Characterisation of anthracyclines from a cosmomycin D-producing species of Streptomyces by collisionally-activated dissociation and ion mobility mass spectrometry

Cultures of cosmomycin D-producing Streptomyces olindensis ICB20 that were propagated for many generations underwent mutations that resulted in production of a range of related anthracyclines by the bacteria. The anthracyclines that retained the two trisaccharide chains of the parent compound were separated by HPLC. Exact mass determination of these compounds revealed that they differed from cosmomycin D (CosD) in that they contained one to three fewer oxygen atoms (loss of hydroxyl groups). Some of the anthracyclines that were separated by HPLC had the same mass. The location from which the hydroxyl groups had been lost relative to CosD (on the aglycone and/or on the sugar residues) was probed by collisionally-activated dissociation using an electrospray ionisation linear quadrupole ion trap mass spectrometer. The presence of anthracyclines with the same mass, but different structure, was confirmed using an electrospray ionisation travelling wave ion mobility mass spectrometer.

Structural elucidation of the nonclassical secondary cell wall polysaccharide from Bacillus cereus ATCC 10987. Comparison with the polysaccharides from Bacillus anthracis and B. cereus type strain ATCC 14579 reveals both unique and common structural features

Nonclassical secondary cell wall polysaccharides constitute a major cell wall structure in the Bacillus cereus group of bacteria. The structure of the secondary cell wall polysaccharide from Bacillus cereus ATCC 10987, a strain that is closely related to Bacillus anthracis, was determined. This polysaccharide was released from the cell wall with aqueous hydrogen fluoride (HF) and purified by gel filtration chromatography. The purified polysaccharide, HF-PS, was characterized by glycosyl composition and linkage analyses, mass spectrometry, and one- and two-dimensional NMR analysis. The results showed that the B. cereus ATCC 10987 HF-PS has a repeating oligosaccharide consisting of a -->6)-alpha-GalNAc-(1-->4)-beta-ManNAc-(1-->4)-beta-GlcNAc-(1--> trisaccharide that is substituted with beta-Gal at O3 of the alpha-GalNAc residue and nonstoichiometrically acetylated at O3 of the N-acetylmannosamine (ManNAc) residue. Comparison of this structure with that of the B. anthracis HF-PS and with structural data obtained for the HF-PS from B. cereus type strain ATCC 14579 revealed that each HF-PS had the same general structural theme consisting of three HexNAc and one Hex residues. A common structural feature in the HF-PSs from B. cereus ATCC 10987 and B. anthracis was the presence of a repeating unit consisting of a HexNAc(3) trisaccharide backbone in which two of the three HexNAc residues are GlcNAc and ManNAc and the third can be either GlcNAc or GalNAc. The implications of these results with regard to the possible functions of the HF-PSs are discussed.

Installation of electron-donating protective groups, a strategy for glycosylating unreactive thioglycosyl acceptors using the preactivation-based glycosylation method

Preactivation-based chemoselective glycosylation is a powerful strategy for oligosaccharide synthesis with its successful application in assemblies of many complex oligosaccharides. However, difficulties were encountered in reactions where glycosyl donors bearing multiple electron-withdrawing groups failed to glycosylate hindered unreactive acceptors. In order to overcome this problem, it was discovered that the introduction of electron-donating protective groups onto the glycosyl donors can considerably enhance their glycosylating power, leading to productive glycosylations even with unreactive acceptors. This observation is quite general and can be extended to a wide range of glycosylation reactions, including one-pot syntheses of chondroitin and heparin trisaccharides. The structures of the reactive intermediates formed upon preactivation were determined through low-temperature NMR studies. It was found that for a donor with multiple electron-withdrawing groups, the glycosyl triflate was formed following preactivation, while the dioxalenium ion was the major intermediate with a donor bearing electron-donating protective groups. As donors were all cleanly preactivated prior to the addition of the acceptors, the observed reactivity difference between these donors was not due to selective activation encountered in the traditional armed-disarmed strategy. Rather, it was rationalized by the inherent internal energy difference between the reactive intermediates and associated oxacarbenium ion like transition states during nucleophilic attack by the acceptor.

Antigen recognition by variable lymphocyte receptors

Variable lymphocyte receptors (VLRs) rather than antibodies play the primary role in recognition of antigens in the adaptive immune system of jawless vertebrates. Combinatorial assembly of leucine-rich repeat (LRR) gene segments achieves the required repertoire for antigen recognition. We have determined a crystal structure for a VLR-antigen complex, VLR RBC36 in complex with the H-antigen trisaccharide from human blood type O erythrocytes, at 1.67 angstrom resolution. RBC36 binds the H-trisaccharide on the concave surface of the LRR modules of the solenoid structure where three key hydrophilic residues, multiple van der Waals interactions, and the highly variable insert of the carboxyl-terminal LRR module determine antigen recognition and specificity. The concave surface assembled from the most highly variable regions of the LRRs, along with diversity in the sequence and length of the highly variable insert, can account for the recognition of diverse antigens by VLRs.

The Stability of Insulin in Solid Formulations Containing Melezitose and Starch. Effects of Processing and Excipients

Solid insulin formulations obtained by different methods of preparation were compared with respect to chemical stability and morphology. Spray- and freeze-drying, solution enhanced dispersion by supercritical fluids (SEDS) and precipitation into starch microspheres were the methods used for preparation of solid powders. The excipients applied were melezitose, starch, and sodium taurocholate. The stability of the samples was evaluated after storage in open containers at 25 degrees C and 30% RH for 6 months. All samples were amorphous after processing and storage as detected by XRD, except for the starch microspheres which were semi-crystalline. The spray- and freeze-dried samples containing melezitose and sodium taurocholate experienced a significant water uptake during storage, resulting in changes in morphology and disappearance of Tg. However, the chemical stability of these samples did not seem to be affected by the water uptake. Changes in morphology were not observed for the SEDS powders and the starch microspheres. The chemical stability of the samples was assessed by HPLC. In general, conventional spray- and freeze drying resulted in samples with higher chemical stability compared to SEDS powders and starch microspheres. Nevertheless, the excipients applied were observed to be of major importance, and further optimization of the formulation as well as processing conditions may lead to slightly different conclusions.

Hydrolysis Kinetics of Trisaccharides Consisting of Glucose, Galactose, and Fructose Residues in Subcritical Water

The hydrolysis kinetics of trisaccharides consisting of glucose, galactose, and fructose residues with different glycosidic bonds, 1-kestose, d-melezitose, d-raffinose, and lactosucrose, in subcritical water were conducted over the temperature range of 150-230 degrees C and at a constant pressure of 10 MPa. The hydrolysis of trisaccharides in subcritical water proceeded consecutively, i.e., one cleavage of the two bonds antedated the other. The preceding cleavage was not expressed by the first-order kinetics, but by the kinetics considering the concentration of the acidic compounds, which were produced by the degradation of the constituent monosaccharides. The hydrolysis of the constituent disaccharides, except sucrose composed of the alpha-Glc-(1-->2)-beta-Fru bond, obeyed first-order kinetics. All of the rate constants of the hydrolytic kinetics were determined, and the values were found to depend on the type of bond.

Oligomannose-coated liposomes activate ERK via Src kinases and PI3K/Akt in J774A.1 cells

We have previously shown that liposomes coated with a neoglycolipid constructed from mannotriose and dipalmitoylphosphatidylethanolamine (Man3-DPPE) activate peritoneal macrophages to induce enhanced expression of co-stimulatory molecules and MHC class II. In this study, we investigated the signaling pathways activated by the Man3-DPPE-coated liposomes (OMLs) in a murine macrophage cell line, J774A.1. In response to OML stimulation, ERK among MAPKs was clearly and transiently phosphorylated in J774 cells. ERK phosphorylation was also induced by treatment of the cells with Man3-DPPE and Man3-BSA, but not by uncoated liposomes. In addition, rapid and transient phosphorylation of Akt and Src family kinases (SFKs) was observed in response to OMLs. OML-induced ERK phosphorylation was inhibited by specific inhibitors of PI3K and SFKs, and OML-induced Akt phosphorylation was inhibited by a inhibitor of SFKs. Therefore, OMLs may activate the PI3K/Akt pathway through phosphorylation of Src family kinases to induce ERK activation.

Which electrospray-based ionization method best reflects protein-ligand interactions found in solution? a comparison of ESI, nanoESI, and ESSI for the determination of dissociation constants with mass spectrometry

We present a comparison of three different electrospray-based ionization techniques for the investigation of noncovalent complexes with mass spectrometry. The features and characteristics of standard electrospray ionization (ESI), chip-based nanoESI, and electrosonic spray ionization (ESSI) mounted onto a hybrid quadrupole time-of-flight mass spectrometer were compared in their performance to determine the dissociation constant (KD) of the model system hen egg white lysozyme (HEWL) binding to N,N',N''-triacetylchitotriose (NAG3). The best KD value compared with solution data were found for ESSI, 19.4 +/- 3.6 microM. Then, we determined the KDs of the two nucleotide binding sites of adenylate kinase (AK), where we obtained KDs of 2.2 +/- 0.8 microM for the first and 19.5 +/- 8.0 microM for the second binding site using ESSI. We found a weak charge state dependence of the KD for both protein-ligand systems, where for all ionization techniques the KD value decreases with increasing charge state. We demonstrate that ESSI is very gentle and insensitive to instrumental parameters, and the KD obtained is in good agreement with solution phase results from the literature. In addition, we tried to determine the KD for the lymphocyte-specific kinase LCK binding to a kinase inhibitor using nanoESI due to the very low amount of sample available. In this case, we found KD values with a strong charge state dependence, which were in no case close to literature values for solution phase.

Application of biselective refocusing soft pulses to the simplification of heteronuclear correlation spectra

The biselective spin echo technique allows the signals of coupled proton pairs to be extracted from crowded liquid state proton NMR spectra. Its use as a preparation sequence in heteronuclear chemical shift correlation experiments requires the removal of the heteronuclear coupling interaction during the biselective echo time. The discrimination between coupled and uncoupled protons signals is achieved by double quantum filtration, which delivers antiphase magnetization states. The latter are not directly compatible with the design of an HSQC-like pulse sequence. The conversion of antiphase to in-phase magnetization states by a second biselective echo sequence solves this problem. The optimization of spin echo delays is also discussed. Lastly, the article presents modified HSQC and HMBC pulses sequences in which information is obtained solely for the biselectively selected proton pairs. A peracetylated trisaccharide was used as a test molecule.

Enzymatic synthesis and identification of two trisaccharides produced from lactulose by transgalactosylation

The enzymatic transgalactosylation during lactulose hydrolysis was studied using the beta-galactosidase from Kluyveromyces lactis and an initial lactulose concentration of 250 g/L. During hydrolysis of lactulose, the formation of two novel trisaccharides was followed by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD). A maximum trisaccharide yield of 14.05% was observed at 91.9% of lactulose hydrolysis. The two novel trisaccharides obtained by transglycosylation of lactulose were isolated and fully characterized by an extensive nuclear magnetic resonance (NMR) study. Complete structure elucidation and full proton and carbon assignment were carried out using 1D ( 1H, 13C, and 1D TOCSY) and 2D (gCOSY, TOCSY, ROESY, gHSQC, and gHMBC) NMR experiments. The trisaccharides were shown to be lactulose-based structures; the main one has a Gal unit linked to C-6 of the galactose moiety, and the other one has a Gal unit linked to C-1 of the fructose moiety. Transglycosylation of lactulose allows for the obtention of galacto-oligosaccharides with new glycosidic structures and would open new routes to the synthesis of prebiotics.

Mimetics of the tri- and tetrasaccharide epitope of GQ1bα as myelin-associated glycoprotein (MAG) ligands

The synthesis of phenoxyphenyl, phenoxybenzyl, biphenyl, and phenyltriazole substituted sialic acid derivatives as mimics of the tri- and tetrasaccharide epitopes of GQ1balpha is described. These synthetically easily available sialosides show comparable or even enhanced affinity to MAG compared with the natural tri- and tetrasaccharide epitopes and form a new class of potential MAG antagonists.

Sequencing and characterization of oligosaccharides using infrared multiphoton dissociation and boronic acid derivatization in a quadrupole ion trap

A simplified method for determining the sequence and branching of oligosaccharides using infrared multiphoton dissociation (IRMPD) in a quadrupole ion trap (QIT) is described. An IR-active boronic acid (IRABA) reagent is used to derivatize the oligosaccharides before IRMPD analysis. The IRABA ligand is designed to both enhance the efficiency of the derivatization reaction and to facilitate the photon absorption process. The resulting IRMPD spectra display oligosaccharide fragments that are formed from primarily one type of diagnostic cleavage, thus making sequencing straightforward. The presence of sequential fragment ions, a phenomenon of IRMPD, permit the comprehensive sequencing of the oligosaccharides studied in a single stage of activation. We demonstrate this approach for two series of oligosaccharides, the lacto-N-fucopentaoses (LNFPs) and the lacto-N-difucohexaoses (LNDFHs).

Practical preparation of 2-azido-2-deoxy-β-d-mannopyranosyl carbonates and their application in the synthesis of oligosaccharides

1-O-Allyloxycarbonyl (or ethyloxycarbonyl)-2-azido-2-deoxy-3-O-benzyl (or allyl, or benzoyl)-4,6-O-isopropylidene-beta-d-mannopyranose derivatives were prepared from the corresponding 2-hydroxy-beta-d-glucopyranosyl carbonates in high yields via triflation of the 2-hydroxyl group and subsequent SN2 displacement with azide ion. An N-acetyl-mannosamine-containing trisaccharide, a fragment of the putative O10 antigen from Acinetobacter baumannii, was efficiently synthesized using these derivatives.

Unraveling the difference between invertases and fructan exohydrolases: a single amino acid (Asp-239) substitution transforms Arabidopsis cell wall invertase1 into a fructan 1-exohydrolase

Plant cell wall invertases and fructan exohydrolases (FEHs) are very closely related enzymes at the molecular and structural level (family 32 of glycoside hydrolases), but they are functionally different and are believed to fulfill distinct roles in plants. Invertases preferentially hydrolyze the glucose (Glc)-fructose (Fru) linkage in sucrose (Suc), whereas plant FEHs have no invertase activity and only split terminal Fru-Fru linkages in fructans. Recently, the three-dimensional structures of Arabidopsis (Arabidopsis thaliana) cell wall Invertase1 (AtcwINV1) and chicory (Cichorium intybus) 1-FEH IIa were resolved. Until now, it remained unknown which amino acid residues determine whether Suc or fructan is used as a donor substrate in the hydrolysis reaction of the glycosidic bond. In this article, we present site-directed mutagenesis-based data on AtcwINV1 showing that the aspartate (Asp)-239 residue fulfills an important role in both binding and hydrolysis of Suc. Moreover, it was found that the presence of a hydrophobic zone at the rim of the active site is important for optimal and stable binding of Suc. Surprisingly, a D239A mutant acted as a 1-FEH, preferentially degrading 1-kestose, indicating that plant FEHs lacking invertase activity could have evolved from a cell wall invertase-type ancestor by a few mutational changes. In general, family 32 and 68 enzymes containing an Asp-239 functional homolog have Suc as a preferential substrate, whereas enzymes lacking this homolog use fructans as a donor substrate. The presence or absence of such an Asp-239 homolog is proposed as a reliable determinant to discriminate between real invertases and defective invertases/FEHs.

One-pot three-enzyme chemoenzymatic approach to the synthesis of sialosides containing natural and non-natural functionalities

Chemoenzymatic synthesis, which combines the flexibility of chemical synthesis and the high selectivity of enzymatic synthesis, is a powerful approach to obtain complex carbohydrates. It is a preferred method for synthesizing sialic acid-containing structures, including those with diverse naturally occurring and non-natural sialic acid forms, different sialyl linkages and different glycans that link to the sialic acid. Starting from N-acetylmannosamine, mannose or their chemically or enzymatically modified derivatives, sialic acid aldolase-catalyzed condensation reaction leads to the formation of sialic acids and their derivatives. These compounds are subsequently activated by a CMP-sialic acid synthetase and transferred to a wide range of suitable acceptors by a suitable sialyltransferase for the formation of sialosides containing natural and non-natural functionalities. The three-enzyme coupled synthesis of sialosides can be carried out in one pot without the isolation of intermediates. The time for synthesis is 4-18 h. Purification and characterization of the product can be completed within 2-3 d.

Characterization of a β-fructofuranosidase from Schwanniomyces occidentalis with transfructosylating activity yielding the prebiotic 6-kestose

beta-Fructofuranosidases are powerful tools in industrial biotechnology. We have characterized an extracellular beta-fructofuranosidase from the yeast Schwanniomyces occidentalis. The enzyme shows broad substrate specificity, hydrolyzing sucrose, 1-kestose, nystose and raffinose, with different catalytic efficiencies (k(cat)/K(m)). Although the main reaction catalysed by this enzyme is sucrose hydrolysis, it also produces two fructooligosaccharides (FOS) by transfructosylation. A combination of (1)H, (13)C and 2D-NMR techniques shows that the major product is the prebiotic trisaccharide 6-kestose. The 6-kestose yield obtained with this beta-fructofuranosidase is, to our concern, higher than those reported with other 6-kestose-producing enzymes, both at the kinetic maximum (76gl(-1)) and at reaction equilibrium (44gl(-1)). The total FOS production in the kinetic maximum was 101gl(-1), which corresponded to 16.4% (w/w) referred to the total carbohydrates in the reaction mixture.

A family 2 pectate lyase displays a rare fold and transition metal-assisted beta-elimination

The family 2 pectate lyase from Yersinia enterocolitica (YePL2A), solved to 1.5A, reveals it to be the first prokaryotic protein reported to display the rare (alpha/alpha)(7) barrel fold. In addition to its apo form, we have also determined the structure of a metal-bound form of YePL2A (to 2.0A) and a trigalacturonic acid-bound substrate complex (to 2.1A) Although its fold is rare, the catalytic center of YePL2A can be superimposed with structurally unrelated families, underlining the conserved catalytic amino acid architecture of the beta-elimination mechanism. In addition to its overall structure, YePL2A also has two other unique features: 1) it utilizes a metal atom other than calcium for catalysis, and 2) its Brønstead base is in an alternate conformation and directly interacts with the uronate group of the substrate.

Synthesis of the 'primer-adaptor' trisaccharide moiety of Escherichia coli O8, O9, and O9a lipopolysaccharide

Described is the synthesis of the trisaccharide alpha-D-Manp-(1-->3)-alpha-D-Manp-(1-->3)-beta-D-GlcpNAcO(CH2)8N3, the glycan portion of which corresponds to the 'adaptor-primer' moiety linking the O-chain and core oligosaccharide in the lipopolysaccharide of several Escherichia coli and Klebsiella pneumoniae serotypes. This report represents the first synthesis of this trisaccharide motif, and in the route involved, a key step is a [2+1] coupling of a protected Manp-(1-->3)-alpha-D-Manp glycosyl donor with a GlcpNAc acceptor. The azido group was included in the target to facilitate future preparation of neoglycoconjugates.

Alternative Neisseria spp. type IV pilin glycosylation with a glyceramido acetamido trideoxyhexose residue

The importance of protein glycosylation in the interaction of pathogenic bacteria with their host is becoming increasingly clear. Neisseria meningitidis, the etiological agent of cerebrospinal meningitis, crosses cellular barriers after adhering to host cells through type IV pili. Pilin glycosylation genes (pgl) are responsible for the glycosylation of PilE, the major subunit of type IV pili, with the 2,4-diacetamido-2,4,6-trideoxyhexose residue. Nearly half of the clinical isolates, however, display an insertion in the pglBCD operon, which is anticipated to lead to a different, unidentified glycosylation. Here the structure of pilin glycosylation was determined in such a strain by "top-down" MS approaches. MALDI-TOF, nanoelectrospray ionization Fourier transform ion cyclotron resonance, and nanoelectrospray ionization quadrupole TOF MS analysis of purified pili preparations originating from N. meningitidis strains, either wild type or deficient for pilin glycosylation, revealed a glycan mass inconsistent with 2,4-diacetamido-2,4,6-trideoxyhexose or any sugar in the databases. This unusual modification was determined by in-source dissociation of the sugar from the protein followed by tandem MS analysis with collision-induced fragmentation to be a hexose modified with a glyceramido and an acetamido group. We further show genetically that the nature of the sugar present on the pilin is determined by the carboxyl-terminal region of the pglB gene modified by the insertion in the pglBCD locus. We thus report a previously undiscovered monosaccharide involved in posttranslational modification of type IV pilin subunits by a MS-based approach and determine the molecular basis of its biosynthesis.

Continuous production of lactosucrose by immobilized Sterigmatomyces elviae mutant

In this study, in order to develop a continuous production process of lactosucrose in a packed-bed reactor, Sterigmatomyces elviae ATCC 18894 was selected and mutated. The mutant strain of S. elviae showed 54.3% higher lactosucrose production than the wild type. Reaction conditions such as temperature, pH, substrate concentration and flow rate were also optimized. Under optimized reaction conditions (50 degrees C, pH 6.0, 25% sucrose and 25% lactose as substrate, flow rate 1.2 ml/min), the maximum concentration of lactosucrose (192 g/l) was obtained. In a packed-bed reactor, continuous production of lactosucrose was performed using S. elviae mutant immobilized in calcium alginate, and about 180 g/l of lactosucrose production was achieved for 48 days.

Synthesis and conformational analysis of an α-d-mannopyranosyl-(1→2)-α-d-mannopyranosyl-(1→6)-α-d-mannopyranose mimic

A mimic of a (1-->2),(1-->6)-mannotrioside was synthesized by replacing the central mannose unit with an enantiomerically pure, conformationally stable trans-diaxial cyclohexanediol. The three-dimensional structure of the molecule was investigated by NMR spectroscopy supported by molecular modelling and was compared to the known features of the natural mannotrioside.

Synthetic studies of complex immunostimulants from Quillaja saponaria: synthesis of the potent clinical immunoadjuvant QS-21Aapi

QS-21 is one of the most promising new adjuvants for immune response potentiation and dose-sparing in vaccine therapy given its exceedingly high level of potency and its favorable toxicity profile. Melanoma, breast cancer, small cell lung cancer, prostate cancer, HIV-1, and malaria are among the numerous maladies targeted in more than 80 recent and ongoing vaccine therapy clinical trials involving QS-21 as a critical adjuvant component for immune response augmentation. QS-21 is a natural product immunostimulatory adjuvant, eliciting both T-cell- and antibody-mediated immune responses with microgram doses. Herein is reported the synthesis of QS-21A(api) in a highly modular strategy, applying novel glycosylation methodologies to a convergent construction of the potent saponin immunostimulant. The chemical synthesis of QS-21 offers unique opportunities to probe its mode of biological action through the preparation of otherwise unattainable nonnatural saponin analogues.

Structure of Saccharomyces cerevisiae chitinase 1 and screening-based discovery of potent inhibitors

Chitinases hydrolyse the beta(1,4)-glycosidic bonds of chitin, an essential fungal cell wall component. Genetic data on a subclass of fungal family 18 chitinases have suggested a role in cell wall morphology. Specific inhibitors of these enzymes would be useful as tools to study their role in cell wall morphogenesis and could possess antifungal properties. Here, we describe the crystallographic structure of a fungal "plant-type" family 18 chitinase, that of Saccharomyces cerevisiae CTS1. The enzyme is active against 4-methylumbelliferyl chitooligosaccharides and displays an unusually low pH optimum for activity. A library screen against ScCTS1 yielded hits with Ki 's as low as 3.2 microM. Crystal structures of ScCTS1 in complex with inhibitors from three series reveal striking mimicry of carbohydrate substrate by small aromatic moieties and a pocket that could be further exploited in optimization of these inhibitors.

Affinities of Shiga toxins 1 and 2 for univalent and oligovalent Pk-trisaccharide analogs measured by electrospray ionization mass spectrometry

The binding stoichiometry and affinities of the Shiga toxins, Stx1 and Stx2, for a series of uni- and oligovalent analogs of the Pk-trisaccharide were measured using the direct electrospray ionization mass spectrometry (ES-MS) assay. Importantly, it is shown that, for a given ligand, Stx1 and Stx2 exhibit similar affinities. The binding data suggest a high degree of similarity in the spatial arrangement and structural characteristics of the Pk binding sites in Stx1 and Stx2. The results confirm that both toxins recognize the alpha-D-Galp(1-->4)-beta-D-Galp(1-->4)-beta-D-Glcp carbohydrate motif of the cell surface glycolipid Gb3. This, taken together with the results of the chemical mapping study, suggests that the nature of the Pk binding interactions with Stx1 and Stx2 are similar. The affinities of Stx1-B(5) and Stx2 for the multivalent ligands reveals that site 2 of Stx2, which shares the same spatial arrangement as site 2 in Stx1, is the primary Pk binding site and that site 1 of Stx1 and of Stx2 can also participate in Pk binding.