Apolactoferrin structure demonstrates ligand-induced conformational change in transferrins

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.

Phylogenetic survey of soluble saxitoxin-binding activity in pursuit of the function and molecular evolution of saxiphilin, a relative of transferrin

Saxiphilin is a soluble protein of unknown function which binds the neurotoxin, saxitoxin (STX), with high affinity. Molecular characterization of saxiphilin from the North American bullfrog, Rana catesbeiana, has previously shown that it is a member of the transferrin family. In this study we surveyed various animal species to investigate the phylogenetic distribution of saxiphilin, as detected by the presence of soluble [3H]STX binding activity in plasma, haemolymph or tissue extracts. We found that saxiphilin activity is readily detectable in a wide variety of arthropods, fish, amphibians, and reptiles. The pharmacological characteristics of [3H]STX binding activity in phylogenetically diverse species indicates that a protein homologous to bullfrog saxiphilin is likely to be constitutively expressed in many ectothermic animals. The results suggest that the saxiphilin gene is evolutionarily as old as an ancestral gene encoding bilobed transferrin, an Fe(2+)-binding and transport protein which has been identified in several arthropods and all the vertebrates which have been studied.

Relationship between antibacterial activity and cell surface binding of lactoferrin in species of genus Micrococcus

Human lactoferrin was bactericidal in vitro for Micrococcus luteus but not for other Micrococcus species (M. radiophilus, M. roseus and M. varians). A correlation between the binding of lactoferrin to the bacterial surface and the antimicrobial action was observed. Viability assays showed that ferric, but not ferrous, salts prevented binding and consequently M. luteus was not killed. The unsaturated form of lactoferrin showed a greater affinity than that of the iron-saturated molecule for lipomannan, a lipoglycan present on the cell wall of M. luteus, supporting the role for lipomannan as one of the possible binding sites of lactoferrin on M. luteus.

Antibacterial peptides of bovine lactoferrin: purification and characterization

Three peptides with antibacterial activity toward enterotoxigenic Escherichia coli have been purified from a pepsin digest of bovine lactoferrin. All peptides were cationic and originated from the N-terminus of the molecule in a region where a bactericidal peptide, lactoferricin B, had been previously identified. The most potent peptide, peptide I, was almost identical to lactoferricin B; the sequence corresponded to residues 17 to 42, and the molecular mass was 3195 as determined by mass spectrometry. A second, less active peptide, peptide II, consisted of two sequences, residues 1 to 16 and 43 to 48 (molecular mass of 2673), linked by a single disulfide bond. The third peptide, peptide III, also a disulfide-linked heterodimer, corresponded to residues 1 to 48 (molecular mass of 5851), cleaved between residues 42 and 43. Peptides I and II displayed antibacterial activity toward a number of pathogenic and food spoilage microorganisms, and peptide I inhibited the growth of Listeria monocytogenes at concentrations as low as 2 microM. Bacterial growth curves showed that bactericidal effects of peptides I and II were observable within 30 min of exposure. The results confirmed and extended those of earlier studies suggesting that the bactericidal domain of lactoferrin was localized in the N-terminus and did not involve iron-binding sites.

Structures and functionalities of milk proteins

During the last decade, marked progress has been made in the study of the fine details of the structures of milk proteins such as caseins, beta-lactoglobulin, alpha-lactalbumin, and lactotransferrin. Many of the functional properties of the individual milk proteins, as well as the milk protein products, may be described at the molecular level. This article is an attempt to thoroughly review the three-dimensional structures of major milk proteins, and to correlate them with the functional aspects of these proteins as food ingredients.

Dynamics of Fusarium solani cutinase investigated through structural comparison among different crystal forms of its variants

In characterizing mutants and covalently inhibited complexes of Fusarium solani cutinase, which is a 197-residue lipolytic enzyme, 34 variant structures, crystallizing in 8 different crystal forms, have been determined, mostly at high resolution. Taking advantage of this considerable body of information, a structural comparative analysis was carried out to investigate the dynamics of cutinase. Surface loops were identified as the major flexible protein regions, particularly those forming the active-site groove, whereas the elements constituting the protein scaffold were found to retain the same conformation in all the cutinase variants studied. Flexibility turned out to be correlated with thermal motion. With a given crystal packing environment, a high flexibility turned out to be correlated with a low involvement in crystal packing contacts. The high degree of crystal polymorphism, which allowed different conformations with similar energy to be detected, made it possible to identify motions which would have remained unidentified if only a single crystal form had been available. Fairly good agreement was found to exist between the data obtained from the structural comparison and those from a molecular dynamics (MD) simulation carried out on the native enzyme. The crystallographic approach used in this study turned out to be a suitable tool for investigating cutinase dynamics. Because of the availability of a set of closely related proteins in different crystal environments, the intrinsic drawback of a crystallographic approach was bypassed. By combining several static pictures, the dynamics of the protein could be monitored much more realistically than what can be achieved on the basis of static pictures alone.

High-yield production of functionally active human serum transferrin using a baculovirus expression system, and its structural characterization

Recently, there has been much interest in expressing recombinant human serum transferrin (HST) and mutants thereof for structural and functional studies. We have developed a baculovirus expression system for the rapid and efficient production of large quantities of HST (> 20 mg/l). Like native HST, the recombinant protein can bind two ferric ions in the presence of bicarbonate, and is actively taken up by receptor-mediated endocytosis. Secondary structure calculations from CD measurements indicate a content of 42% alpha-helix and 28% beta-sheet. This is the first reported use of a non-mammalian expression system to produce functional HST, and will provide a practical tool to allow expression of a wide range of HST variants for mutagenesis studies.

Heterogeneity in utilization of N-glycosylation sites Asn624 and Asn138 in human lactoferrin: a study with glycosylation-site mutants

Human lactoferrin (hLF) is a glycoprotein involved in the host defence against infection and excessive inflammation. Our objective was to determine to what extent each of the three sequons for N-linked glycosylation in hLF is actually used. Human kidney-derived 293(S) cell lines expressing recombinant hLF (rhLF) or glycosylation-site mutants were produced. The mutations involved replacement of asparagine residues with glutamine at one or more sequons for N-glycosylation (Asn138, Asn479 and Asn624). Comparative SDS/PAGE analyses of rhLF, mutated rhLF and human-milk-derived (natural) hLF led us to propose that glycosylation of hLF occurs at two sites (at Asn138 and Asn479) in approx. 85% of all hLF molecules. Glycosylation at a single site (Asn479) or at all three sites occurs in approx, 5% and 9% of hLF respectively. The extent of glycosylation at Asn624 was increased to approx. 29% and 40% of Asn479 and Asn138/479 mutant molecules respectively, which indicates that glycosylation at Asn624 in natural hLF might be limited by glycosylation at Asn479. The presence in supernatant of unglycosylated hLF (approx. 60% of the total) after mutations of Asn138 and Asn479 suggests that glycosylation of hLF is not an absolute requirement for its secretion. The pronounced degradation of unglycosylated hLF in supernatant after mutation at all three glycosylation sites (Asn138/479/624 mutant) but not after mutation at both Asn138 and Asn479 suggests that an altered conformation rather than the lack of glycosylation has rendered the Asn138/479/624 mutant susceptible to intra- and/or extra-cellular degradation.

Copper binding selectivity of N- and C-sites in serum (human)- and ovo-transferrin

Copper binding selectivity of the N- and C-sites in serum (human)- and ovo-transferrin was investigated through copper binding constants, copper dissociation rate constants, and EPR spectra. At pH 7.4, stepwise copper binding constants of serum (human)-transferrin were K1 = 1.8 (+/- 0.6) x 10(12) M-1 and K2 = 1.2 (+/- 0.5) x 10(11) M-1, and those of ovo-transferrin were K1 = 1.9 (+/- 0.5) x 10(11) M-1 and K2 = 2.1 (+/- 0.4) x 10(11) M-1. Absorbance changes resulting from copper binding to the C- or N-site at various ratios of Cu2+/apo-transferrin were separated by a kinetic method. It was clearly indicated that, in serum (human)-transferrin, the copper binding affinity for the C-site was much larger than that for the N-site, whereas in ovo-transferrin, the C- and N-sites have almost the same affinity for copper ions. In the presence of anions (0.1 M KCl or 0.1 M NaClO4), the stepwise copper binding constants of serum (human)-transferrin were almost 10-times smaller than those in the absence of the anions. The selectivity in binding the copper ions to both sites of serum (human)-transferrin in the presence of 0.1 M NaClO4 is much smaller than that in the presence of 0.1 M KCl or in the absence of the anions (0.1 M KCl and 0.1 M NaClO4). EPR spectra of the copper ions of the N-site in dicupric serum-transferrin are dramatically changed respectively by the addition of 0.1 M KCl, 0.1 M NaCl, and 0.1 M NaClO4. This suggests that the change in the coordination geometry of the copper ions occurs at the N-site.

The three-dimensional structure of Escherichia coli porphobilinogen deaminase at 1.76-A resolution

Porphobilinogen deaminase (PBGD) catalyses the polymerization of four molecules of porphobilinogen to form the 1-hydroxymethylbilane, preuroporphyrinogen, a key intermediate in the biosynthesis of tetrapyrroles. The three-dimensional structure of wild-type PBGD from Escherichia coli has been determined by multiple isomorphous replacement and refined to a crystallographic R-factor of 0.188 at 1.76 A resolution. the polypeptide chain of PBGD is folded into three alpha/beta domains. Domains 1 and 2 have a similar overall topology, based on a five-stranded, mixed beta-sheet. These two domains, which are linked by two hinge segments but otherwise make few direct interactions, form an extensive active site cleft at their interface. Domain 3, an open-faced, anti-parallel sheet of three strands, interacts approximately equally with the other two domains. The dipyrromethane cofactor is covalently attached to a cysteine side-chain borne on a flexible loop of domain 3. The cofactor serves as a primer for the assembly of the tetrapyrrole product and is held within the active site cleft by hydrogen-bonds and salt-bridges that are formed between its acetate and propionate side-groups and the polypeptide chain. The structure of a variant of PBGD, in which the methionines have been replaced with selenomethionines, has also been determined. The cofactor, in the native and functional form of the enzyme, adopts a conformation in which the second pyrrole ring (C2) occupies an internal position in the active site cleft. On oxidation, however, this C2 ring of the cofactor adopts a more external position that may correspond approximately to the site of substrate binding and polypyrrole chain elongation. The side-chain of Asp84 hydrogen-bonds the hydrogen atoms of both cofactor pyrrole nitrogens and also potentially the hydrogen atom of the pyrrole nitrogen of the porphobilinogen molecule bound to the proposed substrate binding site. This group has a key catalytic role, possibly in stabilizing the positive charges that develop on the pyrrole nitrogens during the ring-coupling reactions. Possible mechanisms for the processive elongation of the polypyrrole chain involve: accommodation of the elongating chain within the active site cleft, coupled with shifts in the relative positions of domains 1 and 2 to carry the terminal ring into the appropriate position at the catalytic site; or sequential translocation of the elongating polypyrrole chain, attached to the cofactor on domain 3, through the active site cleft by the progressive movement of domain 3 with respect to domains 1 and 2. Other mechanisms are considered although the amino acid sequence comparisons between PBGDs from all species suggest they share the same three-dimensional structure and mechanism of activity.

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.

Di-iron-carboxylate proteins

Di-iron centers bridged by carboxylate residues and oxide/hydroxide groups have so far been seen in four classes of proteins involved in dioxygen chemistry or phosphoryl transfer reactions. The dinuclear iron centers in these proteins are coordinated by histidines and additional carboxylate ligands. Recent structural data on some of these enzymes, combined with spectroscopic and kinetic data, can now serve as a base for detailed mechanistic suggestions. The di-iron sites in the major class of hydroxylase-oxidase enzymes, which contains ribonucleotide reductase and methane monooxygenase, show significant flexibility in the geometry of their coordination of three or more carboxylate groups. This flexibility, combined with a relatively low coordination number, and a buried environment suitable for reactive oxygen chemistry, explains their efficient harnessing of the oxidation power of molecular oxygen.

Mechanisms of iron uptake by mammalian cells

On the basis of the discussion in this paper, the process of transferrin and iron (transferrin-bound iron and non transferrin-bound iron) uptake and transferrin release by reticulocytes are summarized diagrammatically. Although we were able to outline the pathways shown in the figure, there is still a long way to go before we achieve total understanding of the mechanisms of iron uptake. In addition, many important questions need to be answered. For example: what is the nature and properties of the iron carrier on the membrane? What is the relationship between the iron carrier and transferrin receptor? Is the iron carrier system in membranes of cells from different tissues similar or different? And how does iron cross the membrane of the endosomes after it is released from transferrin? All of these questions merit further investigation.

Crystal structure of the cell cycle-regulatory protein suc1 reveals a beta-hinge conformational switch

The Schizosaccharomyces pombe cell cycle-regulatory protein suc1, named as the suppressor of cdc2 temperature-sensitive mutations, is essential for cell cycle progression. To understand suc1 structure-function relationships and to help resolve conflicting interpretations of suc1 function based on genetic studies of suc1 and its functional homologs in both lower and higher eukaryotes, we have determined the crystal structure of the beta-interchanged suc1 dimer. Each domain consists of three alpha-helices and a four-stranded beta-sheet, completed by the interchange of terminal beta-strands between the two subunits. This beta-interchanged suc1 dimer, when compared with the beta-hairpin single-domain folds of suc1, reveals a beta-hinge motif formed by the conserved amino acid sequence HVPEPH. This beta-hinge mediates the subunit conformation and assembly of suc1: closing produces the intrasubunit beta-hairpin and single-domain fold, whereas opening leads to the intersubunit beta-strand interchange and interlocked dimer assembly reported here. This conformational switch markedly changes the surface accessibility of sequence-conserved residues available for recognition of cyclin-dependent kinase, suggesting a structural mechanism for beta-hinge-mediated regulation of suc1 biological function. Thus, suc1 belongs to the family of domain-swapping proteins, consisting of intertwined and dimeric protein structures in which the dual assembly modes regulate their function.

Mechanism of corepressor-mediated specific DNA binding by the purine repressor

The modulation of the affinity of DNA-binding proteins by small molecule effectors for cognate DNA sites is common to both prokaryotes and eukaryotes. However, the mechanisms by which effector binding to one domain affects DNA binding by a distal domain are poorly understood structurally. In initial studies to provide insight into the mechanism of effector-modulated DNA binding of the lactose repressor family, we determined the crystal structure of the purine repressor bound to a corepressor and purF operator. To extend our understanding, we have determined the structure of the corepressor-free corepressor-binding domain of the purine repressor at 2.2 A resolution. In the unliganded state, structural changes in the corepressor-binding pocket cause each subunit to rotate open by as much as 23 degrees, the consequences of which are the disengagement of the minor groove-binding hinge helices and repressor-DNA dissociation.

Dynamic structure of human serum transferrin from transient electric birefringence experiments

In order to investigate the secondary, tertiary, and dynamic structure of the iron-free (apo) and iron-saturated (holo) forms of human serum transferrin and its amino (N)-terminal lobe at the physiologically relevant pHs 7.4 and 5.0, we have combined ultraviolet circular dichroism (CD) spectroscopy with transient-electric birefringence (TEB) measurements. No significant changes are found in the protein's secondary structure under the different conditions studied. The tertiary structure as monitored by near-UV CD is affected by iron binding, but does not change upon decrease in pH. In contrast, TEB results indicate dramatic changes in the dynamic structure of transferrin both upon binding of iron and decrease of pH. In apotransferrin freedom of movement is found for the lobes with respect to each other, and for the domains within the lobes. The interlobal flexibility is considerably enhanced at the lower pH. Holotransferrin is found to behave as a rigid molecule.

Antibacterial activity of lysozyme and lactoferrin is inhibited by binding of advanced glycation-modified proteins to a conserved motif

Why diabetes is associated with abnormally high susceptibility to infection remains unknown, although two major antibacterial proteins, lysozyme and lactoferrin, have now been shown to specifically bind glucose-modified proteins bearing advanced glycation end products (AGEs). Exposure to AGE-modified proteins inhibits the enzymatic and bactericidal activity of lysozyme, and blocks the bacterial agglutination and bacterial killing activities of lactoferrin. Peptide mapping revealed a single AGE binding domain in lysozyme and two AGE binding domains in lactoferrin; each domain contains a 17- to 18- amino acid cysteine-bounded loop motif (CX15-16C) that is markedly hydrophilic. Synthetic peptides corresponding to these motifs in lysozyme and lactoferrin exhibited AGE binding activity, and similar domains are also present in other antimicrobial proteins. These results suggest that elevated levels of AGEs in tissues and serum of diabetic patients may inhibit endogenous antibacterial proteins by binding to this conserved AGE-binding cysteine-bounded domain 'ABCD' motif, thereby increasing susceptibility to bacterial infections in the diabetic population.

The effect of salt and site-directed mutations on the iron(III)-binding site of human serum transferrin as probed by EPR spectroscopy

The effects of site-directed mutation and salt on the iron(III)-binding site of the recombinant half-molecule of the N-terminal lobe (hTf/2N) of human transferrin was studied by EPR spectroscopy. Changes were observed in the EPR spectra of all variants investigated (D63S, D63C, G65R, K206Q, H207E, H249E, H249Q, K296E and K296Q) compared with that of the wild-type protein. The most pronounced changes in the metal site were caused by replacement of the coordinating residues, Asp-63 and His-249, and the non-coordinating residue Lys-296, which is located in the hinge region of the iron-binding cleft. The EPR spectral changes from replacement of other non-coordinating residues were more subtle, indicating small changes in Fe3+ coordination to the protein. The EPR spectrum of variant G65R suggests that it adopts two distinct conformations in solution, one in which the two domains forming the iron-binding cleft are closed and one in which they are open; in the latter instance Asp-63 is no longer coordinated to the Fe3+. Chloride-binding studies on hTf/2N, K206Q, H207E, K296Q and K296E showed similar binding isotherms, indicating that none of the hinge region residues replaced, i.e. Lys-206, His-207 or Lys-296, are the sites of chloride binding. The results show that the coordination environment of the Fe3+ is sensitive to structural changes from site-directed mutation of both remote and coordinated residues and also to chloride-binding and ionic strength effects.

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.

Periodate modification of human serum transferrin Fe(III)-binding sites. Inhibition of carbonate insertion into Fe(III)- and Cu(II)-chelator-transferrin ternary complexes

Periodate modification of human serum transferrin produces a species that binds Fe(III) weakly at pH 7.4 contrary to previous reports that Fe(III)-binding activity is completely lost. Ternary complexes of periodate-modified transferrin and either Fe(III) with nitrilotriacetate (NTA), oxalate, citrate, or EDTA, or of Cu(II) with oxalate could be formed. Peak wavelength maxima of these spectral bands are identical to those reported for native transferrin in the absence of bicarbonate. No carbonate ternary complexes of periodate-modified transferrin with Fe(III), Al(III), Cu(II), or Zn(II) could be formed. Conditional (Fe(NTA)) binding constants (log K) for C- and N-terminal modified sites are 7.33 and 7.54, respectively. The respective extinction coefficients at 470 nm are decreased 45% compared with the native protein. The electron paramagnetic resonance spectrum of the complex closely resembles that of the Fe(III)-NTA ternary complex formed with native transferrin in the absence of bicarbonate. Anions, including bicarbonate, at high concentrations destabilize formation of this Fe(III)-NTA ternary complex, while Fe(III) chelators readily remove the bound Fe(III). Bicarbonate, sulfate, and pyrophosphate still bind to the modified binding sites in the absence of metal although with slightly lower affinity and with lower molar difference absorptivities. Results are interpreted as an inhibition of a crucial protein conformational change by an intramolecular cross-link, preventing formation of the particularly stable metal-carbonate ternary complex from the less stable metal-chelate ternary complex. The method can be used to produce monosited transferrins.

Structural determination of two N-linked glycans isolated from recombinant human lactoferrin expressed in BHK cells

A full-length cDNA coding for human lactoferrin was isolated from a mammary gland library and the recombinant protein was expressed in BHK cells as described by Stowell K. M. et al. [1991, Biochem. J. 276, 349-355]. Two N-linked glycans from purified recombinant lactoferrin were released by hydrazinolysis and analyzed by 400-MHz 1H-NMR spectroscopy. The identified structures corresponded to N-acetyllactosaminic biantennary glycans and were alpha-2,3-disialylated forms (80%) or alpha-2,3-monosialylated (20%) forms. Moreover, 70% of total glycans were alpha-1,6-fucosylated at the GlcNAc residue linked to asparagine. In regard to its glycan moiety, the recombinant glycoprotein is close to native lactoferrins from milk or leucocytes but shows specific structural features which should be taken into account prior to in vivo and in vitro biological studies.

Movie of the structural changes during a catalytic cycle of nucleoside monophosphate kinases

BACKGROUND

There are 17 crystal structures of nucleoside monophosphate kinases known. As expected for kinases, they show large conformational changes upon binding of substrates. These are concentrated in two chain segments, or domains, of 30 and 38 residues that are involved in binding of the substrates N1TP and N2MP (nucleoside tri- and monophosphates with bases N1 and N2), respectively.

RESULTS

After aligning the 17 structures on the main parts of their polypeptide chains, two domains in various conformational states were revealed. These states were caused by bound substrate (or analogues) and by crystal-packing forces, and ranged between a 'closed' conformation and a less well defined 'open' conformation. The structures were visually sorted yielding an approximately evenly spaced series of domain states that outlines the closing motions when the substrates bind. The packing forces in the crystals are weak, leaving the natural domain trajectories essentially intact. Packing is necessary, however, to produce stable intermediates. The ordered experimental structures were then recorded as still pictures of a movie and animated to represent the motions of the molecule during a catalytic cycle. The motions were smoothed out by adding interpolated structures to the observed ones. The resulting movies are available through the World Wide Web (http:@bio5.chemie.uni-freiburg.de/ak movie.html).

CONCLUSIONS

Given the proliferating number of homologous proteins known to exist in different conformational states, it is becoming possible to outline the motions of chain segments and combine them into a movie, which can then represent protein action much more effectively than static pictures alone are able to do.

A system for production of commercial quantities of human lactoferrin: a broad spectrum natural antibiotic

We previously reported the production of limited quantities of biologically active recombinant human lactoferrin in the filamentous fungus Aspergillus oryzae. In the present study, we report a modification of this production system combined with a classical strain improvement program that has enabled production of levels of recombinant human lactoferrin in excess of 2 g/l. The protein was expressed in Aspergillus awamori as a glucoamylase fusion polypeptide which was secreted into the growth medium and processed to mature human lactoferrin by an endogenous KEX-2 peptidase. The recombinant protein retains full biological activity in terms of its ability to bind iron and human enterocyte receptors. Furthermore, the recombinant protein functions as a potent broad spectrum antimicrobial protein.

A comparison of infrared spectra of proteins in solution and crystalline forms

Fourier transform infrared spectroscopy has been used to compare the structure of a range of proteins in solution and in the form of single crystals. An infrared microscope was used to record the spectra of single crystals of the proteins. The proteins studied in this way were hen egg white lysozyme, bovine pancreatic ribonuclease A, bovine gamma-II crystallin, human serum amyloid P component, Endothia parasitica pepsin and Mucor pusillus pepsin. The amide I and amide II bands in the FTIR spectra of these proteins were analysed using derivative procedures thereby providing information on the secondary structure. The crystals were held under a vapour of mother liquor to reduce the effects of dehydration. A comparison of the spectra revealed that spectra recorded from crystals of lysozyme, ribonuclease A and gamma-II crystallin are nearly identical to those recorded from the proteins in solution. However, differences are observed between the spectra of serum amyloid P component, Endothia parasitica pepsin and Mucor pusillus pepsin in solution compared with that of the crystalline form These differences are suggested to be due to rearrangements of turn structures within the protein structure.

Copper incorporation into ceruloplasmin in rat livers

Ceruloplasmin, a blue copper oxidase found in plasma, is synthesized in hepatocytes as a single polypeptide chain consisting of a 19 amino acid leader peptide plus 1046 amino acids of mature protein (132 kDa). Holoceruloplasmin is secreted into the plasma with 6-7 atoms of copper bound per molecule. In this study we identified apo- and holoceruloplasmin and examined the mechanism of copper incorporation during ceruloplasmin biosynthesis using the Long-Evans Cinnamon (LEC) rat which does not incorporate copper into newly synthesized ceruloplasmin. We followed the conversion from ceruloplasmin precursor (with little or no carbohydrate) to the larger product (after carbohydrate addition), which occurred in the secretory compartments of hepatocytes, by native gel electrophoresis. We found that copper accumulates in the hepatocellular Golgi apparatus of LEC rats due to a disorder in the process of copper incorporation. The data indicate that copper is incorporated into ceruloplasmin late in the course of its transport through the secretory compartments.

Characterization of saxitoxin binding to saxiphilin, a relative of the transferrin family that displays pH-dependent ligand binding

Saxiphilin is a 91 kDa saxitoxin-binding protein that is homologous to members of the transferrin family of Fe(3+)-binding proteins noted for pH-dependent release of Fe3+. The mechanism of toxin binding to purified native saxiphilin from the bullfrog (Rana catesbeiana) was studied using [3H]saxitoxin. At pH 7.4 and 0 degrees C [3H]saxitoxin binds to a single site on saxiphilin with a KD of approximately 0.2 nM. The pH dependence of [3H]saxitoxin binding follows a one-site titration curve in the range of pH 9-4 with maximal binding from pH 9 to 7 and half-inhibition at pH 5.7. Inhibition of toxin binding at low pH is the combined result of a decrease in the rate of toxin association and an increase in the rate of toxin dissociation. The dependence of the apparent rate constants for [3H]saxitoxin association and dissociation on [H+] can be accounted for by a four-state model of allosteric interaction between the toxin-binding site and a single titratable residue of saxiphilin with a pKa of 7.2 in the toxin-free form and 4.3 in the toxin-bound form. From 0 to 25 degrees C, the temperature dependence of [3H]saxitoxin binding to saxiphilin is characterized by delta H degrees = -8.3 kcal mol-1, delta S degrees = 13.8 cal mol-1 K-1, and activation energies of 22.5 kcal mol-1 for dissociation and 11.1 kcal mol-1 for association. Binding of [3H]saxitoxin to saxiphilin is competitively inhibited with low affinity by a variety of divalent metal and lanthanide cations. Inhibition of toxin binding by the carboxyl-methylating reagent trimethyloxonium is prevented by pre-equilibration with [3H]saxitoxin, implicating the presence of one or more carboxyl groups in the binding site. Functional similarities suggest that the saxitoxin-binding site of saxiphilin is located in an interdomain cleft analogous to the location of one of the two homologous Fe(3+)-binding sites of transferrins. On the basis of residue substitutions between saxiphilin and transferrins, it is proposed that the saxitoxin-binding site is located in the carboxy terminal lobe of saxiphilin and that binding is modulated by protonation of a conserved histidine residue.

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

The role of protonation of amino acid ligands involved in iron release from human serum transferrin, previously saturated with nitrilotriacetatoiron(III) complex, has been elucidated in acidic media. Iron loss occurs first from the N-terminal site at pH < 6 and is followed at pH < 4 by iron release from the C-terminal iron-binding site. Nitrilotriacetatoiron(III) release from the N-terminal site is controlled by the slow protonation of the mixed protein/nitrilotriacetatoiron(III) complex; the second-order rate constant was k3a = 9.95 +/- 0.35 x 10(4) M-1.s-1. Protonation of an amino acid ligand in the C-terminal site leads to a new protein-site-C-loaded mixed complex with dissociation constant K4 = 0.300 +/- 0.025 mM. Nitrilotriacetatoiron(III) release is the result of mixed complex dissociation and the slow rate-limiting protonation of the iron-free protein with a proton dissociation constant K5a = 0.100 +/- 0.010 mM and a second-order rate constant k5a = 4.20 +/- 0.40 x 10(3) M-1.s-1. The mechanism of iron uptake and release seems to imply that slow proton transfers can induce complex formation between iron and the amino acid ligands of each of the protein iron-binding sites. These slow proton transfers may be controlled by the change of conformation of the binding sites upon iron loss.

Structural mechanisms for domain movements in proteins

We survey all the known instances of domain movements in proteins for which there is crystallographic evidence for the movement. We explain these domain movements in terms of the repertoire of low-energy conformation changes that are known to occur in proteins. We first describe the basic elements of this repertoire, hinge and shear motions, and then show how the elements of the repertoire can be combined to produce domain movements. We emphasize that the elements used in particular proteins are determined mainly by the structure of the interfaces between the domains.

Interaction of transferrin and its iron-binding fragments with heparin

The interaction of heparin with transferrin (Tf; bovine and rat) and the isolated iron-binding lobes of bovine Tf were investigated. Affinity chromatography of rat Tf on heparin-agarose showed that interaction depended on both the iron content of Tf and the pH of the medium. Both the iron-free and iron-saturated forms of Tf were strongly bound by the column at pH 5.6, but only the iron-free form revealed significant affinity at pH 7.4. Desialylation of Tf moderately promoted interaction, treatment with cyclohexanedione moderately reduced interaction, and succinylation abolished it altogether. In the presence of heparin, iron release from the N-terminal lobe of native bovine Tf was accelerated and from the C-terminal lobe it was slightly reduced. The heparin effect remained qualitatively the same on each lobe after their separation by tryptic digestion and DEAE-cellulose chromatography. The affinity of native bovine Tf for heparin was very close to that of its isolated N-terminal lobe, thus suggesting that it is this portion of the molecule that binds to the glycosaminoglycan. It is concluded that the consequences for iron-binding strength of the two transferrin lobes are diagonally opposite when Tf is bound to heparin as opposed to its natural cell-surface receptor.

Electron paramagnetic resonance and difference ultraviolet studies of Mn2+ binding to serum transferrin

Serum transferrin is the mammalian protein whose normal function is to transport ferric ions through the blood among sites of absorption, storage, and utilization. It has two specific metal-binding sites that bind a variety of metal ions in addition to ferric ion. The macroscopic equilibrium constant for the binding of the first equivalent of Mn2+ to apotransferrin has been determined by electron paramagnetic resonance spectroscopy (EPR) to be logKM1 = 4.06 +/- 0.13 at pH 7.4 in 0.1 M N-(2-hydroxyethyl)piperazine-N'-2-ethanesulfonic acid (Hepes). An equilibrium constant for nonspecific binding of Mn2+ to apotransferrin of logKns = 2.93 +/- 0.13 has also been obtained by using EPR. Binding of Mn2+ to apotransferrin and to both C- and N-terminal nonferric transferrin has also been studied by difference UV spectroscopy. The second stepwise macroscopic equilibrium constant for the formation of Mn2Tf is logKM2 = 2.96 +/- 0.13. The site-specific microconstants for Mn2+ binding are logkN = 3.13 +/- 0.09 for the N-terminal site and logkC = 3.80 +/- 0.09 for the C-terminal site. There does not appear to be any significant cooperativity between the two sites with respect to metal binding. An equilibrium model for the speciation of Mn2+ in serum has been developed which estimates that almost 90% of Mn2+ is bound to serum proteins, but only approximately 1% is bound to transferrin. The weak binding of Mn2+ to apotransferrin and the obvious inability of transferrin to compete with albumin indicates that the appearance of Mn-transferrin as a major serum species in vivo must involve oxidation of the metal to form the much more stable Mn(3+)-transferrin complex. The computer model confirms that albumin has a sufficient binding affinity to complex most of the Mn(II) in serum in competition with the common low molecular weight ligands in serum. However, there is insufficient data to rule out the possibility that some other protein, such as alpha 2-macroglobulin, may compete with albumin for Mn(II).

pH-induced structural changes in human serum apotransferrin. pKa values of histidine residues and N-terminal amino group determined by 1H-NMR spectroscopy

The binding of apotransferrin (80 kDa) to the transferrin receptor is known to be highly pH-dependent. We have investigated pH-induced structural changes in human serum apotransferrin over the pH* (meter reading in D2O solutions) range 2.5-11 using 1H-NMR spectroscopy. The pKa values of 14 (possibly 15) of the 19 His residues in the protein have been determined as well as that of the terminal amino group (Val1, 7.75). About eight His residues deprotonate when the pH* is raised from the endosomal value of about 5.5 to the blood plasma value (7.4). Four His residues have pKa < 6. Sharp discontinuities in the His titration curves were observed below pH 4.3 and at pH 3.5 molten globule states were detected.

Structures of a legume lectin complexed with the human lactotransferrin N2 fragment, and with an isolated biantennary glycopeptide: role of the fucose moiety

BACKGROUND

Lectins mediate cell-cell interactions by specifically recognizing oligosaccharide chains. Legume lectins serve as mediators for the symbiotic interactions between plants and nitrogen-fixing microorganisms, an important process in the nitrogen cycle. Lectins from the Viciae tribe have a high affinity for the fucosylated biantennary N-acetyllactosamine-type glycans which are to be found in the majority of N-glycosylproteins. While the structures of several lectins complexed with incomplete oligosaccharides have been solved, no previous structure has included the complete glycoprotein.

RESULTS

We have determined the crystal structures of Lathyrus ochrus isolectin II complexed with the N2 monoglycosylated fragment of human lactotransferrin (18 kDa) and an isolated glycopeptide (2.1 kDa) fragment of human lactotransferrin (at 3.3 A and 2.8 A resolution, respectively). Comparison between the two structures showed that the protein part of the glycoprotein has little influence on either the stabilization of the complex or the sugar conformation. In both cases the oligosaccharide adopts the same extended conformation. Besides the essential mannose moiety of the monosaccharide-binding site, the fucose-1' of the core has a large surface of interaction with the lectin. This oligosaccharide conformation differs substantially from that seen in the previously determined isolectin I-octasaccharide complex. Comparison of our structure with that of concanavalin A (ConA) suggests that the ConA binding site cannot accommodate this fucose.

CONCLUSIONS

Our results explain the observation that Viciae lectins have a higher affinity for fucosylated oligosaccharides than for unfucosylated ones, whereas the affinity of ConA for these types of oligosaccharides is similar. This explanation is testable by mutagenesis experiments. Our structure shows a large complementary surface area between the oligosaccharide and the lectin, in contrast with the recently determined structure of a complex between the carbohydrate recognition domain of a C-type mammalian lectin and an oligomannoside, where only the non-reducing terminal mannose residue interacts with the lectin.

Expression of human lactoferrin in milk of transgenic mice

The expression of human lactoferrin (hLF) in the milk of transgenic mice is described. Regulatory sequences derived from the bovine alpha S1-casein gene were fused to the coding sequence of the hLF cDNA and several lines of transgenic mice were generated. Human LF RNA was detected exclusively in the mammary gland of lactating females and only after the onset of lactation. No aberrant RNA products could be detected using northern blotting and primer extension analysis. The hLF concentrations in the milk ranged from less than 0.1 to 36 micrograms ml-1. Human LF thus expressed did not differ from human milk derived LF, with respect to molecular mass and immunoreactivity with monoclonal and polyclonal antibodies.

Calorimetric studies of serum transferrin and ovotransferrin. Estimates of domain interactions, and study of the kinetic complexities of ferric ion binding

Human serum transferrin and hen ovotransferrin have been studied by differential scanning calorimetry (DSC), in an effort to quantitatively estimate the free energy of interaction of the N- and C-domains in each protein and to further understand their interaction with chelated ferric ions. In the case of serum transferrin, separate DSC transitions are observed for the two domains while only a single, coupled transition is seen for ovotransferrin. Although domain interactions are somewhat larger for ovotransferrin (-4100 cal/mol) than for serum transferrin (-3100 cal/mol), the major cause of separated transitions for serum transferrin is that the difference in intrinsic folding stability of the N- and C-domains is about 4-fold larger than for ovotransferrin. Chelated ferric ions bind strongly to each site in both proteins and produce changes in Tm by as much as 30 degrees C. When apparent binding constants are estimated from DSC results, these appear to be substantially larger than those estimated previously from equilibrium methods at low temperatures, where very long equilibrium times must be used because of slow ligand release. Although second DSC upscans on each protein show good "reversibility", downscans on serum transferrin revealed that liganded forms of the protein are in fact not in true equilibrium during upscanning, which causes Tm values during upscans to be higher than the true reversible Tm values. The likely reason for this kinetic control over unfolding is the slow release of bound ferric ions and those effects, for technical reasons, cannot be totally eliminated by lowering the scan rate.

Isolation of the four forms of transferrin with respect to iron by high-performance liquid chromatography: comparison of three mammalian species

A method, based on anion exchange combined with high-performance liquid chromatography, is presented for the separation of the four different forms of transferrin with respect to iron, i.e. diferric, monoferric with Fe in the C-terminal lobe, monoferric with Fe in the N-terminal lobe, and apo (iron-free). Depending on the size of column applied, these forms can be obtained on a preparative scale amounting to milligrams. The procedure was also found to free transferrin from loosely bound iron and protein aggregates. Comparative studies with human, murine and rat transferrins showed that optimal resolution of these proteins depended on establishing a distinct elution programme for each species. The order in which the four forms referred to above were displaced from the exchanger differed from species to species.