Stabilization of short collagen-like triple helices by protein engineering

Recombinant expression of collagens and fragments of collagens is often difficult, as their biosynthesis requires specific post-translational enzymes, in particular prolyl 4-hydroxylase. Although the use of hydroxyproline-deficient variants offers one possibility to overcome this difficulty, these proteins usually differ markedly in stability when compared with the hydroxyproline-containing analogs. Here, we report a method to stabilize collagen-like peptides by fusing them to the N terminus of the bacteriophage T4 fibritin foldon domain. The isolated foldon domain and the chimeric protein (GlyProPro)(10)foldon were expressed in a soluble form in Escherichia coli. The recombinant proteins and the synthetic (ProProGly)(10) peptide were characterized by circular dichroism (CD) spectroscopy, differential scanning calorimetry, and analytical ultracentrifugation. We show that the foldon domain, which comprises only 27 amino acid residues, forms an obligatory trimer with a high degree of thermal stability. The CD thermal unfolding profiles recorded from foldon are monophasic and completely reversible upon cooling. Similar Van't Hoff and calorimertic enthalpy values of trimer formation indicated a cooperative all-or-none transition. As reported previously, (ProProGly)(10) peptides form collagen triple helices of only moderate stability. When fused to the foldon domain, however, triple helix formation of (GlyProPro)(10) is concentration independent, and the midpoint temperature of the triple helix unfolding is significantly increased. The stabilizing function of the trimeric foldon domain is explained by the close vicinity of its N termini, which induce a high local concentration in the range of 1 M for the C termini of the collagen-like-peptide. Collagen-foldon fusion proteins should be potentially useful to study receptor-collagen interactions.

Periplasmic peptidyl prolyl cis-trans isomerases are not essential for viability, but SurA is required for pilus biogenesis in Escherichia coli

In Escherichia coli, FkpA, PpiA, PpiD, and SurA are the four known periplasmic cis-trans prolyl isomerases. These isomerases facilitate proper protein folding by increasing the rate of transition of proline residues between the cis and trans states. Genetic inactivation of all four periplasmic isomerases resulted in a viable strain that exhibited a decreased growth rate and increased susceptibility to certain antibiotics. Levels of the outer membrane proteins LamB and OmpA in the quadruple mutant were indistinguishable from those in the surA single mutant. In addition, expression of P and type 1 pili (adhesive organelles produced by uropathogenic strains of E. coli and assembled by the chaperone/usher pathway) were severely diminished in the absence of the four periplasmic isomerases. Maturation of the usher was significantly impaired in the outer membranes of strains devoid of all four periplasmic isomerases, resulting in a defect in pilus assembly. Moreover, this defect in pilus assembly and usher stability could be attributed to the absence of SurA. The data presented here suggest that the four periplasmic isomerases are not essential for growth under laboratory conditions but may have significant roles in survival in environmental and pathogenic niches, as indicated by the effect on pilus production.

Glycosylation/Hydroxylation-induced stabilization of the collagen triple helix. 4-trans-hydroxyproline in the Xaa position can stabilize the triple helix

We have shown recently that glycosylation of threonine in the peptide Ac-(Gly-Pro-Thr)(10)-NH(2) with beta-d-galactose induces the formation of a collagen triple helix, whereas the nonglycosylated peptide does not. In this report, we present evidence that a collagen triple helix can also be formed in the Ac-(Gly-Pro-Thr)(10)-NH(2) peptide, if the proline (Pro) in the Xaa position is replaced with 4-trans-hydroxyproline (Hyp). Furthermore, replacement of Pro with Hyp in the sequence Ac-(Gly-Pro-Thr(beta-d-Gal))(10)-NH(2) increases the T(m) of the triple helix by 15.7 degrees C. It is generally believed that Hyp in the Xaa position destabilizes the triple helix because (Pro-Pro-Gly)(10) and (Pro-Hyp-Gly)(10) form stable triple helices but the peptide (Hyp-Pro-Gly)(10) does not. Our data suggest that the destabilizing effect of Hyp relative to Pro in the Xaa position is only true in the case of (Hyp-Pro-Gly)(10). Increasing concentrations of galactose in the solvent stabilize the triple helix of Ac-(Gly-Hyp-Thr)(10)-NH(2) but to a much lesser extent than that achieved by covalently linked galactose. The data explain some of the forces governing the stability of the annelid/vestimentiferan cuticle collagens.

Sweet is stable: glycosylation stabilizes collagen

For most collagens, the melting temperature (T(m)) of the triple-helical structure of collagen correlates with the total content of proline (Pro) and 4-trans-hydroxyproline (Hyp) in the Xaa and Yaa positions of the -Gly-Xaa-Yaa- triplet repeat. The cuticle collagen of the deep-sea hydrothermal vent worm Riftia pachyptila, despite a very low content of Pro and Hyp, has a relatively high thermal stability. Rather than Hyp occupying the Yaa position, as is normally found in mammalian collagens, this position is occupied by threonine (Thr) which is O-glycosylated. We compare the triple-helix forming propensities in water of two model peptides, Ac-(Gly-Pro-Thr)(10)-NH(2) and Ac-(Gly-Pro-Thr(Galbeta))(10)-NH(2), and show that a collagen triple-helix structure is only achieved after glycosylation of Thr. Thus, we show for the first time that glycosylation is required for the formation of a stable tertiary structure and that this modification represents an alternative way of stabilizing the collagen triple-helix that is independent of the presence of Hyp.

Protein dynamics and 1/f noise

It has long been recognized that protein dynamical processes occur over a wide temporal range. However, the functionality of this spectrum of events remains unclear. In this work, a generalized noise function analysis is applied to a collection of diverse protein dynamical systems. It is shown that a power law model with an oscillatory component can adequately describe the time course of a variety of processes. These results suggest that under the appropriate conditions, proteins are in a metastable state. A microscopic, chemical kinetic model based on a Poisson distribution of activation energies is presented. From this model specific functional forms for the parameters of the generalized noise model can be derived. Additionally, a model is presented to described kinetic hole burning effects observed at low temperatures. Scaling laws are derived for these models that provide a connection with the generalized noise analysis.

Real-time and equilibrium (19)F-NMR studies reveal the role of domain-domain interactions in the folding of the chaperone PapD

PapD is a periplasmic chaperone essential for P pilus formation in pyelonephritic strains of E. coli. It is composed of two domains, each of which contains a tryptophan residue (Trp-36 and Trp-128, in the N- and C-terminal domains, respectively). To explore the role of domain-domain interactions during folding, the protein was labeled with 6-fluorotryptophan for use in (19)F-NMR experiments. (19)F-NMR data collected as a function of urea concentration revealed the presence of a resonance caused by Trp-128 that was distinct from either the folded or unfolded resonances. The time course of refolding from urea was monitored by stopped-flow fluorescence, CD, and (19)F-NMR, each method showing multiple kinetic phases. The (19)F-NMR stopped-flow spectra, collected at 70 microM of protein with a fluorine cryoprobe, demonstrated that the intermediate was populated early in the folding process (<5 s). The slow disappearance of the intermediate and unfolded resonance occurred at the same rate as the appearance of the native resonances of both domains. The data are consistent with a model in which the C-terminal domain collapses rapidly to an intermediate, whereas the stabilization of the final structure is slow and requires folding of the N-terminal domain with concomitant readjustment of the C-terminal domain structure.

Chicken FK506-binding protein, FKBP65, a member of the FKBP family of peptidylprolyl cis-trans isomerases, is only partially inhibited by FK506

The chicken FK506-binding protein FKBP65, a peptidylprolyl cis-trans isomerase, is a rough endoplasmic reticulum protein that contains four domains homologous to FKBP13, another rough endoplasmic reticulum PPIase. Analytical ultracentrifugation suggests that in FKBP65 these four domains are arranged in a linear extended structure with a length of about 26 nm and a diameter of about 3 nm. All four domains are therefore expected to be accessible to substrates. The specificity of FKBP65 towards a number of peptide substrates was determined. The specific activity of FKBP65 is generally lower than that of FKBP12 when expressed as a per domain activity. The substrate specificity of FKBP65 also differs from that of FKBP12. Inhibition studies show that only one of the four domains can be inhibited by FK506, a powerful inhibitor of all other known FKBPs. Furthermore, the same domain seems to be susceptible to inhibition by cyclosporin A. No other FKBPs were shown to be inhibited by cyclosporin A. It is also shown that FKBP65 can catalyse the re-folding of type III collagen in vitro with a kcat/Km = 4.3 x 10(3) M-1.s-1.

Catalysis of protein folding by chaperones in pathogenic bacteria

Molecular chaperones are thought to inhibit off-pathway interactions such as aggregation from occurring without influencing the on-pathway formation of native structure. Here, we present a mechanism whereby the family of PapD-like chaperones, which are involved in the formation of adhesive pili in pathogenic bacteria, function by suppressing aggregation while simultaneously catalyzing the folding of subunits that make up the pilus. We also show that the Arg-8 residue, invariant in the cleft of all known PapD-like chaperones, makes up part of the active site of the chaperone. The data argue for a temporal mechanism of catalyzed folding. The terminal carboxylate group of a pilus subunit anchors to the active site of the chaperone by hydrogen bonding. This bonding spatially fixes the COOH terminus of the subunit in the correct context for beta-sheet formation, using the edge of the NH(2)-terminal domain of the chaperone as a nucleation site.

Anthrax toxin protective antigen--insights into molecular switching from prepore to pore

The protective antigen is a key component of the anthrax toxin, as it allows entry of the enzymatic components edema factor and lethal factor into the host cell, through the formation of a membrane spanning pore. This event is absolutely critical for the pathogenesis of anthrax, and although we have yet to understand the mechanism of pore formation, recent developments have provided key insights into how this process may occur. Based on the available data, a model is proposed for the kinetic steps for protective antigen conversion from prepore to pore. In this model, the driving force for pore formation is the formation of the phi (ϕ)-clamp, a region that forms a leak-free seal around the translocating polypeptide. Formation of the ϕ-clamp elicits movements within the prepore that provide steric freedom for the subsequent conformational changes required to form the membrane spanning pore.

Effect of 2-fluorohistidine labeling of the anthrax protective antigen on stability, pore formation, and translocation

The action of anthrax toxin relies in part upon the ability of the protective antigen (PA) moiety to form a heptameric pore in the endosomal membrane, providing a portal for entry of the enzymic moieties of the toxin into the cytosol. Pore formation is dependent on a conformational change in the heptameric prepore that occurs in the neutral to mildly acidic pH range, and it has been hypothesized that protonation of one or more histidine residues triggers this transition. To test this hypothesis, we used biosynthetic methods to incorporate the unnatural amino acid analogue 2-fluorohistidine (2-FHis) into PA. 2-FHis is isosteric with histidine but resists protonation at physiological pH values due to a dramatically reduced side-chain pKa ( approximately 1). We found that 2-FHis-labeled PA was biologically inactive, as judged by its inability to deliver a model intracellular effector, LFN-DTA, to the cytosol of CHO-K1 cells. However, whereas 2-FHis blocked a conformational transition in the full-length PA83 protein in the pH 5-6 range, the pH dependence of prepore-to-pore conversion of (PA63)7 was unchanged from the wild-type protein, implying that this conversion is not dependent on His protonation. Consistent with this result, the labeled, trypsin-activated PA was able to permeabilize liposomes to K+ and retained pore-forming activity in planar phospholipid bilayers. The pores in planar bilayers were incapable, however, of translocating a model ligand in response to a transmembrane pH gradient or elevated voltage. The results indicate that protonation of residues other than His, presumably Glu and/or Asp side chains, triggers pore formation in vitro, but His residues are nonetheless important for PA functioning in vivo.

Y-position collagen II mutation disrupts cartilage formation and skeletal development in a transgenic mouse model of spondyloepiphyseal dysplasia

Mice were generated by pronuclear injection of a type II collagen transgene harboring an Arg789Cys (R789C) mutation that has been found in patients with spondyloepiphyseal dysplasia (SED). Expression was directed to cartilage by the murine Col2a1 promoter to examine the consequences of mutations involving the Y-position of the collagen helix Gly-X-Y triplet on skeletogenesis. The transgenic mice had very short limbs, short trunk, short snout, and cleft palate; they died at birth. Their growth plates were disorganized and collagen fibrils were sparse in cartilage matrix. When the transgene was expressed in RCS cells, there was no evidence that R789C-bearing collagen chains were incorporated into stable collagen molecules. Molecular modeling of the mutation raised the possibility that it destabilizes the collagen triple helix. Together our results suggest that Y-position mutations, such as R789C, can act in a dominant negative manner to destabilize collagen molecules during assembly, reducing their availability to form fibrils, the deficiency of which profoundly disturbs the template functions of cartilage during skeletogenesis.

3S-fluoroproline as a probe to monitor proline isomerization during protein folding by 19F-NMR

Variable-temperature inversion transfer NMR is used to determine the kinetic and thermodynamic parameters of cis-trans isomerization of N-Ac-(3R) and (3S)-fluoroproline-OMe.

Evidence that histidine protonation of receptor-bound anthrax protective antigen is a trigger for pore formation

The protective antigen (PA) component of the anthrax toxin forms pores within the low pH environment of host endosomes through mechanisms that are poorly understood. It has been proposed that pore formation is dependent on histidine protonation. In previous work, we biosynthetically incorporated 2-fluorohistidine (2-FHis), an isosteric analogue of histidine with a significantly reduced pK(a) ( approximately 1), into PA and showed that the pH-dependent conversion from the soluble prepore to a pore was unchanged. However, we also observed that 2-FHisPA was nonfunctional in the ability to mediate cytotoxicity of CHO-K1 cells by LF(N)-DTA and was defective in translocation through planar lipid bilayers. Here, we show that the defect in cytotoxicity is due to both a defect in translocation and, when bound to the host cellular receptor, an inability to undergo low pH-induced pore formation. Combining X-ray crystallography with hydrogen-deuterium (H-D) exchange mass spectrometry, our studies lead to a model in which hydrogen bonds to the histidine ring are strengthened by receptor binding. The combination of both fluorination and receptor binding is sufficient to block low pH-induced pore formation.

Demystifying fluorine chemical shifts: electronic structure calculations address origins of seemingly anomalous (19)F-NMR spectra of fluorohistidine isomers and analogues

Fluorine NMR spectroscopy is a powerful tool for studying biomolecular structure, dynamics, and ligand binding, yet the origins of (19)F chemical shifts are not well understood. Herein, we use electronic structure calculations to describe the changes in (19)F chemical shifts of 2F- and 4F-histidine/(5-methyl)-imidazole upon acid titration. While the protonation of the 2F species results in a deshielded chemical shift, protonation of the 4F isomer results in an opposite, shielded chemical shift. The deshielding of 2F-histidine/(5-methyl)-imidazole upon protonation can be rationalized by concomitant decreases in charge density on fluorine and a reduced dipole moment. These correlations do not hold for 4F-histidine/(5-methyl)-imidazole, however. Molecular orbital calculations reveal that for the 4F species, there are no lone pair electrons on the fluorine until protonation. Analysis of a series of 4F-imidazole analogues, all with delocalized fluorine electron density, indicates that the deshielding of (19)F chemical shifts through substituent effects correlates with increased C-F bond polarity. In summary, the delocalization of fluorine electrons in the neutral 4F species, with gain of a lone pair upon protonation may help explain the difficulty in developing a predictive framework for fluorine chemical shifts. Ideas debated by chemists over 40 years ago, regarding fluorine's complex electronic effects, are shown to have relevance for understanding and predicting fluorine NMR spectra.

Domain 4 of the anthrax protective antigen maintains structure and binding to the host receptor CMG2 at low pH

Domain 4 of the anthrax protective antigen (PA) plays a key role in cellular receptor recognition as well as in pH-dependent pore formation. We present here the 1.95 A crystal structure of domain 4, which adopts a fold that is identical to that observed in the full-length protein. We have also investigated the structural properties of the isolated domain 4 as a function of pH, as well as the pH-dependence on binding to the von Willebrand factor A domain of capillary morphogenesis protein 2 (CMG2). Our results provide evidence that the isolated domain 4 maintains structure and interactions with CMG2 at pH 5, a pH that is known to cause release of the receptor on conversion of the heptameric prepore (PA(63))(7) to a membrane-spanning pore. Our results suggest that receptor release is not driven solely by a pH-induced unfolding of domain 4.

Folding and domain-domain interactions of the chaperone PapD measured by 19F NMR

The folding of the two-domain bacterial chaperone PapD has been studied to develop an understanding of the relationship between individual domain folding and the formation of domain-domain interactions. PapD contains six phenylalanine residues, four in the N-terminal domain and two in the C-terminal domain. To examine the folding properties of PapD, the protein was both uniformly and site-specifically labeled with p-fluoro-phenylalanine ((19)F-Phe) for (19)F NMR studies, in conjunction with those of circular dichroism and fluorescence. In equilibrium denaturation experiments monitored by (19)F NMR, the loss of (19)F-Phe native intensity for both the N- and C-terminal domains shows the same dependence on urea concentration. For the N-terminal domain the loss of native intensity is mirrored by the appearance of separate denatured resonances. For the C-terminal domain, which contains residues Phe 168 and Phe 205, intermediate as well as denatured resonances appear. These intermediate resonances persist at denaturant concentrations well beyond the loss of native resonance intensity and appear in kinetic refolding (19)F NMR experiments. In double-jump (19)F NMR experiments in which proline isomerization does not affect the refolding kinetics, the formation of domain-domain interactions is fast if the protein is denatured for only a short time. However, with increasing time of denaturation the native intensities of the N- and C-terminal domains decrease, and the denatured resonances of the N-terminal domain and the intermediate resonances of the C-terminal domain accumulate. The rate of loss of the N-terminal domain resonances is consistent with a cis to trans isomerization process, indicating that from an equilibrium denatured state the slow refolding of PapD is due to the trans to cis isomerization of one or both of the N-terminal cis proline residues. The data indicate that both the N- and C-terminal domains must fold into a native conformation prior to the formation of domain-domain interactions.

Role of carbohydrate in stabilizing the triple-helix in a model for a deep-sea hydrothermal vent worm collagen

The glycopeptide Ac-(Gly-Pro-Thr(beta-Gal))(10)-NH(2) forms a collagen-like triple-helix. A (1)H NMR structural analysis is reported for the peptides Ac-(Gly-Pro-Thr)(n)-NH(2) and Ac-(Gly-Pro-Thr(beta-Gal))(n)-NH(2), where n = 1, 5, and 10. NMR assignments for the individual peptides are made using one- and two-dimensional TOCSY, ROESY, and NOESY experiments. The NMR and corroborating CD data show that Ac-(Gly-Pro-Thr)(n)-NH(2), n = 1, 5, or 10, as well as Ac-(Gly-Pro-Thr(beta-Gal))(n)-NH(2), n = 1 or 5 peptides are unable to form collagen-like triple-helical structures. Furthermore, the equilibrium ratio of cis to trans isomers of the Pro residues is unaffected by the presence of carbohydrate. For Ac-(Gly-Pro-Thr(beta-Gal))(10)-NH(2), the kinetics of amide (1)H exchange with solvent deuterium indicate a slow rate of exchange for both the Gly and the Thr amide. The data are thus consistent with a model in which the carbohydrate stabilizes the triple helix through an occlusion of water molecules and by hydrogen bonding but not through an influence on the cis to trans isomer ratio.

Seneca Valley Virus Exploits TEM8, a Collagen Receptor Implicated in Tumor Growth

Recent studies reveal that Seneca Valley Virus (SVV) exploits tumor endothelial marker 8 (TEM8) for cellular entry, the same surface receptor pirated by bacterial-derived anthrax toxin. This observation is particularly significant as SVV is a known oncolytic virus which selectively infects and kills tumor cells, particularly those of neuroendocrine origin. TEM8 is a transmembrane glycoprotein that is preferentially upregulated in some tumor cell and tumor-associated stromal cell populations. Both TEM8 and SVV have been evaluated for targeting of tumors of multiple origins, but the connection between the two was previously unknown. Here, we review currently understood interactions between TEM8 and SVV, anthrax protective antigen (PA), and collagen VI, a native binding partner of TEM8, with an emphasis on potential therapeutic directions moving forward.

Comparison of the structural stability and dynamic properties of recombinant anthrax protective antigen and its 2-fluorohistidine-labeled analogue

Protective antigen (PA) is the primary protein antigenic component of both the currently used anthrax vaccine and related recombinant vaccines under development. An analogue of recombinant PA (2-FHis rPA) has been recently shown to block the key steps of pore formation in the process of inducing cytotoxicity in cells, and thus can potentially be used as an antitoxin or a vaccine. This rPA analogue was produced by fermentation to incorporate the unnatural amino acid 2-fluorohistidine (2-FHis). In this study, the effects of 2-FHis labeling on rPA antigen's conformational stability and dynamic properties were investigated by various biophysical techniques. Temperature/pH stability profiles of rPA and 2-FHis rPA were analyzed by the empirical phase diagram (EPD) approach, and physical stability differences between them were identified. Results showed that rPA and 2-FHis rPA had similar stability at pH 7-8. With decreasing solution pH, however, 2-FHis rPA was found to be more stable. Dynamic sensitive measurements of the two proteins at pH 5 found that 2-FHis rPA was more dynamic and/or differentially hydrated under acidic pH conditions. The biophysical characterization and stability data provide information useful for the potential development of 2-FHis rPA as a more stable rPA vaccine candidate.

Monitoring anthrax toxin receptor dissociation from the protective antigen by NMR

The binding of the Bacillus anthracis protective antigen (PA) to the host cell receptor is the first step toward the formation of the anthrax toxin, a tripartite set of proteins that include the enzymatic moieties edema factor (EF), and lethal factor (LF). PA is cleaved by a furin-like protease on the cell surface followed by the formation of a donut-shaped heptameric prepore. The prepore undergoes a major structural transition at acidic pH that results in the formation of a membrane spanning pore, an event which is dictated by interactions with the receptor and necessary for entry of EF and LF into the cell. We provide direct evidence using 1-dimensional (13)C-edited (1)H NMR that low pH induces dissociation of the Von-Willebrand factor A domain of the receptor capillary morphogenesis protein 2 (CMG2) from the prepore, but not the monomeric full length PA. Receptor dissociation is also observed using a carbon-13 labeled, 2-fluorohistidine labeled CMG2, consistent with studies showing that protonation of His-121 in CMG2 is not a mechanism for receptor release. Dissociation is likely caused by the structural transition upon formation of a pore from the prepore state rather than protonation of residues at the receptor PA or prepore interface.

(19)F nuclear magnetic resonance and crystallographic studies of 5-fluorotryptophan-labeled anthrax protective antigen and effects of the receptor on stability

The anthrax protective antigen (PA) is an 83 kDa protein that is one of three protein components of the anthrax toxin, an AB toxin secreted by Bacillus anthracis. PA is capable of undergoing several structural changes, including oligomerization to either a heptameric or octameric structure called the prepore, and at acidic pH a major conformational change to form a membrane-spanning pore. To follow these structural changes at a residue-specific level, we have conducted initial studies in which we have biosynthetically incorporated 5-fluorotryptophan (5-FTrp) into PA, and we have studied the influence of 5-FTrp labeling on the structural stability of PA and on binding to the host receptor capillary morphogenesis protein 2 (CMG2) using ¹⁹F nuclear magnetic resonance (NMR). There are seven tryptophans in PA, but of the four domains in PA, only two contain tryptophans: domain 1 (Trp65, -90, -136, -206, and -226) and domain 2 (Trp346 and -477). Trp346 is of particular interest because of its proximity to the CMG2 binding interface, and because it forms part of the membrane-spanning pore. We show that the ¹⁹F resonance of Trp346 is sensitive to changes in pH, consistent with crystallographic studies, and that receptor binding significantly stabilizes Trp346 to both pH and temperature. In addition, we provide evidence that suggests that resonances from tryptophans distant from the binding interface are also stabilized by the receptor. Our studies highlight the positive impact of receptor binding on protein stability and the use of ¹⁹F NMR in gaining insight into structural changes in a high-molecular weight protein.

pH effects on binding between the anthrax protective antigen and the host cellular receptor CMG2

The anthrax protective antigen (PA) binds to the host cellular receptor capillary morphogenesis protein 2 (CMG2) with high affinity. To gain a better understanding of how pH may affect binding to the receptor, we have investigated the kinetics of binding as a function of pH to the full-length monomeric PA and to two variants: a 2-fluorohistidine-labeled PA (2-FHisPA), which is ∼1 pH unit more stable to variations in pH than WT, and an ∼1 pH unit less stable variant in which Trp346 in the domain 2β(3) -2β(4) loop is substituted with a Phe (W346F). We show using stopped-flow fluorescence that the binding rate increases as the pH is lowered for all proteins, with little influence on the rate of dissociation. In addition, we have crystallized PA and the two variants and examine the influence of pH on structure. In contrast to previous X-ray studies, the domain 2β(3) -2β(4) loop undergoes little change in structure from pH ∼8 to 5.5 for the WT protein, but for the 2-FHis labeled and W346F mutant there are changes in structure consistent with previous X-ray studies. In accord with pH stability studies, we find that the average B-factor values increase by ∼20-30% for all three proteins at low pH. Our results suggest that for the full-length PA, low pH increases the binding affinity, likely through a change in structure that favors a more "bound-like" conformation.