Energetics of a hydrogen bond (charged and neutral) and of a cation-pi interaction in apoflavodoxin.

Anabaena apoflavodoxin contains a single histidine residue (H34) that interacts with two aromatic residues (F7 and Y47). The histidine and phenylalanine rings are almost coplanar and they can establish a cation-pi interaction when the histidine is protonated. The histidine and tyrosine side-chains are engaged in a hydrogen bond, which is their only contact. We analyse the energetics of these interactions using p Ka-shift analysis, double-mutant cycle analysis at two pH values, and X-ray crystallography. The H/F interaction is very weak when the histidine is neutral, but it is strengthened by 0.5 kcal mol-1on histidine protonation. Supporting this fact, the histidine p Kain a F7L mutant is 0.4 pH units lower than in wild-type. The strength of the H/Y hydrogen bond is 0.7 kcal mol-1when the histidine is charged, and it becomes stronger (1.3 kcal mol-1) when the histidine is neutral. This is consistent with our observation that the (H34)Nepsilon2-OH(Y47) distance is slightly shorter in the apoflavodoxin structure at pH 9.0 than in the previously reported structure at pH 6.0. It is also consistent with a histidine p Kavalue 0.6 pH units higher in a Y47F mutant than in the wild-type protein. We suggest that the higher stability of the neutral hydrogen bond could be due to a higher desolvation penalty of the charged hydrogen bond that would offset its more favourable enthalpy of formation. The relationship between hydrogen bond strength and the contribution of hydrogen bonds to protein stability is discussed.     

The testosterone binding mechanism of an antibody derived from a naïve human scFv library

A testosterone binding scFv antibody was isolated from a naïve human library with a modest size of 10⁸ clones. The crystal structure of the Fab fragment form of the 5F2 antibody clone complexed with testosterone determined at 1.5 Å resolution shows that the hapten is bound deeply in the antibody binding pocket. In addition to the interactions with framework residues only CDR‐L3 and CDR‐H3 loops interact with testosterone and the heavy chain forms the majority of the contacts with the hapten. The testosterone binding site of the 5F2 antibody with a high abundance of aromatic amino acid residues shows similarity with an in vitro affinity matured antibody having around 300 times higher affinity. The moderate affinity of the 5F2 antibody originates from the different orientation of the hapten and few light chain contacts. This is the first three‐dimensional structure of a human steroid hormone binding antibody that has been isolated from a naïve human repertoire. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

A focused antibody library for improved hapten recognition

The topography of the antigen-binding site as well as the number and the positioning of the antigen contact residues are strongly correlated with the size of the antigen with which the antibody interacts. On the basis of these considerations, we have designed a focused scFv repertoire biased for haptens, designated the cavity library. The hapten-specific scFv, FITC8, was used as a scaffold for library construction. FITC8, like other hapten binders, displays a characteristic cavity in its paratope into which the hapten binds. In five of the six complementarity-determining regions, diversity-carrying residues were selected rationally on the basis of a model structure of FITC8 and on known antibody structure-function relationships, resulting in variation of 11 centrally located, cavity-lining residues. L3 was allowed to carry a more complex type of diversity. In addition, length variation was introduced into H2, as longer versions of this loop have been shown to correlate with increased hapten binding. The library was screened, using phage display, against a panel of five different haptens, yielding diverse and highly specific binders to four of the antigens. Parallel selections were performed with a library having diversity spread onto a greater area, including more peripherally located residues. This resulted in the isolation of binders, which, in contrast to the clones selected from the cavity library, were not able to bind to the soluble hapten in the absence of the carrier protein. Thus, we have shown that by focusing diversity to the hotspots of interaction a library with improved hapten-binding ability can be created. The study supports the notion that it is possible to create antibody libraries that are biased for the recognition of antigens of pre-defined size.     

Identification of iron ligands in tyrosine hydroxylase by mutagenesis of conserved histidinyl residues.

Tyrosine hydroxylase catalyzes the hydroxylation of tyrosine and other aromatic amino acids using a tetrahydropterin as the reducing substrate. The enzyme is a homotetramer; each monomer contains a single nonheme iron atom. Five histidine residues are conserved in all tyrosine hydroxylases that have been sequenced to date and in the related eukaryotic enzymes phenylalanine and tryptophan hydroxylase. Because histidine has been suggested as a ligand to the iron in these enzymes, mutant tyrosine hydroxylase proteins in which each of the conserved histidines had been mutated to glutamine or alanine were expressed in Escherichia coli. The H192Q, H247Q, and H317A mutant proteins contained iron in comparable amounts to the wild-type enzyme, about 0.6 atoms/sub-unit. In contrast, the H331 and H336 mutant proteins contained no iron. The first three mutant enzymes were active, with Vmax values 39, 68, and 7% that of the wild-type enzyme, and slightly altered V/Km values for both tyrosine and 6-methyltetrahydropterin. In contrast, the H331 and H336 mutant enzymes had no detectable activity. The EPR spectra of the H192Q and H247Q enzymes are indistinguishable from that of wild-type tyrosine hydroxylase, whereas that of the H317A enzyme indicated that the ligand field of the iron had been slightly perturbed. These results are consistent with H331 and H336 being ligands to the active site iron atom.     

NMR study of the complexes between a synthetic peptide derived from the B subunit of cholera toxin and three monoclonal antibodies against it

The contact interactions between a synthetic peptide and three different anti-peptide monoclonal antibodies have been studied by nuclear magnetic resonance (NMR). The synthetic peptide is CTP3 (residues 50-64 of the B subunit of cholera toxin) suggested as a possible epitope for synthetic vaccine against cholera. The hybridoma cell lines TE33 and TE32 derived after immunization with CTP3 produce antibodies cross-reactive with the native toxin. The cell line TE34 produces anti-CTP3 antibodies that do not bind the toxin. Selective deuteriation of the antibodies has been used to simplify the proton NMR spectra and to assign resonances to specific types of amino acids. The difference spectra between the proton NMR spectrum of the peptide-Fab complex and that of Fab indicate that the combining site structures of TE32 and TE33 are very similar but differ considerably from the combining site structure of TE34. By magnetization transfer experiments with selectively deuteriated Fab fragment of the antibody, we have found that in TE32 and TE33 the histidine residue of the peptide is buried in a hydrophobic pocket of the antibody combining site, formed by a tryptophan and two tyrosine residues. The hydrophobic nature of the pocket is further demonstrated by the lack of any pH titration effect on the chemical shift of the C4H of the bound peptide histidine. In contrast, for TE34 we have found only one tyrosine residue in contact with the histidine of the peptide.(ABSTRACT TRUNCATED AT 250 WORDS)     

High-throughput generation of synthetic antibodies from highly functional minimalist phage-displayed libraries

We have previously established a minimalist approach to antibody engineering by using a phage-displayed framework to support complementarity determining region (CDR) diversity restricted to a binary code of tyrosine and serine. Here, we systematically augmented the original binary library with additional levels of diversity and examined the effects. The diversity of the simplest library, in which only heavy chain CDR positions were randomized by the binary code, was expanded in a stepwise manner by adding diversity to the light chain, by diversifying non-paratope residues that may influence CDR conformations, and by adding additional chemical diversity to CDR-H3. The additional diversity incrementally improved the affinities of antibodies raised against human vascular endoethelial growth factor and the structure of an antibody-antigen complex showed that tyrosine side-chains are sufficient to mediate most of the interactions with antigen, but a glycine residue in CDR-H3 was critical for providing a conformation suitable for high-affinity binding. Using new high-throughput procedures and the most complex library, we produced multiple high-affinity antibodies with dissociation constants in the single-digit nanomolar range against a wide variety of protein antigens. Thus, this fully synthetic, minimalist library has essentially recapitulated the capacity of the natural immune system to generate high-affinity antibodies. Libraries of this type should be highly useful for proteomic applications, as they minimize inherent complexities of natural antibodies that have hindered the establishment of high-throughput procedures. Furthermore, analysis of a large number of antibodies derived from these well-defined and simplistic libraries allowed us to uncover statistically significant trends in CDR sequences, which provide valuable insights into antibody library design and into factors governing protein-protein interactions.     

Probing the influence on folding behavior of structurally conserved core residues in P. aeruginosa apo-azurin

The effects on folding kinetics and equilibrium stability of core mutations in the apo-mutant C112S of azurin from Pseudomonas aeruginosa were studied. A number of conserved residues within the cupredoxin family were recognized by sequential alignment as constituting a common hydrophobic core: I7, F15, L33, W48, F110, L50, V95, and V31. Of these, I7, V31, L33, and L50 were mutated for the purpose of obtaining information on the transition state and a potential folding nucleus. In addition, residue V5 in the immediate vicinity of the common core, as well as T52, separate from the core, were mutated as controls. All mutants exhibited a nonlinear dependence of activation free energy of folding on denaturant concentration, although the refolding kinetics of the V31A/C112S mutant indicated that the V31A mutation destabilizes the transition state enough to allow folding via a parallel transition state ensemble. Phi-values could be calculated for three of the six mutants, V31A/C112S, L33A/C112S, and L50A/C112S, and the fractional values of 0.63, 0.33, and 0.50 (respectively) obtained at 0.5 M GdmCl suggest that these residues are important for stabilizing the transition state. Furthermore, a linear dependence of ln k(obs)(H2O) on DeltaG(U-N)(H2O) of the core mutations and the putative involvement of ground-state effects suggest the presence of native-like residual interactions in the denatured state that bias this ensemble toward a folding-competent state.     

Epitope mapping of a monoclonal antibody directed against the α‐subunit of phosphofructokinase‐1 from Saccharomyces cerevisiae by screening phage display libraries

Phosphofructokinase‐1 from Saccharomyces cerevisiae is composed of two types of subunits, α and β. Subunit‐specific monoclonal antibodies were raised to elucidate structural and functional properties of both subunits. One monoclonal antibody, α‐F3, binds to an epitope either at the C‐terminal or at the N‐terminal part of the α‐polypeptide chain. By screening a heptapeptide library with this monoclonal antibody, a set of heptapeptides was selected, which contained the consensus sequences D–A–F and D–S–F. Two heptapeptides with these motifs were synthesized in order assess their capacity to inhibit the binding of antibody α‐F3 to native phosphofructokinase‐1. The peptide G–I–K–D–A–F–L inhibited the binding more strongly (IC₅₀ = 1.5 µM) than the peptide A–P–W–H–D–S–F (IC₅₀ = 33.3 µM). Sequence matching revealed the presence of the D–A–F motif in the polypeptide chain of phosphofructokinase‐1 at amino acid position 172–174. As a control, the nonapeptide A–P–T–S–K–D–A–F–L which corresponds to the sequence of the putative epitope was tested in the inhibition assay. In view of the high inhibitory capacity (IC₅₀ = 0.3 µM) it was concluded that this nonapeptide represents the continuous epitope of phosphofructokinase‐1 that is recognized by antibody α‐F3. Copyright © 1999 John Wiley & Sons, Ltd.     

A triple mutation in the second transmembrane domain of mouse dopamine transporter markedly decreases sensitivity to cocaine and methylphenidate

Previously, we reported that Phe105 in transmembrane domain 2 of the mouse dopamine transporter (DAT) is crucial for high-affinity cocaine binding. In the current study, we investigated whether other residues surrounding Phe105 also affect the potency of cocaine inhibition. After three rounds of sequential random mutagenesis at these residues, we found a triple mutant (L104V, F105C and A109V) of mouse DAT that retained over 50% uptake activity and was 69-fold less sensitive to cocaine inhibition when compared with the wild-type mouse DAT. The triple mutation also resulted in a 47-fold decrease in sensitivity to methylphenidate inhibition, suggesting that the binding sites for cocaine and methylphenidate may overlap. In contrast, the inhibition of dopamine uptake by amphetamine or methamphetamine was not significantly changed by the mutations, suggesting that the binding sites for the amphetamines differ from those for cocaine and methylphenidate. Such functional but cocaine-insensitive DAT mutants can be used to generate a knock-in mouse line to study the role of DAT in cocaine addiction.     

A homologous expression system for obtaining engineered cytochrome ba3 from Thermus thermophilus HB8

Cytochrome ba3 is an integral membrane protein that serves as a terminal oxidase of the respiratory chain in some prokaryotes. We have cloned the complete cba operon of Thermus thermophilus HB8 in an Escherichia coli/T. thermophilus shuttle vector. The ba3-encoding operon, cba, was eliminated from the chromosome of T. thermophilus strain MT111 using the pyrE system of Yamagishi and co-workers. Expression of functional cytochrome ba3 occurred in cells grown at reduced dioxygen levels. A hepta-histidine tag was placed at the N-terminus of subunit I, and a purification method for this form of the enzyme was developed. Growth conditions were investigated for moderate sized cultures (2L) with typical yields of approximately 2 mg of highly pure enzyme per liter of culture medium. The physical properties and enzymatic activities of these recombinant enzymes were compared with those of native enzyme. Recombinant enzyme lacking the histidine tag is spectrally identical to wild-type enzyme. Histidine-tagged cytochrome ba3 shows minor differences from wild-type, and it appears be somewhat less active as a cytochrome c552 oxidase. Exemplary mutants were also produced and compared to native protein. Tyrosine I-237, previously found to be covalently bonded to I-His-233, was changed to phenylalanine (I-Y237F) and to histidine (I-Y237H) in the hepta-histidine tagged cytochrome ba3. The Y to F mutant is devoid of enzyme activity whereas the Y to H mutant possesses approximately 5% wild-type oxidase activity; their properties are compared with those of wild-type enzyme. The above versions of the histidine-tagged enzyme have been crystallized, and our analysis of a 2.3 angstrom resolution electron-density map will be discussed elsewhere.     

Design of a pH-dependent cellulose-binding domain

Protein-carbohydrate interactions typically rely on aromatic stacking interactions of tyrosine, phenylalanine and tryptophan side chains with the sugar rings whereas histidine residues are rarely involved. The small cellulose-binding domain of the Cel7A cellobiohydrolase (formerly CBHI) from Trichoderma reesei binds to crystalline cellulose primarily using a planar strip of three tyrosine side chains. Binding of the wild-type Cel7A CBD is practically insensitive to pH. Here we have investigated how histidine residues mediate the binding interaction and whether the protonation of a histidine side chain makes the binding sensitive to pH. Protein engineering of the Cel7A CBD was thus used to replace the tyrosine residues in two different positions with histidine residues. All of the mutants exhibited a clear pH-dependency of the binding, in clear contrast to the wild-type. Although the binding of the mutants at optimal pH was less than for the wild-type, in one case, Y31H, this binding almost reached the wild-type level.     

Changes in the apomyoglobin folding pathway caused by mutation of the distal histidine residue

Factors governing the folding pathways and the stability of apomyoglobin have been examined by replacing the distal histidine at position 64 with phenylalanine (H64F). Acid and urea-induced unfolding experiments using CD and fluorescence techniques reveal that the mutant H64F apoprotein is significantly more stable than wild-type apoMb. Kinetic refolding studies of this variant also show a significant difference from wild-type apoMb. The amplitude of the burst phase ellipticity in stopped-flow CD measurements is increased over that of wild-type, an indication that the secondary structure content of the earliest kinetic intermediate is greater in the mutant than in the wild-type protein. In addition, the overall rate of folding is markedly increased. Hydrogen exchange pulse labeling was used to establish the structure of the initial intermediate formed during the burst phase of the H64F mutant. NMR analysis of the samples obtained at different refolding times indicates that the burst phase intermediate contains a stabilized E helix as well as the A, G, and H helices previously found in the wild-type kinetic intermediate. Replacement of the polar distal histidine residue with a nonpolar residue of similar size and shape appears to stabilize the E helix in the early stages of folding due to improved hydrophobic packing. The presence of a hydrophilic histidine at position 64 thus exacts a price in the stability and folding efficiency of the apoprotein, but this residue is nevertheless highly conserved among myoglobins due to its importance in function.     

Differential effects of mutations in the chromophore pocket of recombinant phytochrome on chromoprotein assembly and Pr-to-Pfr photoconversion.

Site-directed mutagenesis was performed with the chromophore-bearing N-terminal domain of oat phytochrome A apoprotein (amino acid residues 1-595). Except for Trp366, which was replaced by Phe (W366F), all the residues exchanged are in close proximity to the chromophore-binding Cys321 (i.e. P318A, P318K, H319L, S320K, H322L and the double mutant L323R/Q324D). The mutants were characterized by their absorption maxima, and the kinetics of chromophore-binding and the Pr-->Pfr conversion. The strongest effect of mutation on the chromoprotein assembly, leading to an almost complete loss of the chromophore binding capability, was found for the exchanges of His322 by Leu (H322L) and Pro318 by Lys (P318K), whereas a corresponding alanine mutant (P318A) showed wild-type behavior. The second histidine (H319) is also involved in chromophore fixation, as indicated by a slower assembly rate upon mutation (H319L). For the other mutants, an assembly process very similar to that of the wild-type protein was found. The light-induced Pr-->Pfr conversion kinetics is altered in the mutations H319L and S320K and in the double mutant L323R/Q324D, all of which exhibited a significantly faster I700 decay and accelerated Pfr formation. P318 is also involved in the Pr-->Pfr conversion, the millisecond steps (formation of Pfr) being significantly slower for P318A. Lacking sufficient amounts of W366F, assembly kinetics could not be determined in this case, while the fully assembled mutant underwent the Pr-->Pfr conversion with kinetics similar to wild-type protein.     

Generation of affinity matured scFv antibodies against mouse neural cell adhesion molecule L1 by phage display

The recognition molecule L1 plays important functional roles in the nervous system and in non-neural tissues. Since antibodies to L1 are of prime importance to study its functional properties, we have generated affinity matured human single chain variable fragment (scFv) antibodies against mouse L1 by introducing random mutations in the complementarity determining regions (CDRs) of a previously isolated scFv antibody heavy chain (CDR1 and CDR2) and light chain (CDR3). After biopanning the mutant library, a clone (5F7) that gave the strongest ELISA signal was expressed, purified, and characterized. The dissociation constant of 5F7 (2.86 x 10(-8)M) was decreased 60-fold compared to the wild type clone G6 (1.72 x 10(-6)M). 5F7 detected L1 by Western blot analysis in mouse brain homogenates and recognized L1 in L1 transfected cells and cryosections from mouse retina and optic nerve by immunofluorescence. Bivalent 5F7 scFv antibody (5F7-Cys) was also generated and showed a dissociation constant of 5.22 x 10(-9)M that is 5.5-fold lower than that of monomeric 5F7 antibody. The bivalent affinity matured L1 scFv antibody thus showed stronger binding by a factor of 310 compared to the wild type clone. This antibody should be useful in various biological assays.     

Synthetic antibody libraries focused towards peptide ligands

Synthetic antibody libraries have proven immensely useful for the de novo isolation of antibodies without the need for animal immunization. Recently, focused libraries designed to recognize particular classes of ligands, such as haptens or proteins, have been employed to facilitate the selection of high-affinity antibodies. Focused libraries are built using V regions encoding combinations of canonical structures that resemble the structural features of antibodies that bind the desired class of ligands and sequence diversity is introduced at residues typically involved in recognition. Here we describe the generation and experimental validation of two different single-chain antibody variable fragment libraries that efficiently generate binders to peptides, a class of molecules that has proven to be a difficult target for antibody generation. First, a human anti-peptide library was constructed by diversifying a scaffold: the human variable heavy chain (V_H) germ line gene 3-23, which was fused to a variant of the human variable light chain (V_L) germ line gene A27, in which L1 was modified to encode the canonical structure found in anti-peptide antibodies. The sequence diversity was introduced into 3-23 (V_H) only, targeting for diversification residues commonly found in contact with protein and peptide antigens. Second, a murine library was generated using the antibody 26-10, which was initially isolated based on its affinity to the hapten digoxin, but also binds peptides and exhibits a canonical structure pattern typical of anti-peptide antibodies. Diversity was introduced in the V_H only using the profile of amino acids found at positions that frequently contact peptide antigens. Both libraries yielded binders to two model peptides, angiotensin and neuropeptide Y, following screening by solution phage panning. The mouse library yielded antibodies with affinities below 20 nM to both targets, although only the V_H had been subjected to diversification.     

Identification of the iron-binding histidine residues in soybean lipoxygenase L-1.

Lipoxygenases constitute a class of non-heme, non-sulfur iron dioxygenases acting upon lipids possessing a 1,4-cis-cis-pentadiene moiety. The iron is known to be essential for activity. A motif of six histidine residues has been found in all of the thirteen lipoxygenases, from both plant and animal sources, whose sequences have been reported. We had previously obtained mutant proteins in which each of the 6 conserved histidines of soybean lipoxygenase L-1 had been replaced and found that the mutants H499Q, H504Q (or H504S) and H690Q had no detectable enzymatic activity. We have now found that these inactive proteins contain no Fe, although they have the same electrophoretic mobility as wild-type L-1 under both denaturing and non-denaturing conditions and react with anti-L-1 antibodies.     

NMR assignments of the four histidines of staphylococcal nuclease in native and denatured states

NMR signals from all four histidine ring C epsilon protons and three of the four histidine C delta protons in the protein staphylococcal nuclease have been assigned by comparing spectra of the wild-type (Foggi strain) protein to spectra of three variants that each lack a different histidine residue. All proteins studied were cloned and overproduced in Escherichia coli. The NMR spectra of the three mutant proteins (H8R, H46Y, and H124L) used to make these assignments were similar to one another and to those of the wild type, except for signals from the mutated residues. The pKa values of those histidines conserved between the wild type and the mutants remained essentially unchanged. Multiple histidine C epsilon proton resonances due to non-native forms of nuclease were observed in both thermally induced and acid-induced unfolding. Residue-specific assignments of H epsilon protons in the thermally denatured forms of the mutant H46Y were obtained from connectivities to the native state by saturation transfer.     

Haemophore-mediated bacterial haem transport: evidence for a common or overlapping site for haem-free and haem-loaded haemophore on its specific outer membrane receptor

Bacterial extracellular haemophores also named HasA for haem acquisition system form an independent family of haemoproteins that take up haem from host haeme carriers and shuttle it to specific receptors (HasR). Haemophore receptors are required for the haemophore-dependent haem acquisition pathway and alone allow free or haemoglobin-bound haem uptake, but the synergy between the haemophore and its receptor greatly facilitates this uptake. The three-dimensional structure of the Serratia marcescens holo-haemophore (HasASM) has been determined previously and revealed that the haem iron atom is ligated by tyrosine 75 and histidine 32. The phenolate of tyrosine 75 is also tightly hydrogen bonded to the Ndelta atom of histidine 83. Alanine mutagenesis of these three HasASM residues was performed, and haem-binding constants of the wild-type protein, the three single mutant proteins, the three double mutant proteins and the triple mutant protein were compared by absorption spectrometry to probe the roles of H32, Y75 and H83 in haem binding. We show that one axial iron ligand is sufficient to ligate haem efficiently and that H83 may become an alternative iron ligand in the absence of Y75 or both H32 and Y75. All the single mutant proteins retained the ability to stimulate haemophore-dependent haem uptake in vivo. Thus, the residues H32, Y75 and H83 are not individually necessary for haem delivery to the receptor. The binding of haem-free and haem-loaded HasASM proteins to HasRSM-producing strains was studied. Both proteins bind to HasRSM with similar apparent Kd. The double mutant H32A-Y75A competitively inhibits binding to the receptor of both holo-HasASM and apo-HasASM, showing that there is a unique or overlapping site on HasRSM for the apo- and holo-haemophores. Thus, we propose a new mechanism for haem uptake, in which haem is exchanged between haem-loaded haemophores and unloaded haemophores bound to the receptor without swapping of haemophores on the receptor.     

Electron paramagnetic resonance characterization of the copper-resistance protein PcoC from <Emphasis Type="Italic">Escherichia coli</Emphasis>

Continuous-wave and pulsed electron paramagnetic resonance have been applied to the study of the Cu(II) site of the copper-resistance protein PcoC from Escherichia coli and certain variant forms. Electron spin echo envelope modulation (ESEEM) experiments confirm the presence of two histidine ligands, His1 and His92, at the Cu(II) site of wild-type PcoC, consistent with the available X-ray crystallographic data for the homolog CopC (67% sequence identity) from Pseudomonas syringae pv. tomato. The variants H1F and H92F each lack one of the histidine residues close to the Cu(II) site. The ESEEM data suggest that the surviving histidine residue remains as a ligand. The nA variant features an extra alanine residue at the N terminus, which demotes the His1 ligand to position 2. At least one of the two histidine residues is bound at the Cu(II) site in this form. Simulation of the (14)N superhyperfine structure in the continuous-wave spectra confirms the presence of at least three nitrogen-based ligands at the Cu(II) sites of the wild-type, H92F and nA forms, while the H1F variant has two nitrogen ligands. The spectra of wild-type form can be fitted adequately with a 3N or a 4N model. The former is consistent with the crystal structure of the CopC homolog, where His1 acts as a bidentate ligand. The latter raises the possibility of an additional unidentified nitrogen ligand. The markedly different spectra of the H1F and nA forms compared with the wild-type and H92F proteins further highlight the integral role of the N-terminal histidine residue in the high-affinity Cu(II) site of PcoC.