig TF). Availability of this structure is critical to a complete understanding of the metal binding properties of each lobe of hTF; the apo-hTF structure suggests that differences in the hinge regions of the N- and C-lobes may influence the rates of iron binding and release. In addition, we evaluate potential interactions between apo-hTF and the human transferrin receptor.

The mechanism of iron release from the transferrin-receptor 1 adduct

We report the determination in cell-free assays of the mechanism of iron release from the N-lobe and C-lobe of human serum transferrin in interaction with intact transferrin receptor 1 at 4.3< or =pH< or =6.5. Iron is first released from the N-lobe in the tens of milliseconds range and then from the C-lobe in the hundreds of seconds range. In both cases, iron loss is rate-controlled by slow proton transfers, rate constant for the N-lobe k(1)=1.20(+/-0.05)x10(6)M(-1)s(-1) and for the C-lobe k(2)=1.6(+/-0.1)x10(3)M(-1)s(-1). This iron loss is subsequent to a fast proton-driven decarbonation and is followed by two proton gains, (pK(1a))/2=5.28 per proton for the N-lobe and (pK(2a))/2=5.10 per proton for the C-lobe. Under similar experimental conditions, iron loss is about 17-fold faster from the N-lobe and is at least 200-fold faster from the C-lobe when compared to holotransferrin in the absence of receptor 1. After iron release, the apotransferrin-receptor adduct undergoes a slow partial dissociation controlled by a change in the conformation of the receptor; rate constant k(3)=1.7(+/-0.1)x10(-3)s(-1). At endosomic pH, the final equilibrated state is attained in about 1000 s, after which the free apotransferrin, two prototropic species of the acidic form of the receptor and apotransferrin interacting with the receptor coexist simultaneously. However, since recycling of the vesicle containing the receptor to the cell surface takes a few minutes, the major part of transferrin will still be forwarded to the biological fluid in the form of the apotransferrin-receptor protein-protein adduct.

The position of arginine 124 controls the rate of iron release from the N-lobe of human serum transferrin. A structural study

Human serum transferrin (hTF) is a bilobal iron-binding and transport protein that carries iron in the blood stream for delivery to cells by a pH-dependent mechanism. Two iron atoms are held tightly in two deep clefts by coordination to four amino acid residues in each cleft (two tyrosines, a histidine, and an aspartic acid) and two oxygen atoms from the "synergistic" carbonate anion. Other residues in the binding pocket, not directly coordinated to iron, also play critical roles in iron uptake and release through hydrogen bonding to the liganding residues. The original crystal structures of the iron-loaded N-lobe of hTF (pH 5.75 and 6.2) revealed that the synergistic carbonate is stabilized by interaction with Arg-124 and that both the arginine and the carbonate adopt two conformations (MacGillivray, R. T. A., Moore, S. A., Chen, J., Anderson, B. F., Baker, H., Luo, Y. G., Bewley, M., Smith, C. A., Murphy, M. E., Wang, Y., Mason, A. B., Woodworth, R. C., Brayer, G. D., and Baker, E. N. (1998) Biochemistry 37, 7919-7928). In the present study, we show that the two conformations are also found for a structure at pH 7.7, indicating that this finding was not strictly a function of pH. We also provide structures for two single point mutants (Y45E and L66W) designed to force Arg-124 to adopt each of the previously observed conformations. The structures of each mutant show that this goal was accomplished, and functional studies confirm the hypothesis that access to the synergistic anion dictates the rate of iron release. These studies highlight the importance of the arginine/carbonate movement in the mechanism of iron release in the N-lobe of hTF. Access to the carbonate via a water channel allows entry of protons and anions, enabling the attack on the iron.

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.

Anion-mediated iron release from transferrins. The kinetic and mechanistic model for N-lobe of ovotransferrin

Iron release process of ovotransferrin N-lobe (N-oTf) to anion/chelators has been resolved using kinetic and mechanistic approach. The iron release kinetics of N-oTf were measured at the endosomal pH of 5.6 with three different anions such as nitrilotriacetate, pyrophosphate, and sulfate using stopped flow spectrofluorimetric method, all yielding clear biphasic progress curves. The two observed rate constants and the corresponding amplitudes obtained from the double exponential curve fit to the biphasic curves varied depending on the type and concentration of anions. Several possible models for the iron release kinetic mechanism were examined on the basis of a newly introduced quantitative equation. Results from the curve fitting analyses were consistent with a dual pathway mechanism that includes the competitive iron release from two different protein states, namely, X and Y, with the respective first order rate constants of K(1) and K(2) (X, domain closed holo N-oTf; Y, anion induced different conformer of holo N-oTf). The reversible interconversions of X to Y and Y to X are driven by the second order rate constant k(3) and the first order rate constant K(4), respectively. The obtained rate constants were greatly variable for the three anions depending on the synergistic or nonsynergistic nature. In the light of the anion-binding sites of N-oTf located crystallographically, the compatible mechanistic model that includes competitive anion binding to the iron coordination sites and to a specific anion site is suggested for the dual pathway iron release mechanism.

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.

Tertiary structural changes and iron release from human serum transferrin

Iron release from human serum transferrin was investigated by comparison of the extent of bound iron, measured by charge transfer absorption band intensity (465 nm), with changes observed by small-angle solution X-ray scattering (SAXS) for a series of equilibrated samples between pH 5.69 and 7.77. The phosphate buffers used in this study promote iron release at relatively high pH values, with an empirical pK of 6.9 for the convolved release from the two sites. The spectral data reveal that the N-lobe release is nearly complete by pH 7.0, while the C-lobe remains primarily metal-laden. Conversely, the radius of gyration, Rg, determined from the SAXS data remains constant between pH 7.77 and 7.05, and the evolution of Rg between its value observed for the diferric protein at pH 7.77 (31.2+/-0.2 A) and that of the apo protein at pH 5.69 (33.9+/-0.4 A) exhibits an empirical pK of 6.6. While Rg is effectively constant in the pH range associated with iron release from the N-lobe, the radius of gyration of cross-section, Rc, increases from 16.9+/-0.2 A to 17.6+/-0.2 A. Model simulations suggest that two different rotations of the NII domain relative to the NI domain about a hinge deep in the iron-binding cleft of the N-lobe, one parallel with and one perpendicular to the plane of the iron-binding site, can be significantly advanced relative to their holo protein positions while yielding constant Rg and increased Rc values consistent with the scattering data. Rotation of the CII domain parallel with the C-lobe iron-binding site plane can partially account for the increased Rg values measured at low pH; however, no reasonable combined repositioning of the NII and CII domains yields the experimentally observed increase in Rg.

Does an acidic pH explain why low density lipoprotein is oxidised in atherosclerotic lesions?

The oxidation of low density lipoprotein (LDL) within atherosclerotic lesions may be involved in atherogenesis. LDL oxidation by cells in the presence of iron is faster at acidic pH. In addition, LDL oxidation by iron alone or iron cysteine in the absence of cells is much faster at acidic pH, even at mildly acidic pH (pH 6.5). The effect of pH on LDL oxidation by copper ions is more complex, in that acidity slows down the initial oxidation, as measured by conjugated dienes, hydroperoxides and thiobarbituric acid-reactive substances, but can increase the later stages of LDL oxidation as measured by increased macrophage uptake. Extensive LDL oxidation by cells in atherosclerotic lesions probably requires a source of iron or copper as catalysts for the oxidation. Iron in plasma is carried by the protein transferrin. Lowering the pH releases some of the iron from transferrin so that it can catalyse LDL oxidation. Copper is carried in plasma on caeruloplasmin and becomes more effective in catalysing LDL oxidation when the caeruloplasmin is preincubated at acidic pH, or even at pH 7.0. These effects can be seen with concentrations of caeruloplasmin and transferrin below those present in plasma. By analogy to other inflammatory and ischaemic sites, atherosclerotic lesions may well have an acidic extracellular pH, particularly within clusters of macrophages where the oxidative stress may also be high. This localised acidic pH may help to explain why atherosclerotic lesions are one of the few sites in the body where extensive LDL oxidation occurs.

A mechanism for iron uptake by transferrin

Iron uptake by transferrin from iron nitrilotriacetate (FeNAc3) in the presence of bicarbonate has been investigated in the pH range 6.5-8. Apotransferrin, in interaction with bicarbonate, extracts iron from FeNAc3, without the formation of an intermediate protein-iron-ligand mixed complex (iron-exchange-equilibrium constant, K1=1 +/- 0.05; direct second-order-rate constant, k1=8.0x10(4) +/- 0.5x10(4)M(-1)s(-1)., reverse second-order-rate constant, k-1=7.5x10(4) +/- 0.5x10(4)M(-1)s(-1). The newly formed iron-protein complex loses a single proton (proton-dissociation constant, Ka=16 +/- 1.5nM) and then undergoes a modification of its conformation followed by loss of two or three protons (first-order-rate constant, K2=2.80 +/- 0.10s-1). This includes a new modification in the conformation (first-order-rate constant, K2=6.2x10(2) +/- 0.3x10(-2)s(-1). This second modification in conformation controls the rate of iron uptake by the N-site of the protein and is followed by loss of one proton (K3a=6.80 nM). Finally, the holoprotein or the monoferric transferrin in its final equilibrated state is produced by a third modification in the conformation that occurs after approximately 3000 s. Iron uptake by the N-site does not occur when the apotransferrin interacts with bicarbonate. Nevertheless, it occurs with the monoferric transferrin, in which iron is bound to the C-site, in its final state of equilibrium by a mechanism similar to that of iron uptake by the C-site of apotransferrin. These modifications in the conformation of the protein occur after iron uptake by the C-site and may be important for the recognition of the protein by its receptor prior to iron delivery by endocytosis.

Transferrin, a mechanism for iron release

Iron release from transferrin has been investigated in mildly acidic and acidic media in the presence of formate, acetate and citrate. It occurs first from the N-terminal iron-binding site (N-site) of the holoprotein. It is independent of the nature and the concentration of competing ligands and is controlled by a slow proton transfer; second-order rate constant k1 = (7.4 +/- 0.5) x 10(4) M-1 s-1 which can be attributed to a rate-limiting slow proton gain by a protein ligand subsequent to a fast decarbonation of the N-site. Iron loss from the C-terminal iron-binding site (C-site) is slower than that from the N-site and occurs by two pathways. The first is favoured below pH 4 and does not involve the formation of an intermediate ternary complex. It can be controlled by a rate-limiting slow proton-triggered decarbonation of the binding site; second-order rate constant k3 = (2.25 +/- 0.05) x 10(4) M-1 s-1. The second pathway is favoured above pH 4 and involves a mixed protein-ligand iron complex. It takes place through the slow protonation of the mixed ternary complex and depends on the nature of the competing ligand. It is faster in the presence of citrate than in that of acetate; second-order rate constant k4 = (1.75 +/- 0.10) x 10(3) M-1 s-1 for citrate and (85 +/- 5) M-1 s-1 for acetate. All these phenomena can possibly describe proton-triggered changes of conformation of the binding sites.