Anion binding by human lactoferrin: results from crystallographic and physicochemical studies

Shongwe, Musa S.; Smith, Clyde A.; Ainscough, Eric W.; Baker, Heather M.; Brodie, Andrew M.; Baker, Edward N.

doi:10.1021/bi00133a010

Cited by 64 publications

(60 citation statements)

References 39 publications

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“…Serum transferrin is composed of two similar but nonidentical globular lobes, with each lobe divided into two domains that consist of ␤-sheets overlaid with ␣-helices (1-3). Each lobe houses an Fe(III)-binding site in a cleft between its domains (4,5).…”

mentioning

confidence: 99%

Binding and release of iron by gel-encapsulated human transferrin: Evidence for a conformational search

Navati

Samuni

Aisen

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

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Human transferrin is a single-chain bilobal protein with each of the two similar but not identical lobes in turn composed of two domains. Each lobe may assume one of two stable structural conformations, open or closed, determined by a rigid rotation of the domains with respect to each other. In solution, the transformation of a lobe between open and closed conformations is associated with the release or binding of an Fe(III) ion. The results of the present study indicate that encapsulation of transferrin within a porous sol-gel matrix allows for a dramatic expansion, to days or weeks, of this interconversion time period, thus providing an opportunity to probe heretofore inaccessible transient intermediates. Sol-gel-encapsulated iron-free transferrin samples are prepared by using two protocols. In the first protocol, the equilibrium form of apotransferrin is encapsulated in the sol-gel matrix, whereas in the second protocol holotransferrin is first encapsulated and then iron is removed from the protein. Results of kinetic and spectroscopic studies allow for distinguishing between two models for iron binding. In the first, iron is assumed to bind to amino acid ligands of one domain, inducing a rigid rotation of the second domain to effect closure of the interdomain cleft. In the second, iron undertakes a conformational search among the thermally accessible states of the lobe, ''choosing'' the state which most nearly approximates the stable closed state when iron is bound. Our experimental results support the second mechanism.T ransferrins are iron-binding proteins that function in the transport of iron among cells that are absorbing, secreting, or in need of this essential element. Serum transferrin is composed of two similar but nonidentical globular lobes, with each lobe divided into two domains that consist of ␤-sheets overlaid with ␣-helices (1-3). Each lobe houses an Fe(III)-binding site in a cleft between its domains (4, 5).Domain motions are central to a variety of protein functions. For apotransferrin, the two domains of each lobe are, on average, in an open jaw conformation. When iron is bound, the domains close to surround the metal with their ligands. A phenolate-to-iron charge transfer transition at 465 nm arising from the binding of iron to two tyrosine ligands is responsible for the characteristic salmon-pink color of ironbearing lobes of transferrin (6). Conversely, exposure of solution-phase holotransferrin to an iron-sequestering buffer at pH Ͻ5.0 results in the release of iron, loss of color, and return to the equilibrium open conformation of the iron-free lobe (7). Molecular events that take place when iron is released or taken up are not easily probed in solution because the rate for conformational changes is likely to be much faster than the rate for iron diffusion or for the stochastic iron-release process (8).Sol-gel encapsulation of proteins (9-11) offers a possible means of overcoming the limitations described above. Sol-gelencapsulated proteins have been shown to retain reactivity with re...

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mentioning

confidence: 99%

Binding and release of iron by gel-encapsulated human transferrin: Evidence for a conformational search

Navati

Samuni

Aisen

et al. 2002

Proc. Natl. Acad. Sci. U.S.A.

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“…As with serum transferrin, the understanding of the process of interaction of lactoferrin with bicarbonate and its behaviour towards proton dissociation is essential for the comprehension of iron uptake by the protein (Bellounis et al, 1996;Pakdaman and El Hage Chahine, 1996). However, only very few reports deal with these interactions (Shongwe et al, 1992;Ainscough et al, 1980). We focus our attention in this article on the proton dissociation of the iron-free bovine lactoferrin (apolactoferrin) and its interaction with bicarbonate or carbonate in neutral and slightly basic media.…”

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confidence: 99%

Transferrin

Pakdaman

Chahine

1997

European Journal of Biochemistry

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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 pK,, 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;' = 3.2 M-'). 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, pK,, of the bicarbonate/ carbonate acid/base equilibrium, which would decrease from pK, 10.35 to pK,, 6.90 in the bicarbonatelactoferrin 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 [HCO,] 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.Keywords; transferrin ; lactoferrin; synergistic anion ; iron metabolism; iron transport.The transferrin family constitutes the most important irontransport system in vertebrates and in some invertebrates, mainly worms and insects (Aisen, 1989;Kurama et al., 1995). These proteins transport iron from the biological fluid into the cytoplasm via the plasma membrane by receptor-mediated endocytosis. With the exception of melanotransferrin, which is overexpressed in melanocytes, the other transferrins of the family are soluble glycoproteins (Baker and Lindley, 1992). These are represented by serum transferrin, responsible for iron transport in mammals, egg white ovotransferrin and milk lactoferrin, which is also present in tears, genital secretion, seminal fluid, sweat, etc. (Aisen, 1989). In addition to their iron-transport function, some transferrins, such as lactoferrins, are involved in the immune response and are bactericidal and bacteriostatic (FurmanCorrespondence to J

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“…The hydrogen bonding between the guanidinium cations and the coordinated carbonate anions does not resemble that in guanidinium bicarbonate (Baldwin et al, 1986), where each guanidinium cation forms two hydrogen bonds to one neighbouring bicarbonate anion and one hydrogen bond to another. It also does not resemble the hydrogen-bonding interaction between arginine and coordinated carbonate in lactoferrin (Shongwe et al, 1992), where Arg 121 forms two hydrogen bonds to one of the coordinated O atoms of the carbonate. The guanidinium cation does not appear to influence the structure of the coordination sphere of the Co m atom.…”

Section: O(7)o(2) and N(5)---h(6)o(4)o(5)mentioning

confidence: 92%

“…[Co (NCS) Denner, Egan & Markwell, 1986), as well as that between the guanidinium group of arginine 121 and the carbonate ligand of Fe nI in lactoferrin (Shongwe et al, 1992).…”

mentioning

confidence: 99%

Guanidinium β-cis-(Carbonato-O,O')(N,N'-ethylenediaminediacetato-N,N',O,O'')cobaltate(III)

Egan¹,

Baldwin²,

Denner³

et al. 1995

Acta Crystallogr C

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Anion binding by human lactoferrin: results from crystallographic and physicochemical studies

Cited by 64 publications

References 39 publications

Binding and release of iron by gel-encapsulated human transferrin: Evidence for a conformational search

Binding and release of iron by gel-encapsulated human transferrin: Evidence for a conformational search

Transferrin

Guanidinium β-cis-(Carbonato-O,O')(N,N'-ethylenediaminediacetato-N,N',O,O'')cobaltate(III)

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