Ligand variation in the transferrin family: the crystal structure of the H249Q mutant of the human transferrin N-lobe as a model for iron binding in insect transferrins

Proteins of the transferrin (Tf) family play a central role in iron homeostasis in vertebrates. In vertebrate Tfs, the four iron-binding ligands, 1 Asp, 2 Tyr, and 1 His, are invariant in both lobes of these bilobal proteins. In contrast, there are striking variations in the Tfs that have been characterized from insect species; in three of them, sequence changes in the C-lobe binding site render it nonfunctional, and in all of them the His ligand in the N-lobe site is changed to Gln. Surprisingly, mutagenesis of the histidine ligand, His249, to glutamine in the N-lobe half-molecule of human Tf (hTf/2N) shows that iron binding is destabilized and suggests that Gln249 does not bind to iron. We have determined the crystal structure of the H249Q mutant of hTf/2N and refined it at 1.85 A resolution (R = 0.221, R(free) = 0.246). The structure reveals that Gln249 does coordinate to iron, albeit with a lengthened Fe-Oepsilon1 bond of 2.34 A. In every other respect, the protein structure is unchanged from wild-type. Examination of insect Tf sequences shows that the K206.K296 dilysine pair, which aids iron release from the N-lobes of vertebrate Tfs, is not present in the insect proteins. We conclude that substitution of Gln for His does destabilize iron binding, but in the insect Tfs this is compensated by the loss of the dilysine interaction. The combination of a His ligand with the dilysine pair in vertebrate Tfs may have been a later evolutionary development that gives more sophisticated pH-mediated control of iron release from the N-lobe of transferrins.

Expression, purification, and characterization of recombinant nonglycosylated human serum transferrin containing a C-terminal hexahistidine tag

Attachment of a hexa-His tag is a common strategy in recombinant protein production. The use of such a tag greatly simplifies the purification of the protein from the complex mixture of other proteins in the media or cell extract. We describe the production of two recombinant nonglycosylated human serum transferrins (hTF-NG), containing a factor Xa cleavage site and a hexa-His tag at their carboxyl-terminal ends. One of the constructs comprises the entire coding region for hTF (residues 1-679), while the other lacks the final three carboxyl-terminal amino acids. After insertion of the His-tagged hTFs into the pNUT vector, transfection into baby hamster kidney (BHK) cells, and selection with methotrexate, the secreted recombinant proteins were isolated from the tissue culture medium. Average maximum expression levels of the His-tagged hTFs were about 40 mg/L compared to an average maximum of 50 mg/L for hTF-NG. The first step of purification involved an anion exchange column. The second step utilized a Poros metal chelate column preloaded with copper from which the His-tagged sample was eluted with a linear imidazole gradient. The His-tagged hTFs were characterized and compared to both recombinant hTF-NG and glycosylated hTF from human serum. The identity of each of the His-tagged hTFs constructs was verified by electrospray mass spectroscopy. In summary, the His-tagged hTF constructs simplify the purification of these metal-binding proteins with minimal effects on many of their physical properties. The His-tagged hTFs share many features common to hTF, including reversible iron binding, reactivity with a monoclonal antibody, and presence as a monomer in solution.

Spectral and metal-binding properties of three single-point tryptophan mutants of the human transferrin N-lobe

Human serum transferrin N-lobe (hTF/2N) contains three conserved tryptophan residues, Trp(8), Trp(128) and Trp(264), located in three different environments. The present report addresses the different contributions of the three tryptophan residues to the UV-visible, fluorescence and NMR spectra of hTF/2N and the effect of the mutations at each tryptophan residue on the iron-binding properties of the protein. Trp(8) resides in a hydrophobic box containing a cluster of three phenylalanine side chains and is H bonded through the indole N to an adjacent water cluster lying between two beta-sheets containing Trp(8) and Lys(296) respectively. The fluorescence of Trp(8) may be quenched by the benzene rings. The apparent increase in the rate of iron release from the Trp(8)-->Tyr mutant could be due to the interference of the mutation with the H-bond linkage resulting in an effect on the second shell network. The partial quenching in the fluorescence of Trp(128) results from the nearby His(119) residue. Difference-fluorescence spectra reveal that any protein containing Trp(128) shows a blue shift upon binding metal ion, and the NMR signal of Trp(128) broadens out and disappears upon the binding of paramagnetic metals to the protein. These data imply that Trp(128) is a major fluorescent and NMR reporter group for metal binding, and possibly for cleft closure in hTF/2N. Trp(264) is located on the surface of the protein and does not connect to any functional residues. This explains the facts that Trp(264) is the major contributor to both the absorbance and fluorescence spectra, has a strong NMR signal and the mutation at Trp(264) has little effect on the iron-binding and release behaviours of the protein.

Crystal structures and iron release properties of mutants (K206A and K296A) that abolish the dilysine interaction in the N-lobe of human transferrin

Human transferrin (Tf) is responsible for the binding and transport of iron in the bloodstream of vertebrates. Delivery of this bound iron to cells occurs by a process of receptor-mediated endocytosis during which Tf releases its iron at the reduced endosomal pH of approximately 5.6. Iron release from Tf involves a large conformational change in which the two domains that enclose the binding site in each lobe move apart. We have examined the role of two lysines, Lys206 and Lys296, that form a hydrogen-bonded pair close to the N-lobe binding site of human Tf and have been proposed to form a pH-sensitive trigger for iron release. We report high-resolution crystal structures for the K206A and K296A mutants of the N-lobe half-molecule of Tf, hTf/2N, and quantitative iron release data on these mutants and the double mutant K206A/K296A. The refined crystal structures (for K206A, R = 19.6% and R(free) = 23.7%; for K296A, R= 21.2% and R(free) = 29.5%) reveal a highly conserved hydrogen bonding network in the dilysine pair region that appears to be maintained even when individual hydrogen bonding groups change. The iron release data show that the mutants retain iron to a pH 1 unit lower than the pH limit of wild type hTf/2N, and release iron much more slowly as a result of the loss of the dilysine interaction. Added chloride ions are shown to accelerate iron release close to the pH at which iron is naturally lost and the closed structure becomes destabilized, and to retard it at higher pH.

Factor X fusion proteins: improved production and use in the release in vitro of biologically active hirudin from an inactive alpha-factor-hirudin fusion protein

Many recombinant proteins are synthesized as fusion proteins containing affinity tags to aid in the downstream processing. After purification, the affinity tag is often removed by using a site-specific protease such as factor Xa (FXa). However, the use of FXa is limited by its expense and availability from plasma. To develop a recombinant source of FXa, we have expressed two novel forms of FXa using baby hamster kidney (BHK) cells as host and the expression vector pNUT. The chimeric protein FIIFX consisted of the prepropeptide and the Gla domain of prothrombin linked to the activation peptide and protease region of FXa, together with a cellulose-binding domain (CBD(Cex)) as an affinity tag. A second variant consisted of the transferrin signal peptide linked to the second epidermal growth factor-like domain and the catalytic domain of FX and a polyhistidine tag. Both FX variants were secreted into the medium, their affinity tags were functional, and following activation, both retained FXa-specific proteolytic activity. However, the yield of the FIIFX-CBD(Cex) fusion protein was 10-fold higher than that of FX-CBD(Cex) and other forms of recombinant FX reported to date. The FXa derivatives were used to cleave two different fusion proteins, including a biologically inactive alpha-factor-hirudin fusion protein secreted by Saccharomyces cerevisiae. After cleavage, the released hirudin demonstrated biological activity in a thrombin inhibition assay, suggesting that this method may be applicable to the production of toxic or unstable proteins. The availability of novel FX derivatives linked to different affinity tags allows the development of a versatile system for processing fusion proteins in vitro.

Fluorescence properties and functional roles of tryptophan residues 60d, 96, 148, 207, and 215 of thrombin

Conservative Trp-to-Phe mutations were individually created in human thrombin at positions 60d, 96, 148, 207, and 215. Fluorescence intensities for these residues varied by a factor of 6. Residues 60d, 96, 148, and 215 transferred energy to the thrombin inhibitor 5-dimethylaminonaphthalene-1-sulfonylarginine-N-(3-ethyl-1,5- pentanediyl)amide efficiently, but residue 207 did not. Intensities correlated inversely with exposure to solvent, and measured and theoretical energy transfer efficiencies agreed well. Function was measured with respect to fibrinogen clotting, platelet and factor V activation, inhibition by antithrombin, and the thrombomodulin-dependent activation of protein C and thrombin-activable fibrinolysis inhibitor (TAFI). All activities of W96F and W207F ranged from 74 to 154% of the wild-type activity. This was also true for W148F, except for inhibition by antithrombin, where it showed 60% activity. W60dF was deficient by 30, 57, and 43% with fibrinogen clotting, platelet activation, and factor V cleavage (Arg(1006)), respectively. W215F was deficient by 90, 55, and 56% with fibrinogen clotting, platelet activation, and factor V cleavage (Arg(1536)). With protein C and TAFI, W96F, W148F, and W207F were normal. W60dF, however, was 76 and 23% of normal levels with protein C and TAFI, respectively. In contrast, W215F was 25 and 124% of normal levels in these reactions. Thus, many activities of thrombin are retained upon substitution of Trp with Phe at positions 96, 148, and 207. Trp(60d), however, appears to be very important for TAFI activation, and Trp(215) appears to very important for clotting and protein C activation.

The chloride effect is related to anion binding in determining the rate of iron release from the human transferrin N-lobe

The major function of human transferrin is to deliver iron from the bloodstream to actively dividing cells. Upon iron release, the protein changes its conformation from 'closed' to 'open'. Extensive studies in vitro indicate that iron release from transferrin is very complex and involves many factors, including pH, the chelator used, an anion effect, temperature, receptor binding and intra-lobe interactions. Our earlier work [He, Mason and Woodworth (1997) Biochem. J. 328, 439-445] using the isolated transferrin N-lobe (recombinant N-lobe of human transferrin comprising residues 1-337; hTF/2N) has shown that anions and pH modulate iron release from hTF/2N in an interdependent manner: chloride retards iron release at neutral pH, but accelerates the reaction at acidic pH. The present study supports this idea and further details the nature of the dual effect of chloride: the anion effect on iron release is closely related to the strength of anion binding to the apoprotein. The negative effect seems to originate from competition between chloride and the chelator for an anion-binding site(s) near the metal centre. With decreasing pH, the strength of anion binding to hTF/2N increases linearly, decreasing the contribution of competition with the chelator. In the meantime, the 'open' or 'loose' conformation of hTF/2N, induced by the protonation of critical residues such as the Lys-206/Lys-296 pair at low pH, enables chloride to enter the cleft and bind to exposed side chains, thereby promoting cleft opening and synergistically allowing removal of iron by the chelator, leading to a positive anion effect. Disabling one or more of the primary anion-binding residues, namely Arg-124, Lys-206 and Lys-296, substantially decreases the anion-binding ability of the resulting mutant proteins. In these cases, the competition for the remaining binding residue(s) is increased, leading to a negative chloride effect or, at most, a very small positive effect, even at low pH.

Mutation of the iron ligand His 249 to Glu in the N-lobe of human transferrin abolishes the dilysine "trigger" but does not significantly affect iron release

Serum transferrin is the major iron transport protein in humans. Its function depends on its ability to bind iron with very high affinity, yet to release this bound iron at the lower intracellular pH. Possible explanations for the release of iron from transferrin at low pH include protonation of a histidine ligand and the existence of a pH-sensitive "trigger" involving a hydrogen-bonded pair of lysines in the N-lobe of transferrin. We have determined the crystal structure of the His249Glu mutant of the N-lobe half-molecule of human transferrin and compared its iron-binding properties with those of the wild-type protein and other mutants. The crystal structure, determined at 2.4 A resolution (R-factor 19.8%, R(free) 29.4%), shows that Glu 249 is directly bound to iron, in place of the His ligand, and that a local movement of Lys 296 has broken the dilysine interaction. Despite the loss of this potentially pH-sensitive interaction, the H249E mutant is only slightly more acid-stable than wild-type and releases iron slightly faster. We conclude that the loss of the dilysine interaction does make the protein more acid stable but that this is counterbalanced by the replacement of a neutral ligand (His) by a negatively charged one (Glu), thus disrupting the electroneutrality of the binding site.

Mutations at the histidine 249 ligand profoundly alter the spectral and iron-binding properties of human serum transferrin N-lobe

Human serum transferrin is an iron-binding and -transport protein which carries iron from the blood stream into various cells. Iron is held in two deep clefts located in the N- and C-lobes by coordinating to four amino acid ligands, Asp 63, Tyr 95, Tyr 188, and His 249 (N-lobe numbering), and to two oxygens from carbonate. We have previously reported the effect on the iron-binding properties of the N-lobe following mutation of the ligands Asp 63, Tyr 95, and Tyr 188. Here we report the profound functional changes which result from mutating His 249 to Ala, Glu, or Gln. The results are consistent with studies done in lactoferrin which showed that the histidine ligand is critical for the stability of the iron-binding site [H. Nicholson, B. F. Anderson, T. Bland, S. C. Shewry, J. W. Tweedie, and E. N. Baker (1997) Biochemistry 36, 341-346]. In the mutant H249A, the histidine ligand is disabled, resulting in a dramatic reduction in the kinetic stability of the protein toward loss of iron. The H249E mutant releases iron three times faster than wild-type protein but shows significant changes in both EPR spectra and the binding of anion. This appears to be the net effect of the metal ligand substitution from a neutral histidine residue to a negative glutamate residue and the disruption of the "dilysine trigger" [MacGillivray, R. T. A., Bewley, M. C., Smith, C. A., He, Q.-Y., Mason, A. B., Woodworth, R. C., and Baker, E. N. (2000) Biochemistry 39, 1211-1216]. In the H249Q mutant, Gln 249 appears not to directly contact the iron, given the similarity in the spectroscopic properties and the lability of iron release of this mutant to the H249A mutant. Further evidence for this idea is provided by the preference of both the H249A and H249Q mutants for nitrilotriacetate rather than carbonate in binding iron, probably because NTA is able to provide a third ligation partner. An intermediate species has been identified during the kinetic interconversion between the NTA and carbonate complexes of the H249A mutant. Thus, mutation of the His 249 residue does not abolish iron binding to the transferrin N-lobe but leads to the appearance of novel iron-binding sites of varying structure and stability.

Crystal structures of two mutants (K206Q, H207E) of the N-lobe of human transferrin with increased affinity for iron

The X-ray crystallographic structures of two mutants (K206Q and H207E) of the N-lobe of human transferrin (hTF/2N) have been determined to high resolution (1.8 and 2.0 A, respectively). Both mutant proteins bind iron with greater affinity than native hTF/2N. The structures of the K206Q and H207E mutants show interactions (both H-bonding and electrostatic) that stabilize the interaction of Lys296 in the closed conformation, thereby stabilizing the iron bound forms.

[13C]Methionine NMR and metal-binding studies of recombinant human transferrin N-lobe and five methionine mutants: conformational changes and increased sensitivity to chloride

The N-lobe of human serum transferrin (hTF/2N) and single point mutants in which each of the five methionine residues was individually mutated have been produced in a mammalian tissue-culture expression system. Since the five methionine residues are well distributed in the transferrin N-lobe, (13)C NMR of the [epsilon-(13)C]methionine-labelled proteins has been used to monitor conformational changes of the protein during metal binding. All five methionine residues have been assigned [Beatty, Cox, Frenkiel, Tam, Mason, MacGillivray, Sadler and Woodworth (1996) Biochemistry 35, 7635-7642]. The tentative two-dimensional NMR assignment for two of the five methionine residues, namely Met(26) and Met(109), has been corrected. A series of NMR spectra for the complexes of (13)C-Met-labelled hTF/2N with six different metal ions, Fe(III), Cu(II), Cr(III), Co(III), Ga(III) and In(III), demonstrate that the conformational change of the protein upon metal binding can be observed by means of the changes in the NMR chemical shifts associated with certain methionine residues, regardless of whether diamagnetic or paramagnetic metals are used. Changing any of the methionine residues should have minimal effects on transferrin function, since structural analysis shows that none of these residues contacts functional amino acids or has any obvious role in iron uptake or release. In fact, UV-visible spectra show little perturbation of the electronic spectra of any of the mutants. Nevertheless, the M109L mutant (Met(109)-->Leu) releases iron at half the rate of the wild-type N-lobe, and chloride shows a significantly greater retarding effect on the rate of iron release from all five mutants. All the methionine mutants (especially in the apo form) show a poor solubility in Hepes buffer lacking anions such as bicarbonate. These findings imply a more general effect of anion binding to surface residues than previously realized.

Dual role of Lys206-Lys296 interaction in human transferrin N-lobe: iron-release trigger and anion-binding site

The unique structural feature of the dilysine (Lys206-Lys296) pair in the transferrin N-lobe (hTF/2N) has been postulated to serve a special function in the release of iron from the protein. These two lysines, which are located in opposite domains, hydrogen bond to each other in the iron-containing hTF/2N at neutral pH but are far apart in the apo-form of the protein. It has been proposed that charge repulsion resulting from the protonation of the dilysines at lower pH may be the trigger to open the cleft and facilitate iron release. The fact that the dilysine pair is positively charged and resides in a location close to the metal-binding center has also led to the suggestion that the dilysine pair is an anion-binding site for chelators. The present report provides comprehensive evidence to confirm that the dilysine pair plays this dual role in modulating release of iron. When either of the lysines is mutated to glutamate or glutamine or when both are mutated to glutamate, release of iron is much slower compared to the wild-type protein. This is due to the fact that the driving force for cleft opening is absent in the mutants or is converted to a lock-like interaction (in the case of the K206E and K296E mutants). Direct titration of the apo-proteins with anions as well as anion-dependent iron release studies show that the dilysine pair is part of an active anion-binding site which exists with the Lys296-Tyr188 interaction as a core. At this site, Lys296 serves as the primary anion-binding residue and Tyr188 is the main reporter for electronic spectral change, with smaller contributions from Lys206, Tyr85, and Tyr95. In iron-loaded hTF/2N, anion binding becomes invisible as monitored by UV-vis difference spectra since the spectral reporters Tyr188 and Tyr95 are bound to iron. Our data strongly support the hypothesis that the apo-hTF/2N exists in equilibrium between the open and closed conformations, because only in the closed form is Lys296 in direct contact with Tyr188. The current findings bring together observations, ideas, and experimental data from a large number of previous studies and shed further light on the detailed mechanism of iron release from the transferrin N-lobe. In iron-containing hTF/2N, Lys296 may still function as a target to introduce an anion (or a chelator) near to the iron-binding center. When the pH is lowered, the protonation of carbonate (synergistic anion for metal binding) and then the dilysine pair form the driving force to loosen the cleft, exposing iron; the nearby anion (or chelator) then binds to the iron and releases it from the protein.

Analysis of gamma-carboxyglutamic acid content of protein, urine, and plasma by capillary electrophoresis and laser-induced fluorescence

When the properties of an analyte are known, the separation system can be designed to make the analyte of interest migrate at either a much faster or a much slower velocity compared to other molecules in the sample matrix. A simple and sensitive method to analyze the gamma-carboxyglutamic acid (Gla) content of protein, urine, and plasma was developed using capillary electrophoresis with laser-induced fluorescence detection (CE-LIF). The separation method is designed according to the specific properties of three amino acids of interest. The number of Gla residues from three vitamin K-dependent proteins were estimated by quantifying the amount of fluorescein thiocarbamyl derivative of Gla after alkaline hydrolysis and fluorescein isothiocyanate labeling. Human prothrombin, blood coagulation factor X, and bovine osteocalcin were calculated to have 10.0 +/- 0.7, 11.0 +/- 0.6, and 2.1 +/- 0.1 Gla residues per mole of protein, respectively, which agreed well with amino acid sequencing data. The analysis of free Gla content in urine and plasma was also demonstrated by this method. It was demonstrated that submicrograms of protein can be characterized by CE-LIF.

Thermodynamic studies on anion binding to apotransferrin and to recombinant transferrin N-lobe half molecules

Equilibrium constants for the binding of anions to apotransferrin, to the recombinant N-lobe half transferrin molecule (Tf/2N), and to a series of mutants of Tf/2N have been determined by difference UV titrations of samples in 0.1 M Hepes buffer at pH 7.4 and 25 degrees C. The anions included in this study are phosphate, sulfate, bicarbonate, pyrophosphate, methylenediphosphonic acid, and ethylenediphosphonic acid. There are no significant differences between anion binding to Tf/2N and anion binding to the N-lobe of apotransferrin. The binding of simple anions like phosphate appears to be essentially equivalent for the two apotransferrin binding sites. The binding of pyrophosphate and the diphosphonates is inequivalent, and the studies on the recombinant Tf/2N show that the stronger binding is associated with the N-terminal site. Anion binding constants for phosphate, pyrophosphate, and the diphosphonates with the N-lobe mutants K206A, K296A, and R124A have been determined. Anion binding tends to be weakest for the K296A mutant, but the variation in log K values among the three mutants is surprisingly small. It appears that the side chains of K206, K296, and R124 all make comparable contributions to anion binding. There are significant variations in the intensities of the peaks in the difference UV spectra that are generated by the titrations of the mutant apoproteins with these anions. These differences appear to be related more to variations in the molar extinction coefficients of the anion-protein complexes rather than to differences in binding constants.

Cloning, mutagenesis, and structural analysis of human pancreatic alpha-amylase expressed in Pichia pastoris

Human pancreatic alpha-amylase (HPA) was expressed in the methylotrophic yeast Pichia pastoris and two mutants (D197A and D197N) of a completely conserved active site carboxylic acid were generated. All recombinant proteins were shown by electrospray ionization mass spectrometry (ESI-MS) to be glycosylated and the site of attachment was shown to be Asn461 by peptide mapping in conjunction with ESI-MS. Treatment of these proteins with endoglycosidase F demonstrated that they contained a single N-linked oligosaccharide and yielded a protein product with a single N-acetyl glucosamine (GlcNAc), which could be crystallized. Solution of the crystal structure to a resolution of 2.0 A confirmed the location of the glycosyl group as Asn461 and showed that the recombinant protein had essentially the same conformation as the native enzyme. The kinetic parameters of the glycosylated and deglycosylated wild-type proteins were the same while the k(cat)/Km values for D197A and D197N were 10(6)-10(7) times lower than the wild-type enzyme. The decreased k(cat)/Km values for the mutants confirm that D197 plays a crucial role in the hydrolytic activity of HPA, presumably as the catalytic nucleophile.

Undercarboxylation of recombinant prothrombin revealed by analysis of gamma-carboxyglutamic acid using capillary electrophoresis and laser-induced fluorescence

The gamma-carboxyglutamic acid (Gla) content of several variants of human prothrombin has been measured by using capillary electrophoresis and laser-induced fluorescence (CE-LIF). Both plasma-derived prothrombin and recombinant prothrombin contain ten residues of Gla per molecule of protein. In contrast, a variant of human prothrombin (containing the second kringle domain of bovine prothrombin) was separated into two populations that differed in their Gla content. Direct measurement of the Gla content showed an association with the presence or absence of the calcium-dependent conformational change that is required for prothombinase function. Thus, the CE-LIF assay is useful in determining the carboxylation status of recombinant proteins.

X-ray crystallography and mass spectroscopy reveal that the N-lobe of human transferrin expressed in Pichia pastoris is folded correctly but is glycosylated on serine-32

The ferric form of the N-lobe of human serum transferrin (Fe(III)-hTF/2N) has been expressed at high levels in Pichia pastoris. The Fe(III)-hTF/2N was crystallized in the space group P41212, and X-ray crystallography was used to solve the structure of the recombinant protein at 2.5 A resolution. This represents only the second P. pastoris-derived protein structure determined to date, and allows the comparison of the structures of recombinant Fe(III)-hTF/2N expressed in P. pastoris and mammalian cells with serum-derived transferrin. The polypeptide folding pattern is essentially identical in all of the three proteins. Mass spectroscopic analyses of P. pastoris- hTF/2N and proteolytically derived fragments revealed glycosylation of Ser-32 with a single hexose. This represents the first localization of an O-linked glycan in a P. pastoris-derived protein. Because of its distance from the iron-binding site, glycosylation of Ser-32 should not affect the iron-binding properties of hTF/2N expressed in P. pastoris, making this an excellent expression system for the production of hTF/2N.

Kinetic studies on the removal of iron and aluminum from recombinant and site-directed mutant N-lobe half transferrins

Kinetic studies have been conducted in pH 7.4 Hepes buffer at 25 degreesC on the removal of Fe(III) and Al(III) from the recombinant N-lobe half molecule of human serum transferrin (Tf/2N) and from the R124A, K206A, and K296A mutants of this protein. The rates of iron removal from Tf/2N by 3-hydroxypyridin-4-one (deferiprone) and nitrilotriacetic acid (NTA) are essentially identical with previous results on N-terminal monoferric transferrin (Tf-FeN). For both Tf/2N and Tf-FeN, iron removal by deferiprone follows simple saturation kinetics, while iron removal by NTA follows simple first-order kinetics. There is some discrepancy between the two proteins with respect to iron removal by PPi, but this may be due to differences in the chloride concentrations among different studies. The addition of Fe(NTA)2 to R124A at ambient bicarbonate concentrations forms the Fe-NTA-Tf ternary complex, but the usual Fe-CO3-Tf complex can be formed by adding ferrous ion in the presence of a larger excess of bicarbonate. This complex releases its iron very rapidly by a mechanism that is first-order with respect to the ligand. This suggests that the first-order component of metal release from transferrin involves the displacement of the synergistic carbonate anion. Since iron removal from K206A and K296A at pH 7.4 is extremely slow, studies have been conducted on the more labile Al3+ complexes of Tf/2N, K206A, and K296A. The removal of Al3+ from Tf/2N by PPi follows the same complex kinetic order with respect to the ligand concentration that is observed for iron removal, while the removal of Al3+ from both K206A and K296A reverts to a simple saturation process. The addition of perchlorate retards the removal of Al3+ from both K206A and K296A, suggesting that these lysine residues are not associated with the allosteric effects of inorganic anions on the rates of metal removal.

Ligand-induced conformational change in transferrins: crystal structure of the open form of the N-terminal half-molecule of human transferrin

Serum transferrin binds ferric ions in the bloodstream and transports them to cells, where they are released in a process involving receptor-mediated endocytosis. Iron release is believed to be pH dependent and is coupled with a large conformational change. To help define the steps in iron release, we have determined the three-dimensional structure of the iron-free (apo) form of the recombinant N-lobe half-molecule of human serum transferrin (ApoTfN) by X-ray crystallography. Two crystal forms were obtained, form 1 with four molecules in the asymmetric unit and form 2 with two molecules in the asymmetric unit. The structures of both forms were determined by molecular replacement and were refined at 2.2 and 3.2 A resolution, respectively. Final R-factors were 0.203 (free R = 0. 292) for form 1 and 0.217 (free R = 0.312) for form 2. All six copies of the ApoTfN structure are essentially identical. Comparison with the holo form (FeTfN) shows that a large rigid-body domain movement of 63 degrees has occurred in ApoTfN, to give an open binding cleft. The extent of domain opening is the same as in the N-lobe of human lactoferrin, showing that it depends on internal constraints that are conserved in both proteins, and that it is unaffected by the presence or absence of the C-lobe. Although the conformational change is primarily a rigid-body motion, several local adjustments occur. In particular, two iron ligands, Asp 63 and His 249, change conformation to form salt bridges, with Lys 296 and Glu 83, respectively, in the binding cleft of the apo protein. Both salt bridges would have to break for iron coordination to occur. Most importantly, the structure, determined at a pH (5.3) that is close to the pH of physiological iron release, indicates that protonation of His 249 is a key step in iron release.

Mutations at nonliganding residues Tyr-85 and Glu-83 in the N-lobe of human serum transferrin. Functional second shell effects

The x-ray crystal structure of the N-lobe of human serum transferrin has shown that there is a hydrogen bond network, the so-called "second shell," around the transferrin iron binding site. Tyrosine at position 85 and glutamic acid at position 83 are two nonliganding residues in this network in the human serum transferrin N-lobe (hTF/2N). Mutation of each of these two amino acids has a profound effect on the metal binding properties of hTF/2N. When Tyr-85 is mutated to phenylalanine, iron release from the resulting mutant Y85F is much more facile than from the parent protein. Elimination of the hydrogen bond between Tyr-85 and Lys-296 appears to interfere with the "di-lysine (Lys-206-Lys-296) trigger," which affects the iron binding stability of the protein. Surprisingly, mutation of Glu-83 to alanine leads to the absence of one of the normal iron binding ligands; introduction of a monovalent anion is able to restore the normal first coordination sphere. The missing ligand appears to be His-249, as revealed by comparison of the metal binding behaviors of mutants H249Q and E83A and structural analysis. Glu-83 has a strong H bond linkage with His-249 in apo-hTF/2N, which helps to hold the His-249 in the proper position for iron binding. Disabling Glu-83 by mutation to an alanine seriously disturbs the H bond network, allowing His-249 to move away. A monovalent anion can help reestablish the normal network by providing a negative charge near the position of Glu-83 to reach charge balance, so that ligand His-249 is available again for iron binding.

Two high-resolution crystal structures of the recombinant N-lobe of human transferrin reveal a structural change implicated in iron release

The N-lobe of human serum transferrin (hTF/2N) has been expressed in baby hamster kidney cells and crystallized in both orthorhombic (P212121) and tetragonal (P41212) space groups. Both crystal forms diffract to high resolution (1.6 and 1.8 A, respectively) and have been solved by molecular replacement. Subsequent refinement resulted in final models for the structure of hTF/2N that had crystallographic R-factors of 18.1 and 19.7% for the two crystal forms, respectively; these models represent the highest-resolution transferrin structures determined to date. The hTF/2N polypeptide has a folding pattern similar to those of other transferrins, including the presence of a deep cleft that contains the metal-binding site. In contrast to other transferrins, both crystal forms of hTF/2N display disorder at the iron-binding site; model building suggests that this disorder consists of alternative conformations of the synergistically bound carbonate anion, the side chain for Arg-124, and several solvent molecules. Subsequent refinement revealed that conformation A has an occupancy of 0.63-0. 65 and corresponds to the structure of the iron-binding site found in other transferrins. The alternative conformation B has an occupancy of 0.35-0.37; in this structure, the carbonate has rotated 30 degrees relative to the iron and the side chain for Arg-124 has moved to accommodate the new carbonate position. Several water molecules appear to stabilize the carbonate anion in the two conformations. These structures are consistent with the protonation of the carbonate and resulting partial removal of the anion from the metal; these events would occur prior to cleft opening and metal release.

Inequivalence of the two tyrosine ligands in the N-lobe of human serum transferrin

Human serum transferrin N-lobe (hTF/2N) has four iron-binding ligands, including one histidine, one aspartate, and two tyrosines. The present report elucidates the inequivalence of the two tyrosine ligands (Tyr 95 and Tyr 188) on the metal-binding properties of hTF/2N by means of site-directed mutagenesis, metal release kinetics, and absorption and electron paramagnetic resonance (EPR) spectroscopies. When the liganding tyrosines were mutated individually to phenylalanine, the resulting mutant Y95F showed a weak binding affinity for iron and no affinity for copper, whereas, mutant Y188F completely lost the ability to bind iron but formed a stable complex with copper. Since other studies have demonstrated that mutations of the other two ligands, histidine and aspartate, did not completely abolish iron binding, the present findings suggest that the tyrosine ligand at position 188 is essential for binding of iron to occur. Replacement of Tyr 188 with phenylalanine created a favorable chemical environment for copper coordination but a fatal situation for iron binding. The positions of the two liganding tyrosines in the metal-binding cleft suggest a reason for the inequivalence.

A kinetically active site in the C-lobe of human transferrin

Release of iron from transferrin, the iron-transporting protein of the circulation, is a concerted process involving remote amino acid residues as well as those at the two specific iron-binding sites of the protein. Previous studies of fluoresceinated transferrin have suggested Lys 569 as a kinetically active site in the C-terminal lobe of the protein. We have therefore turned to site-directed mutagenesis to investigate the role of Lys 569 in the release process at pH 5.6, the pH of the endosome where iron is transferred from transferrin to the iron-dependent cell. Mutation of positively charged Lys 569 to an uncharged Gln results in a protein in which release of iron from the mutated lobe to pyrophosphate is slowed by a factor of 15-20 and in which release kinetics switch from a complex saturation-linear to a simple saturation function. Acceleration of release by chloride is also substantially less than in native transferrin. When Lys 569 is replaced by a positively charged Arg, in contrast, observed release rates and chloride dependence are close to those of the native protein. The mechanism of release from the C-lobe site therefore appears to be sensitive to positive charge at position 569. Binding of chloride or other simple anion accelerates and is essential for release from the C-lobe; a muted response of K569Q to chloride concentration suggests that Lys 569 may function as a kinetically active anion-binding residue in the C-lobe. Despite the kinetic effects of the K569 mutation on iron release, rates of iron uptake by K562 cells from the C-lobes of native, K569Q, and K569R proteins are almost identical. In contrast to the C-lobe, iron release from the N-lobe is insensitive to charge at residue 233, the site in that lobe homologous to residue 569, with chloride retarding rather than accelerating release. K233, therefore, is not a kinetically active anion-binding site in the N-lobe. Release mechanisms differ substantially in the two lobes of transferrin despite the identity of ligands and their nearly identical arrangements in the lobes.

Receptor recognition sites reside in both lobes of human serum transferrin

The binding of iron by transferrin leads to a significant conformational change in each lobe of the protein. Numerous studies have shown that the transferrin receptor discriminates between iron-saturated and iron-free transferrin and that it modulates the release of iron. Given these observations, it seems likely that there is contact between each lobe of transferrin and the receptor. This is the case with chicken transferrin, in which it has been demonstrated unambiguously that both lobes are required for binding and iron donation to occur [Brown-Mason and Woodworth (1984) J. Biol. Chem. 259, 1866-1873]. Further support to this contention is added by the ability of both N- and C-domain-specific monoclonal antibodies to block the binding of a solution containing both lobes [Mason, Brown and Church (1987) J. Biol. Chem. 262, 9011-9015]. In the present study a similar conclusion is reached for the binding of human serum transferrin to the transferrin receptor. With the use of recombinant N- and C-lobes of human transferrin produced in a mammalian expression system, we show that both lobes are required to achieve full binding. (Production of recombinant C-lobe in the baby hamster kidney cell system is reported here for the first time.) Each lobe is able to donate iron to transferrin receptors on HeLa S3 cells in the presence of the contralateral lobe. The results are not identical with the chicken system, because the C-lobe alone shows a limited ability to bind to receptors and to donate iron. Further complications arise from the relatively weak re-association between the two lobes of human transferrin compared with the re-association of the ovotransferrin lobes. However, domain-specific monoclonal antibodies to either lobe block the binding of N- and C-lobe mixtures in the human system, thus substantiating the need for both.

Effects of mutations of aspartic acid 63 on the metal-binding properties of the recombinant N-lobe of human serum transferrin

Mutations of the aspartic acid residue at position 63 of the N-lobe of human serum transferrin substantially alter the metal ion- and anion-binding properties of the protein. Substitution of serine, asparagine, glutamic acid, or alanine results in the loss of a key component of the interface in the interdomain cleft and the metal-binding ligand, aspartic acid, leading in all cases to an increased preference for NTA rather than carbonate as the "synergistic" anion relative to the wild-type protein. Excess bicarbonate is required to eliminate the NTA and obtain the "correct" visible spectrum. Carbonate replaces NTA via an intermediate. Blue shifts for the characteristic absorption band of each mutant show a range of effects on the Fe-O (Tyr) interaction. Titration with Co(III) yielded the molecular absorption coefficient for each mutant except D63A, where Co(III) appeared to oxidize the tyrosine residues and damage the ability of the mutant to bind metal. The chelator, Tiron, removes iron from hTF/2N with a simple saturation kinetic mode with respect to the ligand concentration. Chloride inhibits the release in an interesting manner: the effect is initially sharp and then levels off with a minimum k(obs) at [KCl] = 0.5 M. However, the reaction of the D63 mutants with Tiron results in the formation of the ternary complexes Fe-hTF/2N-Tiron. Significant red shifts for the characteristic absorption bands of these complexes suggest a different ligation of Tiron in the mutants from that in wild-type hTF/2N.

Functional characterization of recombinant human meizothrombin and Meizothrombin(desF1). Thrombomodulin-dependent activation of protein C and thrombin-activatable fibrinolysis inhibitor (TAFI), platelet aggregation, antithrombin-III inhibition

Recombinant human prothrombin (rII) and two mutant forms (R155A, R271A,R284A (rMZ) and R271A,R284A (rMZdesF1)) were expressed in mammalian cells. Following activation and purification, recombinant thrombin (rIIa) and stable analogues of meizothrombin (rMZa) and meizothrombin(desF1) (rMZdesF1a) were obtained. Studies of the activation of protein C in the presence of recombinant soluble thrombomodulin (TM) show TM-dependent stimulation of protein C activation by all three enzymes and, in the presence of phosphatidylserine/phosphatidylcholine phospholipid vesicles, rMZa is 6-fold more potent than rIIa. In the presence of TM, rMZa was also shown to be an effective activator of TAFI (thrombin-activatable fibrinolysis inhibitor) (Bajzar, L., Manuel, R., and Nesheim, M. E. (1995) J. Biol. Chem. 270, 14477-14484). All three enzymes were capable of inducing platelet aggregation, but 60-fold higher concentrations of rMZa and rMZdesF1a were required to achieve the effects obtained with rIIa. Second order rate constants (M-1.min-1) for inhibition by antithrombin III (AT-III) were 2.44 x 10(5) (rIIa), 6.10 x 10(4) (rMZa), and 1.05 x 10(5) (rMZdesF1a). The inhibition of rMZa and rMZdesF1a by AT-III is not affected by heparin. All three enzymes bound similarly to hirudin. The results of this and previous studies imply that full-length meizothrombin has marginal procoagulant properties compared to thrombin. However, meizothrombin has potent anticoagulant properties, expressed through TM-dependent activation of protein C, and can contribute to down-regulation of fibrinolysis through the TM-dependent activation of TAFI.

A new method for characterization and epitope determination of a lupus anticoagulant-associated neutralizing antiprothrombin antibody

A patient had both lupus anticoagulant hypoprothrombinemia syndrome and celiac disease. The presence of a neutralizing antiprothrombin antibody in the patient's serum was demonstrated by coagulation tests, immunoadsorption, and Western blot analysis. The probable cause for the severe hypoprothrombinemia was clearance of prothrombin-antibody complexes from the circulation. Studies showed the antiprothrombin antibody binding to human prothrombin was phospholipid- and Ca(++)-independent; the antibody did not bind to human thrombin. The target epitope of the antibody was studied by Western blot analysis of mutated recombinant human prothrombin molecules. The antibody reacted with the fragment 2-A region of prothrombin, spanning the second kringle domain and the thrombin A chain within prothrombin. Based on this new method, the proposed mechanism for the neutralizing action of the antibody is impairment of prothrombin activation by the prothrombinase complex, either by steric hindrance of the hydrolysis of prothrombin by factor Xa or by interference of the interaction of prothrombin with factor Va; both reactions are required for efficient conversion of prothrombin to thrombin.

Association of the two lobes of ovotransferrin is a prerequisite for receptor recognition. Studies with recombinant ovotransferrins

Different recombinant N-lobes of chicken ovotransferrin (oTF/2N) have been isolated from the tissue-culture medium of baby hamster kidney cells transfected with the plasmid pNUT containing the relevant DNA coding sequence. Levels of up to 40, 55 and 30 mg/1 oTF/2N were obtained for constructs defining residues 1-319, 1-332 and 1-337-(Ala)3 respectively. In addition, a full-length non-glycosylated oTF was expressed at a maximum of 80 mg/1 and a foreshortened oTF consisting of residues 1-682 was expressed at a level of 95 mg/l. These preparations were then used to produce, proteolytically, two different C-lobes (oTF/2C) comprising residues 342-686 and 342-682. The purified recombinant N-lobes (oTF/2N) are similar to the proteolytically derived half-molecule with regard to immunoreactivity and spectral properties; they show some interesting differences in thermal stability. A sequence analysis of the cDNA revealed six changes at the nucleotide level that led to six differences in the amino acid sequence compared with that reported by Jeltsch and Chambon [(1982) Eur. J. Biochem. 122, 291-295]. Electrospray mass spectrometry gives results consistent with these six changes. Interaction between the various N- and C-lobes was measured by titration calorimetry. Studies show that only those lobes that associate in solution are able to bind to the receptors on chick embryo red blood cells. These findings do not support a previous report by Oratore et al.

Production and isolation of the recombinant N-lobe of human serum transferrin from the methylotrophic yeast Pichia pastoris

The N-lobe of human serum transferrin has been expressed in the methylotrophic yeast Pichia pastoris by placing the hTF/2N cDNA under the control of the methanol-inducible alcohol oxidase promoter. Following induction with methanol, the N-lobe was efficiently secreted into a basal salt medium in shake flasks at a level of 150-240 mg/liter. As judged by mobility on SDS-PAGE, immunoreactivity with two domain-specific monoclonal antibodies, and both thermal stability and spectral properties (indictative of correct folding and ability to bind iron), the recombinant N-lobe produced by the yeast cells appears to be identical to that produced in a mammalian expression system. Electrospray-mass spectrometry and a third domain specific antibody, however, show that approximately 80% of the protein from the yeast cells contains one or two hexose residues.

Interlobe communication in 13C-methionine-labeled human transferrin

[1H, 13C] NMR investigations of metal-induced conformational changes in the blood serum protein transferrin (80 kDa) are reported. These are thought to play an important role in the recognition of this protein by its cellular receptors. [1H, 13C] NMR resonance assignments are presented for all nine methionine 13CH3 groups of recombinant deglycosylated human transferrin on the basis of studies of recombinant N-lobe (40 kDa, five Met residues), NOESY-relayed [1H, 13C] HMQC spectra, and structural considerations. The first specific assignments for C-lobe resonances of transferrin are presented. Using methionine 13CH3 resonances as probes, it is shown that, with oxalate as the synergistic anion, Ga3+ binds preferentially to the C-lobe and subsequently to the N-lobe. The NMR shifts of Met464, which is in the Trp460-centered hydrophobic patch of helix 5 in the C-lobe in contact with the anion and metal binding site, show that Ga3+ binding causes movement of side chains within this helix, as is also the case in the N-lobe. The C-lobe residue Met382, which contacts the N-lobe hinge region, is perturbed when Ga3+ binds to the N-lobe, indicative of interlobe communication, a feature which may control the recognition of fully-metallated transferrin by its receptor. These results demonstrate that selective 13C labeling is a powerful method for probing the structure and dynamics of high-molecular-mass proteins.

Calcium ion modulation of meizothrombin autolysis at Arg55-Asp56 and catalytic activity

When a recombinant variant of prothrombin with the cleavage site mutations R155A, R271A, and R284A (rMZ) is exposed to either prothrombinase or ecarin, a form of meizothrombin (rMZa) is generated that is stable for weeks in the presence of Ca2+ (Côté, H. C. F., Stevens, W. K., Bajzar, L., Banfield, D. K., Nesheim, M. E., and MacGillivray, R. T. A. (1994) J. Biol. Chem. 269, 11374-11380). In the absence of Ca2+ however, rMZa is rapidly cleaved within a disulfide bonded loop in the F1 domain at Arg55 in the sequence RTPR downward arrowDKL, yielding a molecule with 3 chains joined by two disulfide bonds (rMZa*). Cleavage kinetics are first order regardless of the rMZa concentration, indicating an intramolecular cleavage. This cleavage does not occur at Ca2+ concentrations in excess of 1.0 mM. To assess the role of the F1 domain in rMZa activity, another variant lacking the R155A mutation (rMZdesF1) was expressed, which when activated yields meizothrombin lacking the F1 domain (rMZdesF1a). Rates of hydrolysis of the tripeptide substrate S2238 by rMZa or rMZa* increase from 60% to 90% that of recombinant thrombin as Ca2+, Mg2+, or Mn2+ concentrations are varied from 0 to 10 mM. Km and kcat values for rMZa in the absence and presence of 5 mM Ca2+ are 1.9 and 2.2 microM and 65 and 105 s-1. TAME esterase activity of rMZa also increases with 5 mM Ca2+. No such metal ion-dependent effects are obtained with either thrombin or rMZdesF1a. Fibrinogen clotting activities, relative to that of thrombin, increase in a manner analogous to those obtained with small substrates, for rMZa and rMZa* but not rMZdesF1a. Complexes of the active site probe dansylarginine N-(3-ethyl-1,5-pentanediyl)amide with rMZa and rMZa*, but not thrombin or rMZdesF1a exhibit large cation-dependent decreases in fluorescence intensity, suggesting that metal ion binding in the F1 domain alters the environment of the probe at the active site. These results indicate that in the absence of divalent cations, the activity of rMZa is inhibited, perhaps by obstruction of the active site by the F1 domain, and that Ca2+ binding to the F1 domain modulates the properties of not only the F1 domain but also the protease domain.

The insulinotropic region of gastric inhibitory polypeptide; fragment analysis suggests the bioactive site lies between residues 19 and 30

Glucose-dependent insulinotropic polypeptide or gastric inhibitory polypeptide (GIP) is a 42 amino acid intestinal hormone, which exhibits several direct and indirect effects on fat and glucose metabolism. To determine the bioactive region(s) of the molecule, synthetic and proteolytic fragments of the hormone were generated and tested for their ability to induce a biological response in the isolated, perfused rat pancreas and stomach. A synthetic fragment corresponding to porcine GIP residues 1-30 retained strong insulinotropic activity in the isolated, perfused rat pancreas but greatly reduced somatostatinotropic activity in the isolated perfused rat stomach. A synthetic fragment corresponding to porcine GIP residues 15 to 30 was biologically inactive. However, enterokinase treatment of the synthetic 15-30 fragment restored partial insulinotropic activity in the isolated, perfused rat pancreas. The hypothesis that the restoration of biological activity was due to the enzymatic removal of the amino-terminal dipeptide (Asp-Lys) of GIP15-30 was supported by the observation that a synthetic fragment lacking these two residues was also insulinotropic. Further fractionation of the molecule generated a biologically active 19-30 fragment, suggesting that the residues necessary for the insulin response are contained within this region.

Aceruloplasminemia: molecular characterization of this disorder of iron metabolism

Ceruloplasmin is an abundant alpha 2-serum glycoprotein that contains 95% of the copper found in the plasma of vertebrate species. We report here on the identification of a genetic defect in the ceruloplasmin gene in a patient previously noted to have a total absence of circulating serum ceruloplasmin in association with late-onset retinal and basal ganglia degeneration. In this patient T2 (transverse relaxation time)-weighted magnetic resonance imaging of the brain revealed basal ganglia densities consistent with iron deposition, and liver biopsy confirmed the presence of excess iron. Although Southern blot analysis of the patient's DNA was normal, PCR amplification of 18 of the 19 exons composing the human ceruloplasmin gene revealed a distinct size difference in exon 7. DNA sequence analysis of this exon revealed a 5-bp insertion at amino acid 410, resulting in a frame-shift mutation and a truncated open reading frame. The validity of this mutation was confirmed by analysis of DNA from the patient's daughter, which revealed heterozygosity for this same 5-bp insertion. The presence of this mutation in conjunction with the clinical and pathologic findings demonstrates an essential role for ceruloplasmin in human biology and identifies aceruloplasminemia as an autosomal recessive disorder of iron metabolism. These findings support previous studies that identified ceruloplasmin as a ferroxidase and are remarkably consistent with recent studies on the essential role of a homologous copper oxidase in iron metabolism in yeast. The clinical and laboratory findings suggest that additional patients with movement disorders and nonclassical Wilson disease should be examined for ceruloplasmin gene mutations.

Nucleotide sequence of the phosphotransacetylase gene of Escherichia coli strain K12

The phosphotransacetylase gene (pta) from Escherichia coli strain K-12 1100 was identified in a cloned fragment of chromosomal DNA (Yamamoto-Otake, H., Matsuyama, A. and Nakano, A. (1990) Appl. Microbiol. Biotechnol. 33, 680-682). Overexpression in E. coli confirmed the presence of the pta gene within the cloned fragment. DNA sequence analysis of the cloned pta gene indicates that the predicted phosphotransacetylase polypeptide chain is 713 amino acids in length. The carboxyterminal region of the E. coli phosphotransacetylase shows 42.6% sequence identity with the corresponding enzyme from Methanosarcina thermophila (142 out of 333 residues in corresponding positions are identical). Several short regions of high sequence identity may be structurally or functionally important for enzymic activity.

Characterization of a stable form of human meizothrombin derived from recombinant prothrombin (R155A, R271A, and R284A)

Meizothrombin is a transient intermediate produced during the activation of prothrombin by the prothrombinase complex. Because meizothrombin is very sensitive to further activation and autolysis, its isolation is possible only in the presence of active site thrombin inhibitors. This complicates studies of the activities and functions of meizothrombin. As a model, we have expressed a mutant human prothrombin cDNA (R155A, R271A, R284A) with three of the cleavage sites modified so that they are no longer cleaved by factor Xa or thrombin. Several stable baby hamster kidney cell lines were isolated that secreted up to 20 micrograms/ml of carboxylated mutant prothrombin. After purification, the mutant prothrombin was activated by the prothrombinase complex or by ecarin, resulting in the formation of a meizothrombin-like molecule. Electrophoretic analysis and NH2-terminal sequence analysis were consistent with cleavage of a single bond between Arg320-Ile321 and proper processing of the prepropeptide. The meizothrombin was stable for weeks at 4 degrees C. Activation in the presence of dansylarginine N-(3-ethyl-1,5-pentanediyl) amide confirmed the conversion of prothrombin via meizothrombin. Compared with human plasma-derived thrombin, recombinant meizothrombin demonstrated approximately 7% clotting activity, 100% p-toluene-sulfonylarginine methyl ester esterase activity, and approximately 35% S2238 amidolytic activity, and could attenuate fibrinolysis.

Evolution of prothrombin: isolation and characterization of the cDNAs encoding chicken and hagfish prothrombin

The cDNA sequences of chicken and hagfish prothrombin have been determined. The sequences predict that prothrombin from both species is synthesized as a prepro-protein consisting of a putative Gla domain, two kringle domains, and a two-chain protease domain. Chicken and hagfish prothrombin share 51.6% amino acid sequence identity (313/627 residues). Both chicken and hagfish prothrombin are structurally very similar to human, bovine, rat, and mouse prothrombin and all six species share 41% amino acid sequence identity. Amino acid sequence alignments of human, bovine, rat, mouse, chicken, and hagfish prothrombin suggest that the thrombin B-chain and the propeptide-Gla domain are the regions most constrained for the common function(s) of vertebrate prothrombins.

Characterization of wild-type and an active-site mutant in Escherichia coli of short-chain acyl-CoA dehydrogenase from Megasphaera elsdenii

The objective of this work is to determine the molecular mechanism and regulation of short-chain acyl-CoA dehydrogenase (SCAD) from Megasphaera elsdenii. To achieve this, the gene coding for SCAD from M. elsdenii was cloned and sequenced. Site-directed mutagenesis was then used to identify an amino acid residue that is required for the proposed mechanism. To clone the gene, the amino acid sequence of the 50 N-terminal residues of SCAD from M. elsdenii was determined. This sequence information was utilized to synthesize two sets of mixed oligonucleotide primers which were then used to generate a 120-bp specific probe from M. elsdenii DNA by the polymerase chain reaction (PCR) method. The 120-bp probe was used to screen a M. elsdenii genomic DNA library cloned into Escherichia coli. The gene encoding M. elsdenii SCAD was identified from this library, sequenced, and expressed. The cloned SCAD gene contained an open reading frame which revealed a high degree of sequence identity with an open reading frame protein sequence of the human SCAD and the rat medium-chain acyl-CoA dehydrogenase (MCAD) (44% and 36% identical residues in paired comparisons for human SCAD and rat MCAD, respectively). Recombinant SCAD expressed from a pUC119 vector accounted for 35% of the cytosolic protein in the Escherichia coli crude extract. The expressed protein had similar activity, redox potential properties, and nearly identical amino acid composition to native M. elsdenii SCAD. In addition, a site-directed Glu367 Gln mutant of SCAD expressed from a pUC119 vector was shown to have minimal reductive and oxidative pathway activity with butyryl-CoA and crotonyl-CoA, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

The role of thrombin's Tyr-Pro-Pro-Trp motif in the interaction with fibrinogen, thrombomodulin, protein C, antithrombin III, and the Kunitz inhibitors

When amino acids Pro60B, Pro60C, and Trp60D are deleted from thrombin, the resulting mutant (des-PPW) exhibits (compared to the wild-type enzyme): a similar second order rate constant of inhibition (k(on)) for diisopropyl fluorophosphate, and a comparable inhibition constant (K(i)) for benzamidine, suggesting that the charge stabilizing system and the primary binding pocket are little altered, if at all, by the mutation. As predicted from the x-ray structure, des-PPW is remarkably sensitive to the bovine pancreatic trypsin inhibitor, with a K(i) over 3 x 10(3) times tighter relative to thrombin, but des-PPW is also markedly less susceptible to inactivation by antithrombin III, with a k(on) that is over 100-fold lower. The catalytic constant (kcat) for most p-nitroanilide substrates tested is preserved or even increased, but the Michaelis constant (Km) increases. In contrast, the Km for the fibrinogen A alpha-chain is essentially unchanged, whereas kcat decreases approximately 50-fold. Unlike thrombin, the rate of fibrinopeptide B release becomes, following a lag phase, comparable to that of fibrinopeptide A. Inasmuch as des-PPW cleaves an additional peptide bond in the bovine fibrin alpha-chain, it remains a highly specific serine protease, which releases a single peptide from denatured casein (versus two with thrombin). Protein C activation by des-PPW is approximately 30 times slower than by thrombin in the absence, as well as in the presence, of calcium and thrombomodulin. Although this study confirms that the B-insertion restricts access to the active site cleft, it also suggests that other motifs and/or discrete amino acids are mainly responsible for the narrow specificity of thrombin.

Transmutation of a heme protein

Residue Asn57 of bovine liver cytochrome b5 has been replaced with a cysteine residue, and the resulting variant has been isolated from recombinant Escherichia coli as a mixture of four major species: A, BI, BII, and C. A combination of electronic spectroscopy, 1H NMR spectroscopy, resonance Raman spectroscopy, electrospray mass spectrometry, and direct electrochemistry has been used to characterize these four major cytochrome derivatives. The red form A (E(m) = -19 mV) is found to possess a heme group bound covalently through a thioether linkage involving Cys57 and the alpha carbon of the heme 4-vinyl group. Form BI has a covalently bound heme group coupled through a thioether linkage involving the beta carbon of the heme 4-vinyl group. Form BII is similar to BI except that the sulfur involved in the thioether linkage is oxidized to a sulfoxide. The green form C (E(m) = 175 mV) possesses a noncovalently bound prosthetic group with spectroscopic properties characteristic of a chlorin. A mechanism is proposed for the generation of these derivatives, and the implications of these observations for the biosynthesis of cytochrome c and naturally occurring chlorin prosthetic groups are discussed.

Expression of glycosylated and nonglycosylated human transferrin in mammalian cells. Characterization of the recombinant proteins with comparison to three commercially available transferrins

The coding sequence for human serum transferrin was assembled from restriction fragments derived from a full-length cDNA clone isolated from a human liver cDNA library. The assembled clone was inserted into the expression vector pNUT and stably transfected into transformed baby hamster kidney (BHK) cells, leading to secretion of up to 125 mg/L recombinant protein into the tissue culture medium. As judged by mobility on NaDodSO4-PAGE, immunoreactivity, spectral properties (indicative of correct folding and iron binding), and the ability to bind to receptors on a human cell line, initial studies showed that the recombinant transferrin, is identical to three commercial human serum transferrin samples. Electrospray mass spectrometry (ESMS), anion-exchange chromatography, and urea gel analysis showed that the recombinant protein has an extremely complex carbohydrate pattern with 16 separate masses ranging from 78,833 to 80,802 daltons. Mutation of the two asparagine carbohydrate linkage sites to aspartic acid residues led to the expression and secretion of up to 25 mg/L nonglycosylated transferrin. ESMS, anion-exchange chromatography, and urea gel analysis showed a single molecular species that was consistent with the expected theoretical mass of 75,143 daltons. In equilibrium binding experiments, the nonglycosylated mutant bound to HeLa S3 cells with the same avidity and to the same extent as the glycosylated protein and the three commercial samples. These studies demonstrate conclusively that carbohydrate has no role in this function.

Preliminary crystallographic analyses of the N-terminal lobe of recombinant human serum transferrin

The N-terminal lobe of recombinant human serum transferrin (residues 1 to 337) has been crystallized in a form suitable for high-resolution three-dimensional X-ray crystallographic analyses. Crystals are of the orthorhombic space group P2(1)2(1)2(1), with unit cell dimensions of a = 44.9 A, b = 57.0 A and c = 135.9 A, and diffract to beyond 2 A resolution. Further studies show that isomorphous crystals of specifically designed mutants of this protein can also be grown. Structural studies of both recombinant and mutant protein forms will provide a basis for understanding the mechanism by which human serum transferrin functions.

High level expression of active human prothrombin in a vaccinia virus expression system

We have worked out an efficient and time saving procedure for the expression of recombinant human prothrombin. The glycoprotein was expressed in the vaccinia virus expression system in several mammalian cell lines. The kidney cell lines Vero and BHK and the human cell line Hela were found to efficiently secrete prothrombin. Expression level of 3-4 micrograms of factor II per 10(6) cells per day corresponding to 18-23 mU per 10(6) cells per day were achieved. Since the expression levels obtained with the vaccinia virus/Vero cell system were comparable to those obtained in amplified transformed CHO cells it provides an alternative system for the efficient expression of human prothrombin and may allow to further elucidate structure-function relationships of (pro)thrombin and its various effectors.

The isolation and characterization of rabbit motilin precursor cDNA

The cDNA sequence of rabbit motilin precursor has been determined. The predicted amino acid sequence indicates that the precursor consists of 133 amino acids and includes a 25 amino acid signal peptide followed by the 22 amino acid motilin sequence and an 86 amino acid motilin associated peptide (MAP). As in the human and porcine precursors, two lysine residues follow motilin in the rabbit sequence. Rabbit motilin shares 64% amino acid sequence identity with human and porcine motilin, and all amino acid substitutions represent conservative changes. Amino acid sequence alignments of the rabbit, human and porcine MAP sequences suggest three functional/structural motifs corresponding to a putative endoproteinase recognition site, a putative PEST site and a potential posttranslational processing recognition element.

Partial characterization of vertebrate prothrombin cDNAs: amplification and sequence analysis of the B chain of thrombin from nine different species

The cDNA sequence of the B chain of thrombin (EC 3.4.21.5) has been determined from nine vertebrate species (rat, mouse, rabbit, chicken, gecko, newt, rainbow trout, sturgeon, and hagfish). The amino acid sequence identities vary from 96.5% (rat vs. mouse) to 62.6% (newt vs. hagfish). Of the 240 amino acids spanned in all the species compared, there is identity at 110 (45.8%) positions. When conservative changes are included, the amino acid similarity increases to 75%. The most conserved portions of the B chain are the active-site residues and adjacent amino acids, the B loop, and the primary substrate-binding region. In addition, the Arg-Gly-Asp motif is conserved in 9 of the 11 species compared, and the chemotactic/growth factor domain is well conserved in all of the 11 species compared. The least conserved regions of the B chain correspond to surface loops, including the putative thrombomodulin-binding sites and one of the hirudin-binding regions. The extent of the amino acid sequence similarity and the conservation of many of the functional/structural motifs suggests that, in addition to their role in blood coagulation, vertebrate thrombins may also play an important role in the general mechanisms of wound repair.

The complete cDNA sequence of bovine coagulation factor V

Lack of availability of a primary structure for bovine factor V has hindered detailed analysis of a vast majority of structure-function correlations on this molecule. To determine the primary structure of bovine factor V, we used liver mRNA as a template for the synthesis of three cDNA libraries. The sequences of seven overlapping cDNA clones infer two bovine factor V variants. Variant 1 results in a 6910-basepair (bp) cDNA including 103 bp of 5'-untranslated sequence, 6633 bp of coding sequence and 171 bp of 3'-untranslated sequence with a putative polyadenylation site. Variant 2 differs only in the size of the coding sequence (6618 bp). The open reading frame translates to factor V consisting of 2211 (or 2206) amino acids including a 28-amino acid signal peptide. Comparison of the amino acid sequences with human factor Va reveals 84% identity for the heavy and 86% for the light chains. In contrast, the B domain (connecting region) exhibits only 59% identity relative to the human molecule. The bovine B domain contains two repeats of a 14-amino acid structure that is contained only once in the human sequence. Bovine factor V lacks one of the nine amino acid repeats and one of the 17 amino acid repeats present in the human B domain. Factor V has little homology to the factor VIII molecule in the B domain. The 17-amino acid repeat missing in bovine factor V allows identification of an 18-amino acid sequence that is homologous to the B domain of human factor VIII. These 18 amino acids may either constitute the unique vestige of a divergent evolution between the B domains of factors V and VIII or reveal the convergent evolution toward a critical epitope involved in the activation of both procofactors.

Expression and initial characterization of five site-directed mutants of the N-terminal half-molecule of human transferrin

Five site-directed mutants of the N-terminal half-molecule of human serum transferrin have been expressed in baby hamster kidney cells and purified to homogeneity. Expression levels and overall yields varied considerably from the wild-type protein, depending on the mutant in question. The mutants are D63S, D63C, G65R, K206Q, and H207E and are based on mutations observed in a variety of transferrins of known sequence. Their molecular masses, determined by electrospray mass spectrometry, agree with theory, except for the D63C mutant, which appears to be cysteinylated. All mutants bind iron but with varying affinities; qualitatively, in increasing order D63S approximately D63C approximately G65R much less than wild type less than or equal to H207E much less than K206Q. In general, reduction of formal negative charge within the binding cleft shifts the visible spectral maximum of the iron complex toward the blue and reduces the affinity for iron, and increasing the formal negative charge shifts the visible maximum toward the red and increases the affinity for iron. The K206Q mutant is exceptional inasmuch as its visible maximum shows a blue shift, but its affinity for iron is the greatest of all of the mutants studied. All mutants reported, in addition to the wild-type protein, exhibit very similar visible molar extinction coefficients for the iron complex and very similar changes in extinction coefficients at 240 nm on binding Fe(III) or Ga(III). These results suggest that in all cases the bound metal ion is coordinated by two tyrosyl side chains.