Biomedical subjects

E N Baker

Publications and source records attributed to E N Baker.

At least 37 records · Page 2Linked to original sources

PubMed2001

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.

Alanine↗

PubMed2001

Outer sphere mutations perturb metal reactivity in manganese superoxide dismutase.

Tyrosine 34 and glutamine 146 are highly conserved outer sphere residues in the mononuclear manganese active site of Escherichia coli manganese superoxide dismutase. Biochemical and spectroscopic characterization of site-directed mutants has allowed functional characterization of these residues in the wild-type (wt) enzyme. X-ray crystallographic analysis of three mutants (Y34F, Q146L, and Q146H) reveal subtle changes in the protein structures. The Y34A mutant, as well as the previously reported Y34F mutant, retained essentially the full superoxide dismutase activity of the wild-type enzyme, and the X-ray crystal structure of Y34F manganese superoxide dismutase shows that mutation of this strictly conserved residue has only minor effects on the positions of active site residues and the organized water in the substrate access funnel. Mutation of the outer sphere solvent pocket residue Q146 has more dramatic effects. The Q146E mutant is isolated as an apoprotein lacking dismutase activity. Q146L and Q146H mutants retain only 5-10% of the dismutase activity of the wild-type enzyme. The absorption and circular dichroism spectra of the Q146H mutant resemble corresponding data for the superoxide dismutase from a hyperthermophilic archaeon, Pyrobaculum aerophilum, which is active in both Mn and Fe forms. Interestingly, the iron-substituted Q146H protein also exhibits low dismutase activity, which increases at lower pH. Mutation of glutamine 146 disrupts the hydrogen-bonding network in the active site and has a greater effect on protein structure than does the Y34F mutant, with rearrangement of the tyrosine 34 and tryptophan 128 side chains.

Binding Sites↗

PubMed2001

Oligomerization and ligand binding in a homotetrameric hemoglobin: two high-resolution crystal structures of hemoglobin Bart's (gamma(4)), a marker for alpha-thalassemia.

Hemoglobin (Hb) Bart's is present in the red blood cells of millions of people worldwide who suffer from alpha-thalassemia. alpha-Thalassemia is a disease in which there is a deletion of one or more of the four alpha-chain genes, and excess gamma and beta chains spontaneously form homotetramers. The gamma(4) homotetrameric protein known as Hb Bart's is a stable species that exhibits neither a Bohr effect nor heme-heme cooperativity. Although Hb Bart's has a higher O(2) affinity than either adult (alpha(2)beta(2)) or fetal (alpha(2)gamma(2)) Hbs, it has a lower affinity for O(2) than HbH (beta(4)). To better understand the association and ligand binding properties of the gamma(4) tetramer, we have solved the structure of Hb Bart's in two different oxidation and ligation states. The crystal structure of ferrous carbonmonoxy (CO) Hb Bart's was determined by molecular replacement and refined at 1.7 A resolution (R = 21.1%, R(free) = 24.4%), and that of ferric azide (N(3)(-)) Hb Bart's was similarly determined at 1.86 A resolution (R = 18.4%, R(free) = 22.0%). In the carbonmonoxy-Hb structure, the CO ligand is bound at an angle of 140 degrees, and with an unusually long Fe-C bond of 2.25 A. This geometry is attributed to repulsion from the distal His63 at the low pH of crystallization (4.5). In contrast, azide is bound to the oxidized heme iron in the methemoglobin crystals at an angle of 112 degrees, in a perfect orientation to accept a hydrogen bond from His63. Compared to the three known quaternary structures of human Hb (T, R, and R2), both structures most closely resemble the R state. Comparisons with the structures of adult Hb and HbH explain the association and dissociation behaviour of Hb homotetramers relative to the heterotetrameric Hbs.

Azides↗

PubMed2001

Isolation and characterization of a gene encoding a drought-induced cysteine protease in tomato (Lycopersicon esculentum).

In a previous study, a 65 kDa protein, TDI-65, was found to be accumulated in the leaves of drought-stressed tomato (Lycopersicon esculentum cv. Starfire) plants. The protein level returns to control level when the drought-stressed plants are rewatered. Antibodies raised against the purified protein were used to elucidate the subcellular localization of the protein. The protein was found to be mainly localized in the nuclei and chloroplasts of drought-stressed leaf cells. To identify the nature of the protein, a cDNA library was constructed and screened by the purified anti-TDI-65 antibody. A cDNA clone designated tdi-65 was isolated and characterized. The deduced amino acid sequences of tdi-65 protein has extensive homology with known cysteine proteases such as actinidin and papain. Northern blot analysis revealed that tdi-65 mRNA is 10-fold higher in drought-stressed plants as compared to control and rewatered plants. Similar results were observed in the tomato cultivar Ailsa and its near isogenic abscisic acid (ABA)-deficient mutant line, flacca, suggesting that the gene does not require ABA for its expression under drought conditions. Based on the previous immunolocalization findings we suggest that tdi-65 encoded cysteine protease functions in relation to drought-induced senescence and programmed cell death.

Abscisic Acid↗

PubMed2000

Crystal structure of a truncated mutant of glucose-fructose oxidoreductase shows that an N-terminal arm controls tetramer formation.

N-terminal or C-terminal arms that extend from folded protein domains can play a critical role in quaternary structure and other intermolecular associations and/or in controlling biological activity. We have tested the role of an extended N-terminal arm in the structure and function of a periplasmic enzyme glucose-fructose oxidoreductase (GFOR) from Zymomonas mobilis. We have determined the crystal structure of the NAD(+) complex of a truncated form of the enzyme, GFORDelta, in which the first 22 residues of the N-terminal arm of the mature protein have been deleted. The structure, refined at 2.7 A resolution (R(cryst)=24.1%, R(free)=28.4%), shows that the truncated form of the enzyme forms a dimer and implies that the N-terminal arm is essential for tetramer formation by wild-type GFOR. Truncation of the N-terminal arm also greatly increases the solvent exposure of the cofactor; since GFOR activity is dependent on retention of the cofactor during the catalytic cycle we conclude that the absence of GFOR activity in this mutant results from dissociation of the cofactor. The N-terminal arm thus determines the quaternary structure and the retention of the cofactor for GFOR activity and during translocation into the periplasm. The structure of GFORDelta also shows how an additional mutation, Ser64Asp, converts the strict NADP(+) specificity of wild-type GFOR to a dual NADP(+)/NAD(+) specificity.

Amino Acid Sequence↗

PubMed2000

Structural and functional similarities in the ADP-forming amide bond ligase superfamily: implications for a substrate-induced conformational change in folylpolyglutamate synthetase.

Comparison of the three-dimensional structures of folylpolyglutamate synthetase (FPGS) and the bacterial cell wall ligase UDP-N-acetylmuramoyl-l-alanine:d-glutamate ligase (MurD) reveals that these two enzymes have a remarkable structural similarity despite a low level of sequence identity. Both enzymes have a modular, multi-domain structure and catalyse a similar ATP-dependent reaction involving the addition of a glutamate residue to a carboxylate-containing substrate, tetrahydrofolate in the case of FPGS, and UDP-N-acetylmuramoyl-l-alanine in the case of MurD. Site-directed mutations of selected residues in the active site of Lactobacillus casei FPGS (P74A, E143A, E143D, E143Q, K185A, D313A, H316A, G411A and S412A) showed that most of these changes resulted in an almost complete loss of activity. Several of these amino acid residues in FPGS are found in structurally equivalent positions to active-site residues in MurD. Some insights into the function of these residues in FPGS activity are proposed, based on the roles surmised from the structures of two MurD. UDP-N-acetylmuramoyl-l-alanine.ADP complexes and a MurD. UDP-N-acetylmuramoyl-l-alanine-d-glutamate complex. Furthermore, the comparison has led us to propose that conformational changes induced by substrate binding in the reaction mechanism of FPGS result in a movement of the domains towards each other to more closely resemble the orientation of the corresponding domains in MurD. This relative domain movement may be a key feature of this new family of ADP-forming amide bond ligases.

Adenosine Diphosphate↗

PubMed2000

Purification and characterization of Ak.1 protease, a thermostable subtilisin with a disulphide bond in the substrate-binding cleft.

Ak.1 protease, a thermostable subtilisin isolated originally from Bacillus st. Ak.1, was purified to homogeneity from the Escherichia coli clone PB5517. It is active against substrates containing neutral or hydrophobic branched-chain amino acids at the P(1) site, such as valine, alanine or phenylalanine. The K(m) and k(cat) of the enzyme decrease with decreasing temperature, though not to the same degree with all substrates, suggesting that specificity changes with temperature. The protease is markedly stabilized by Ca(2+) ions. At 70 degrees C, a 10-fold increase in Ca(2+) concentration increases the half-life by three orders of magnitude. Ak.1 protease is stabilized by Ca(2+) to a greater extent than is thermitase. This may be due, in part, to the presence of an extra Ca(2+)-binding site in Ak.1 protease. Other metal ions, such as Sr(2+), increase the thermostability of the enzyme, but to a significantly lower degree than does Ca(2+). The structure of the protease showed the presence of a disulphide bond located within the active-site cleft. This bond influences both enzyme activity and thermostability. The disulphide bond appears to have a dual role: maintaining the integrity of the substrate-binding cleft and increasing the thermostability of the protease. The protease was originally investigated to determine its usefulness in the clean-up of DNA at high temperatures. However, it was found that this protease has a limited substrate specificity, so this application was not explored further.

Disulfides↗

PubMed2000

Crystal structure and iron-binding properties of the R210K mutant of the N-lobe of human lactoferrin: implications for iron release from transferrins.

Lactoferrin (Lf) and serum transferrin (Tf) combine high-affinity iron binding with an ability to release this iron at reduced pH. Lf, however, retains iron to significantly lower pH than Tf, giving the two proteins distinct functional roles. In this paper, we compared the iron-release profiles for human Lf, Tf, and their N-lobe half-molecules Lf(N) and Tf(N) and showed that half of the difference in iron retention at low pH ( approximately 1.3 pH units) results from interlobe interactions in Lf. To probe factors intrinsic to the N-lobes, we further examined the specific role of two basic residues that are proposed to form a pH-sensitive dilysine trigger for iron release in the N-lobe of Tf [Dewan, J. C., Mikami, B., Hirose, M., and Sacchettini, J. C. (1993) Biochemistry 32, 11963-11968] by mutating Arg 210 to Lys in the N-lobe half-molecule Lf(N). The R210K mutant was expressed, purified, and crystallized, and its crystal structure was determined and refined at 2.0-A resolution to a final R factor (R(free)) of 19.8% (25.0%). The structure showed that Lys 210 and Lys 301 in R210K do not form a dilysine interaction like that between Lys 206 and Lys 296 in human Tf. The R210K mutant retained iron to lower pH than Tf(N), consistent with the absence of the dilysine interaction but released iron at approximately 0.7 pH units higher than Lf(N). We conclude that (i) the ability of Lf to retain iron to significantly lower pH than Tf is due equally to interlobe interactions and to the absence in Lfs of an interaction analogous to the dilysine pair in Tfs, even when two lysines are present at the corresponding sequence positions, and (ii) an appropriately positioned basic residue (Arg 210 in human Lf) modulates iron release by inhibiting protonation of the N-lobe iron ligands, specifically His 253.

Binding Sites↗

PubMed2000

Conservation and variation in superantigen structure and activity highlighted by the three-dimensional structures of two new superantigens from Streptococcus pyogenes.

Bacterial superantigens (SAgs) are a structurally related group of protein toxins secreted by Staphylococcus aureus and Streptococcus pyogenes. They are implicated in a range of human pathologies associated with bacterial infection whose symptoms result from SAg-mediated stimulation of a large number (2-20%) of T-cells. At the molecular level, bacterial SAgs bind to major histocompatability class II (MHC-II) molecules and disrupt the normal interaction between MHC-II and T-cell receptors (TCRs). We have determined high-resolution crystal structures of two newly identified streptococcal superantigens, SPE-H and SMEZ-2. Both structures conform to the generic bacterial superantigen folding pattern, comprising an OB-fold N-terminal domain and a beta-grasp C-terminal domain. SPE-H and SMEZ-2 also display very similar zinc-binding sites on the outer concave surfaces of their C-terminal domains. Structural comparisons with other SAgs identify two structural sub-families. Sub-families are related by conserved core residues and demarcated by variable binding surfaces for MHC-II and TCR. SMEZ-2 is most closely related to the streptococcal SAg SPE-C, and together they constitute one structural sub-family. In contrast, SPE-H appears to be a hybrid whose N-terminal domain is most closely related to the SEB sub-family and whose C-terminal domain is most closely related to the SPE-C/SMEZ-2 sub-family. MHC-II binding for both SPE-H and SMEZ-2 is mediated by the zinc ion at their C-terminal face, whereas the generic N-terminal domain MHC-II binding site found on many SAgs appears not to be present. Structural comparisons provide evidence for variations in TCR binding between SPE-H, SMEZ-2 and other members of the SAg family; the extreme potency of SMEZ-2 (active at 10(-15) g ml-1 levels) is likely to be related to its TCR binding properties. The smez gene shows allelic variation that maps onto a considerable proportion of the protein surface. This allelic variation, coupled with the varied binding modes of SAgs to MHC-II and TCR, highlights the pressure on SAgs to avoid host immune defences.

Alleles↗

PubMed2000

Crystal structure of the zymogen form of the group A Streptococcus virulence factor SpeB: an integrin-binding cysteine protease.

Pathogenic bacteria secrete protein toxins that weaken or disable their host, and thereby act as virulence factors. We have determined the crystal structure of streptococcal pyrogenic exotoxin B (SpeB), a cysteine protease that is a major virulence factor of the human pathogen Streptococcus pyogenes and participates in invasive disease episodes, including necrotizing fasciitis. The structure, determined for the 40-kDa precursor form of SpeB at 1.6-A resolution, reveals that the protein is a distant homologue of the papain superfamily that includes the mammalian cathepsins B, K, L, and S. Despite negligible sequence identity, the protease portion has the canonical papain fold, albeit with major loop insertions and deletions. The catalytic site differs from most other cysteine proteases in that it lacks the Asn residue of the Cys-His-Asn triad. The prosegment has a unique fold and inactivation mechanism that involves displacement of the catalytically essential His residue by a loop inserted into the active site. The structure also reveals the surface location of an integrin-binding Arg-Gly-Asp (RGD) motif that is a feature unique to SpeB among cysteine proteases and is linked to the pathogenesis of the most invasive strains of S. pyogenes.

Bacterial Proteins↗

PubMed2000

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.

Binding Sites↗

PubMed2000

Metal substitution in transferrins: specific binding of cerium(IV) revealed by the crystal structure of cerium-substituted human lactoferrin.

Proteins of the transferrin family play a key role in iron homeostasis through their extremely strong binding of iron, as Fe3+. They are nevertheless able to bind a surprisingly wide variety of other metal ions. To investigate how metal ions of different size, charge and coordination characteristics are accommodated, we have determined the crystal structure of human lactoferrin (Lf) complexed with Ce4+. The structure, refined at 2.2 A resolution (R=20.2%, Rfree=25.7%) shows that the two Ce4+ ions occupy essentially the same positions as do Fe3+, and that the overall protein structure is unchanged; the same closed structure is formed for Ce2Lf as for Fe2Lf. The larger metal ion is accommodated by small shifts in the protein ligands, made possible by the presence of water molecules adjacent to each binding site. The two Ce4+ sites are equally occupied, indicating that the known difference in the pH-dependent release of Ce4+ arises from a specific protonation event, possibly of the His ligand in one of the binding sites. Comparing the effects of binding Ce4+ with those for the binding of other metal ions, we conclude that the ability of transferrins to accommodate metal ions other than Fe3+ depends on an interplay of charge, size, coordination and geometrical preferences of the bound metal ion. However, it is the ability to accept the six-coordinate, approximately octahedral, site provided by the protein that is of greatest importance.

Cerium↗

PubMed2000

Crystal structure of the protein disulfide bond isomerase, DsbC, from Escherichia coli.

DsbC is one of five Escherichia coli proteins required for disulfide bond formation and is thought to function as a disulfide bond isomerase during oxidative protein folding in the periplasm. DsbC is a 2 x 23 kDa homodimer and has both protein disulfide isomerase and chaperone activity. We report the 1.9 A resolution crystal structure of oxidized DsbC where both Cys-X-X-Cys active sites form disulfide bonds. The molecule consists of separate thioredoxin-like domains joined via hinged linker helices to an N-terminal dimerization domain. The hinges allow relative movement of the active sites, and a broad uncharged cleft between them may be involved in peptide binding and DsbC foldase activities.

Amino Acid Sequence↗

PubMed2000

Crystallization and preliminary X-ray analysis of native and recombinant human bile-salt dependent lipase: strategies for improvement of diffraction quality.

Human bile-salt dependent lipase (BSDL), secreted into both the digestive tract and human milk, is integral to the effective absorption of dietary lipids. In attempts to obtain crystals suitable for high-resolution X-ray crystallographic studies, various forms of the enzyme have been crystallized, including native and desialidated human milk BSDL and both intact recombinant BSDL and a truncated form lacking the heavily glycosylated C-terminal repeat region. Trigonal crystals of native BSDL, with unit-cell parameters a = b = 90.0, c = 156.1 A, were obtained using 15-20%(w/v) PEG 8000 as precipitant. These crystals diffract to 3.5 A along the unique axis, but to only 5-7 A in orthogonal directions. Crystals of recombinant truncated BSDL grown from 15-20%(w/v) PEG 6000 are orthorhombic, space group P2(1)2(1)2(1), with unit-cell parameters a = 59.2, b = 90.0, c = 107.7 A, and diffract to 2.6 A resolution. These are suitable for structural analysis by X-ray crystallography.

Crystallization↗

PubMed2000

Structure of XynB, a highly thermostable beta-1,4-xylanase from Dictyoglomus thermophilum Rt46B.1, at 1.8 A resolution.

Microorganisms employ a large array of enzymes to break down the cellulose and hemicelluloses of plant biomass. These enzymes, especially those with high thermal stability, have many uses in biotechnology. We have solved the crystal structure of a beta-1, 4-xylanase, XynB, from the extremely thermophilic bacterium Dictyoglomus thermophilum, isolate Rt46B.1. The protein crystallized from 1.6 M ammonium sulfate, 0.2 M HEPES pH 7.2 and 10% glycerol, with unit-cell parameters a = b = 91.3, c = 44.9 A and space group P4(3). The structure was solved at high resolution (1.8 A) by X-ray crystallography, using the method of isomorphous replacement with a single mercury derivative, and refined to a final R factor of 18.3% (R(free) = 22.1%). XynB has the single-domain fold typical of family 11 xylanases, comprising a jelly roll of two highly twisted beta-sheets that create a deep substrate-binding cleft. The two catalytic residues, Glu90 and Glu180, occupy this cleft. Compared with other family 11 xylanases, XynB has a greater proportion of polar surface and has a slightly extended C-terminus that, combined with the extension of beta-strand A5, gives additional hydrogen bonding and hydrophobic packing. These factors may account for the enhanced thermal stability of the enzyme.

Amino Acid Sequence↗

PubMed1999

Calcium-mediated thermostability in the subtilisin superfamily: the crystal structure of Bacillus Ak.1 protease at 1.8 A resolution.

Proteins of the subtilisin superfamily (subtilases) are widely distributed through many living species, where they perform a variety of processing functions. They are also used extensively in industry. In many of these enzymes, bound calcium ions play a key role in protecting against autolysis and thermal denaturation. We have determined the crystal structure of a highly thermostable protease from Bacillus sp. Ak.1 that is strongly stabilized by calcium. The crystal structure, determined at 1.8 A resolution (R=0. 182, Rfree=0.247), reveals the presence of four bound cations, three Ca(2+) and one Na(+). Two of the Ca(2+) binding sites, Ca-1 and Ca-2, correspond to sites also found in thermitase and the mesophilic subtilisins. The third calcium ion, however, is at a novel site that is created by two key amino acid substitutions near Ca-1, and has not been observed in any other subtilase. This site, acting cooperatively with Ca-1, appears to give substantially enhanced thermostability, compared with thermitase. Comparisons with the mesophilic subtilisins also point to the importance of aromatic clusters, reduced hydrophobic surface and constrained N and C termini in enhancing the thermostability of thermitase and Ak.1 protease. The Ak.1 protease also contains an unusual Cys-X-Cys disulfide bridge that modifies the active site cleft geometry.

Amino Acid Sequence↗

PubMed1999

Crystal structures of Bacillus caldovelox arginase in complex with substrate and inhibitors reveal new insights into activation, inhibition and catalysis in the arginase superfamily.

BACKGROUND: Arginase is a manganese-dependent enzyme that catalyzes the hydrolysis of L-arginine to L-ornithine and urea. In ureotelic animals arginase is the final enzyme of the urea cycle, but in many species it has a wider role controlling the use of arginine for other metabolic purposes, including the production of creatine, polyamines, proline and nitric oxide. Arginase activity is regulated by various small molecules, including the product L-ornithine. The aim of these structural studies was to test aspects of the catalytic mechanism and to investigate the structural basis of arginase inhibition. RESULTS: We report here the crystal structures of arginase from Bacillus caldovelox at pH 5.6 and pH 8.5, and of binary complexes of the enzyme with L-arginine, L-ornithine and L-lysine at pH 8.5. The arginase monomer comprises a single compact alpha/beta domain that further associates into a hexameric quaternary structure. The binary complexes reveal a common mode of ligand binding, which places the substrate adjacent to the dimanganese centre. We also observe a conformational change that impacts on the active site and is coupled with the occupancy of an external site by guanidine or arginine. CONCLUSIONS: The structures reported here clarify aspects of the active site and indicate key features of the catalytic mechanism, including substrate coordination to one of the manganese ions and an orientational role for a neighboring histidine residue. Stereospecificity for L-amino acids is found to depend on their precise recognition at the active-site rim. Identification of a second arginine-binding site, remote from the active site, and associated conformational changes lead us to propose a regulatory role for this site in substrate hydrolysis.

Arginase↗

PubMed1999

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.

Animals↗