Gln Residues Research Articles

Conformational studies of immunodominant myelin basic protein 1–11 analogues using NMR and molecular modeling

Τwo dimensional nuclear magnetic resonance studies complimented by molecular dynamics simulations were conducted to investigate the conformation of the immunodominant epitope of acetylated myelin basic protein residues 1-11 (Ac-MBP(1-11)) and its altered peptide ligands, mutated at position 4 to an alanine (Ac-MBP(1-11)[4A]) or a tyrosine residue (Ac-MBP(1-11)[4Y]). Conformational analysis of the three analogues indicated that they adopt an extended conformation in DMSO solution as no long distance NOE connectivities were observed and seem to have a similar conformation when bound to the active site of the major histocompatibility complex (MHC II). The interaction of each peptide with MHC class II I-A(u) was further investigated in order to explore the molecular mechanism of experimental autoimmune encephalomyelitis induction/inhibition in mice. The present findings indicate that the Gln(3) residue, which serves as a T-cell receptor (TCR) contact site in the TCR/peptide/I-A(u) complex, has a different orientation in the mutated analogues especially in the Ac-MBP(1-11)[4A] peptide. In particular the side chain of Gln(3) is not solvent exposed as for the native Ac-MBP(1-11) and it is not available for interaction with the TCR.

An Extended β7α7 Substrate-Binding Loop Is Essential for Efficient Catalysis by 3-Deoxy-d-manno-Octulosonate 8-Phosphate Synthase

The enzyme 3-deoxy-D-manno-octulosonate 8-phosphate (KDO8P) synthase catalyzes the reaction between phosphoenolpyruvate and arabinose 5-phosphate (A5P) in the first committed step in the biosynthetic pathway for the formation of 3-deoxy-D-manno-octulosonate, an important component in the cell wall of Gram-negative bacteria. KDO8P synthase is evolutionarily related to the first enzyme of the shikimate pathway, 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAH7P) synthase, which uses erythrose 4-phosphate in place of A5P. The A5P binding site in KDO8P synthase is formed by three long loops that extend from the core catalytic (β/α)(8) barrel, β2α2, β7α7, and β8α8. The extended β7α7 loop is always present in KDO8P synthase yet is not observed for DAH7P synthase. Modeling of this loop indicated interactions between this loop and the extended β2α2 loop; both loops provide key hydrogen-bonding contacts with A5P. The two absolutely conserved residues on the β7α7 loop (Gln and Ser) were mutated to Ala in both the metal-dependent KDO8P synthase from Acidithiobacillus ferrooxidans and the metal-independent KDO8P synthase from Neisseria meningitidis. In addition, mutants were constructed for both enzymes with the extended β7α7 loop excised to match the DAH7P synthase architecture. Removal of the loop extension severely hindered efficient catalysis, dramatically increasing the K(m)(A5P) and reducing the k(cat) for both enzymes. Excision of the complete loop was far more detrimental to catalysis than the double mutations of the two conserved Gln and Ser residues. Therefore, the presence of the entire extended β7α7 loop is important for efficient catalysis by KDO8P synthase, with the loop acting to promote efficient and productive binding of A5P.

Negative control in two-component signal transduction by transmitter phosphatase activity

Bifunctional sensor transmitter modules of two-component systems exert both positive and negative control on the receiver domain of the cognate response regulator. In negative control, the transmitter module accelerates the rate of phospho-receiver dephosphorylation. This transmitter phosphatase reaction serves the important physiological functions of resetting response regulator phosphorylation level and suppressing cross-talk. Although the biochemical reactions underlying positive control are reasonably well understood, the mechanism for transmitter phosphatase activity has been unknown. A recent hypothesis is that the transmitter phosphatase reaction is catalysed by a conserved Gln, Asn or Thr residue, via a hydrogen bond between the amide or hydroxyl group and the nucleophilic water molecule in acyl-phosphate hydrolysis. This hypothetical mechanism closely resembles the established mechanisms of auxiliary phosphatases such as CheZ and CheX, and may be widely conserved in two-component signal transduction. In addition to the proposed catalytic residues, transmitter phosphatase activity also requires the correct transmitter conformation and appropriate interactions with the receiver. Evidence suggests that the phosphatase-competent and autokinase-competent states are mutually exclusive, and the corresponding negative and positive activities are likely to be reciprocally regulated through dynamic control of transmitter conformations.

Human inter-α-inhibitor is a substrate for factor XIIIa and tissue transglutaminase

In this study, we show that inter-α-inhibitor is a substrate for both factor XIIIa and tissue transglutaminase. These enzymes catalyze the incorporation of dansylcadaverine and biotin–pentylamine, revealing that inter-α-inhibitor contains reactive Gln residues within all three subunits. These findings suggest that transglutaminases catalyze the covalent conjugation of inter-α-inhibitor to other proteins. This was demonstrated by the cross-linking between inter-α-inhibitor and fibrinogen by either factor XIIIa or tissue transglutaminase. Finally, using quantitative mass spectrometry, we show that inter-α-inhibitor is cross-linked to the fibrin clot in a 1:20 ratio relative to the known factor XIIIa substrate α2-antiplasmin. This interaction may protect fibrin or other Lys-donating proteins from adventitious proteolysis by increasing the local concentration of bikunin. In addition, the reaction may influence the TSG-6/heavy Chain 2-mediated transfer of heavy chains observed during inflammation.

Comparative fibril formation of analogs corresponding to the (12–24) segment of the β-amyloid peptide

The (1-42) β-amyloid peptide is a main component of the plaques found in the brain of patients suffering from the Alzheimer's disease. As the single substitution of Glu for Gln at position 22 of this peptide seems to be responsible for the manifestation of the more severe amyloidosis (Dutch-type), we decided to evaluate the aggregation characteristics of peptide analogs interchanging Glu and Gln residues at positions 22 and also 15 in the minor (12-24) (VHHQ(15)KLVFFAE(22)DV) fragment. The Q15Q22, E15E22, E15Q22 and the native Q15E22 were compared to the (1-42) β-amyloid peptide in terms of fibril or structured aggregates formation propensity. In contrast to a rather similar solubility data measured of all analogs, fluorescence and light scattering methods indicated that only Q15E22 and Q15Q22 displayed relevant fibril formation capacity. Conversely, E15E22 and E15Q22 were not capable of the formation of this type of structure thus suggesting a key role for the Q(15) residue in the unique aggregation characteristic of the β-amyloid peptide.

Crystal Structure of Human ISG15 Protein in Complex with Influenza B Virus NS1B

ISG15 (interferon-stimulated gene 15), the first ubiquitin-like protein (UBL) identified, has emerged as an important cellular antiviral factor. It consists of two UBL domains with a short linker between them. The covalent attachment of ISG15 to host and viral proteins to modify their functions, similar to ubiquitylation, is named ISGylation. Influenza B virus NS1B protein antagonizes human but not mouse ISGylation because NS1B exhibits species specificity; it only binds human and non-human primate ISG15. Previous studies have demonstrated that the N-terminal UBL domain and linker of ISG15 are required for the binding by NS1B and that the linker plays a large role in the species specificity, but the structural basis for them has not been elucidated. Here we report the crystal structure of human ISG15 in complex with NS1B at a resolution of 2.0 Å. A loop in the ISG15 N-terminal UBL domain inserts into a pocket in the NS1B dimer, forming a high affinity binding site. The nonspecific van der Waals contacts around the ISG15 linker form a low affinity site for NS1B binding. However, sequence alignment reveals that residues in the high affinity site are highly conserved in primate and non-primate ISG15. We propose that the low affinity binding around the ISG15 linker is important for the initial contact with NS1B and that the stable complex formation is largely contributed by the following high affinity interactions between ISG15 N-terminal UBL domain and NS1B. This provides a structural basis for the species-specific binding of ISG15 by the NS1B protein.

A New Approach to Obtaining N α‐t‐Boc‐Amino Acid Aldehydes from Asparagine and Glutamine for Reduced Amide Pseudopeptide Solid‐Phase Synthesis

Reduced amide pseudopeptides have been proposed as structural probes that could be useful as potential malarial vaccine components. However, designing determined pseudopeptide sequences containing isoster peptide bonds, either on an asparagine (Asn) or on a glutamine (Gln) residues, can become difficult because these precursor amino acid aldehydes are obtained in yields lower than 0.5%. This work presents a new strategy for obtaining both Asn and Gln aldehydes based on a controlled side-chain protection approach as well as a suitable solvent partition procedure. FT-IR, (1) H-NMR and (13) C-NMR were used for molecule characterization and identification. Amino acid aldehydes were successfully incorporated into a 20-mer peptide from a malarial-relevant sequence, and their impact on the molecule's conformational properties was assessed.

Interaction with Polyglutamine-expanded Huntingtin Alters Cellular Distribution and RNA Processing of Huntingtin Yeast Two-hybrid Protein A (HYPA)

Huntington disease (HD) is an autosomal inherited disorder that causes the deterioration of brain cells. The polyglutamine (polyQ) expansion of huntingtin (Htt) is implicated in the pathogenesis of HD via interaction with an RNA splicing factor, Htt yeast two-hybrid protein A/forming-binding protein 11 (HYPA/FBP11). Besides the pathogenic polyQ expansion, Htt also contains a proline-rich region (PRR) located exactly in the C terminus to the polyQ tract. However, how the polyQ expansion influences the PRR-mediated protein interaction and how this abnormal interaction leads to the biological consequence remain elusive. Our NMR structural analysis indicates that the PRR motif of Htt cooperatively interacts with the tandem WW domains of HYPA through domain chaperoning effect of WW1 on WW2. The polyQ-expanded Htt sequesters HYPA to the cytosolic location and then significantly reduces the efficiency of pre-mRNA splicing. We propose that the toxic gain-of-function of the polyQ-expanded Htt that causes dysfunction of cellular RNA processing contributes to the pathogenesis of HD.

Site-Specific Modification Of Proteins Mediated By Transglutaminase

AbstractTransglutaminase (TGase) catalyzes an acyl transfer reaction between the [gamma]-carboxamide group of glutamine (Gln) and the [epsilon]-amino group of lysine (Lys) residues to form a stable amide bond. The TGase reaction can be used for bioconjugation of an amino-derivative of poly-ethylene glycol (PEG) to protein drugs, leading to PEGylated proteins that display increased bioactivity and stability. The procedure was shown to lead to site-specific bioconjugation of few proteins, thus offering a valid alternative to the chemical methods of PEGylation in current use (1). Moreover, TGase can be used for site-specifically labeling proteins with fluorescent groups at the level of Gln or Lys for diagnostic applications. The TGase attack is not entirely dependent on a specific consensus sequence around the Gln or Lys residues. However, a correlation between the TGase-mediated sites of PEGylation and the chain flexibility has been observed (2). Our study was focused to elucidate the molecular features favoring the TGase mediated site-specific reactions on two extensively studied model proteins, i.e. apomyoglobin (apoMb) and lysozyme. Besides amino-PEG, we used dansyl-cadaverine as an acyl acceptor and N-carbobenzoxy-Gln-Gly-OH as acyl donor for the TGase reactions. The sites of protein modification were determined by fingerprinting and ESI mass spectrometry. Myoglobin in its holo form is not susceptible to TGase reactions due to its rigid conformation,but the apo form was conjugated with PEG and fluorescent labels at the level of helix F (chain segment 82–99). The NMR study on apoMb had earlier demonstrated increased flexibility of the helix F (3) and, moreover, several proteases cleave the 153-residue chain of apoMb at the level of helix F only (4). Therefore, the chain region attacked by both TGase and proteases is a flexible or unfolded site(s). Lysozyme in its disulfide crosslinked native state is highly resistant to proteases and TGase attack. However, the more dynamic three-disulphide derivative of lysozyme, lacking the Cys6–Cys127 disulfide bridge, is susceptible to TGase-mediated reactions, as well as limited proteolysis. These results indicate that the sites of TGase reactions and the sites of limited proteolysis have a clear analogy for their presence in flexible/disordered regions of protein substrates. Overall, our studies clearly demonstrate that TGase-mediated reactions occur only at disordered chain regions, as evidenced by the correlation between sites of the TGase reaction and sites of enhanced chain flexibility, the latter deduced from the crystallographic Bfactor.

Studies on Lactoferricin-derived Escherichia coli Membrane-active Peptides Reveal Differences in the Mechanism of N-Acylated Versus Nonacylated Peptides

To improve the low antimicrobial activity of LF11, an 11-mer peptide derived from human lactoferricin, mutant sequences were designed based on the defined structure of LF11 in the lipidic environment. Thus, deletion of noncharged polar residues and strengthening of the hydrophobic N-terminal part upon adding a bulky hydrophobic amino acid or N-acylation resulted in enhanced antimicrobial activity against Escherichia coli, which correlated with the peptides' degree of perturbation of bacterial membrane mimics. Nonacylated and N-acylated peptides exhibited different effects at a molecular level. Nonacylated peptides induced segregation of peptide-enriched and peptide-poor lipid domains in negatively charged bilayers, although N-acylated peptides formed small heterogeneous domains resulting in a higher degree of packing defects. Additionally, only N-acylated peptides perturbed the lateral packing of neutral lipids and exhibited increased permeability of E. coli lipid vesicles. The latter did not correlate with the extent of improvement of the antimicrobial activity, which could be explained by the fact that elevated binding of N-acylated peptides to lipopolysaccharides of the outer membrane of gram-negative bacteria seems to counteract the elevated membrane permeabilization, reflected in the respective minimal inhibitory concentration for E. coli. The antimicrobial activity of the peptides correlated with an increase of membrane curvature stress and hence bilayer instability. Transmission electron microscopy revealed that only the N-acylated peptides induced tubular protrusions from the outer membrane, whereas all peptides caused detachment of the outer and inner membrane of E. coli bacteria. Viability tests demonstrated that these bacteria were dead before onset of visible cell lysis.

Cellular and molecular action of the mitogenic protein-deamidating toxin from Pasteurella multocida.

The mitogenic toxin from Pasteurella multocida (PMT) is a member of the dermonecrotic toxin family, which includes toxins from Bordetella, Escherichia coli and Yersinia. Members of the dermonecrotic toxin family modulate G-protein targets in host cells through selective deamidation and/or transglutamination of a critical active site Gln residue in the G-protein target, which results in the activation of intrinsic GTPase activity. Structural and biochemical data point to the uniqueness of PMT among these toxins in its structure and action. Whereas the other dermonecrotic toxins act on small Rho GTPases, PMT acts on the α subunits of heterotrimeric G(q) -, G(i) - and G(12/13) -protein families. To date, experimental evidence supports a model in which PMT potently stimulates various mitogenic and survival pathways through the activation of G(q) and G(12/13) signaling, ultimately leading to cellular proliferation, whilst strongly inhibiting pathways involved in cellular differentiation through the activation of G(i) signaling. The resulting cellular outcomes account for the global physiological effects observed during infection with toxinogenic P. multocida, and hint at potential long-term sequelae that may result from PMT exposure.

Influence of TiO2 on prebiotic thermal synthesis of the Gly-Gln polymer

The role of the titanium dioxide (rutile and anatase) with and without room light on the thermal synthesis of the glycine-L-glutamine (Gly-Gln) polymer is described. The efficiency in percentage of polymerization with room light was increased in 6% in the presence of rutile and in 23% in the presence of anatase. The thermal synthesis in the molten state was carried out in the absence and presence of both oxides. In all cases, the vibrational spectra showed characteristic group frequencies corresponding to a polypeptide structure. No spectral differences were observed by room light effect on the polymer on rutile. However, the polymer obtained in the presence of anatase and room light shows spectral changes associated with the formation of shorter new abundant and conformationally different species compared with the original polymer. The SEM-EDX characterization of the solid phase involved in the thermal synthesis showed that the morphology of the polypeptide is different in the presence of rutile compared to anatase. The SDS-PAGE and GPC results suggest that smaller chains are formed in the presence of both oxides and the distribution of the size and weight of each polymer molecule is completely different when the condensation is performed in the presence of anatase or rutile. Nuclear magnetic resonance analyses confirmed the incorporation of both Gly and Gln residues in the polymers, with a prevalence of Gly. Both possible sequences N-GlyGln-C and N-GlnGly-C were also detected.

Structural Insights into the Novel Diadenosine 5′,5‴- P1, P4-Tetraphosphate Phosphorylase from Mycobacterium tuberculosis H37Rv

Rv2613c is a diadenosine 5′,5‴- P 1, P 4-tetraphosphate (Ap 4A) phosphorylase from Mycobacterium tuberculosis H37Rv. Sequence analysis suggests that Rv2613c belongs to the histidine triad (HIT) motif superfamily, which includes HIT family diadenosine polyphosphate (Ap n A) hydrolases and Ap 4A phosphorylases. However, the amino acid sequence of Rv2613c is more similar to that of HIT family Ap n A hydrolases than to that of typical Ap 4A phosphorylases. Here, we report the crystal structure of Rv2613c, which is the first structure of a protein with Ap n A phosphorylase activity, and characterized the structural basis of its catalytic activity. Our results showed that the structure of Rv2613c is similar to those of other HIT superfamily proteins. However, Asn139, Gly146, and Ser147 in the active site of Rv2613c replace the corresponding Gln, Gln, and Thr residues that are normally found in HIT family Ap n A hydrolases. Furthermore, analyses of Rv2613c mutants revealed that Asn139, Gly146, and Ser147 are important active-site residues and that Asn139 has a critical role in catalysis. The position of Gly146 might influence the phosphorylase activity. In addition, the tetrameric structure of Rv2613c and the presence of Trp160 might be essential for the formation of the Ap 4A binding site. These structural insights into Rv2613c may facilitate the development of novel structure-based inhibitors for treating tuberculosis.

Designed Coiled Coils Promote Folding of a Recombinant Bacterial Collagen

Collagen triple helices fold slowly and inefficiently, often requiring adjacent globular domains to assist this process. In the Streptococcus pyogenes collagen-like protein Scl2, a V domain predicted to be largely α-helical, occurs N-terminal to the collagen triple helix (CL). Here, we replace this natural trimerization domain with a de novo designed, hyperstable, parallel, three-stranded, α-helical coiled coil (CC), either at the N terminus (CC-CL) or the C terminus (CL-CC) of the collagen domain. CD spectra of the constructs are consistent with additivity of independently and fully folded CC and CL domains, and the proteins retain their distinctive thermal stabilities, CL at ∼37 °C and CC at >90 °C. Heating the hybrid proteins to 50 °C unfolds CL, leaving CC intact, and upon cooling, the rate of CL refolding is somewhat faster for CL-CC than for CC-CL. A construct with coiled coils on both ends, CC-CL-CC, retains the ∼37 °C thermal stability for CL but shows less triple helix at low temperature and less denaturation at 50 °C. Most strikingly however, in CC-CL-CC, the CL refolds slower than in either CC-CL or CL-CC by almost two orders of magnitude. We propose that a single CC promotes folding of the CL domain via nucleation and in-register growth from one end, whereas initiation and growth from both ends in CC-CL-CC results in mismatched registers that frustrate folding. Bioinformatics analysis of natural collagens lends support to this because, where present, there is generally only one coiled-coil domain close to the triple helix, and it is nearly always N-terminal to the collagen repeat.

A residue in the TRPM2 channel outer pore is crucial in determining species-dependent sensitivity to extracellular acidic pH

Acidic pH is an important parameter regulating ion channel activity and its biological function. This study investigated inhibition of the hTRPM2 channels by extracellular acidic pH and compared the sensitivity of human (h) and mouse (m) TRPM2 channel to such an inhibition. The initial inhibition of hTRPM2 channel currents was substantially reversible, but the reversibility progressively diminished as the exposure to acidic pH was prolonged and it was essentially lost in the steady state, suggesting that extracellular acidic pH induces initial reversible inhibition and subsequent irreversible inactivation. Like the hTRPM2 channel, the mTRPM2 channel was sensitive to inhibition by pH4.0-5.5, but the kinetics was significantly slower. Moreover, in contrast to the complete inhibition of the hTRPM2 channel, the mTRPM2 channel was insensitive to pH6.0. Replacement of residue Gln(992) in the outer pore with the equivalent residue His(995) in the hTRPM2 channel resulted in a mutant mTRPM2 channel with the pH sensitivity and kinetics of inhibition of the wild-type hTRPM2 channel. Conversely, the reciprocal mutation H995Q in the hTRPM2 channel dramatically slowed down the kinetics of inhibition. Swapping other residues in the pore region failed to produce such opposing effects. Taken together, our results suggest a crucial role of residue His(995)/Gln(992) in the outer pore of TRPM2 channels in determining species-dependent effects of extracellular acidic pH.

Binding of Quercetin to Lysozyme as Probed by Spectroscopic Analysis and Molecular Simulation

The binding of quercetin to lysozyme (LYSO) in aqueous solution was investigated by fluorescence spectroscopy, UV-vis absorption spectroscopy and molecular simulation at pH7.4. The fluorescence quenching of LYSO by addition of quercetin is due to static quenching, the binding constants, K ( a ), were 3.63 × 10(4), 3.31 × 10(4) and 2.85 × 10(4)L·mol(-1) at 288, 298 and 308K, respectively. The thermodynamic parameters, enthalpy change, ∆H, and entropy change, ∆S, were noted to be -7.56kJ·mol(-1) and 61.07J·mol(-1)·K(-1). The results indicated that hydrophobic interaction may play a major role in the binding process. The distance r between the donor (LYSO) and acceptor (quercetin) was determined as 3.34nm by the fluorescence resonance energy transfer. The synchronous fluorescence spectroscopy showed the polarity around the tryptophan residues increased and the hydrophobicity decreased. Furthermore, the study of molecular simulation indicated that quercetin could bind to the active site (a pocket made up of 24 amino-acid residues) of LYSO mainly via hydrophobic interactions and that there were hydrogen interactions between the residues (Gln 57, Ile 98) of LYSO and quercetin. The accessible surface area (ASA) calculation verified the important roles of tryptophan (Trp) residues during the binding process.

Sugar Binding Residue Affects Apparent Na+ Affinity and Transport Stoichiometry in Mouse Sodium/Glucose Cotransporter Type 3B

SGLT1 is a sodium/glucose cotransporter that moves two Na(+) ions with each glucose molecule per cycle. SGLT3 proteins belong to the same family and are described as glucose sensors rather than glucose transporters. Thus, human SGLT3 (hSGLT3) does not transport sugar, but extracellular glucose depolarizes the cell in which it is expressed. Mouse SGLT3b (mSGLT3b), although it transports sugar, has low apparent sugar affinity and partially uncoupled stoichiometry compared with SGLT1, suggesting that mSGLT3b is also a sugar sensor. The crystal structure of the Vibrio parahaemolyticus SGLT showed that residue Gln(428) interacts directly with the sugar. The corresponding amino acid in mammalian proteins, 457, is conserved in all SGLT1 proteins as glutamine. In SGLT3 proteins, glutamate is the most common residue at this position, although it is a glycine in mSGLT3b and a serine in rat SGLT3b. To test the contribution of this residue to the function of SGLT3 proteins, we constructed SGLT3b mutants that recapitulate residue 457 in SGLT1 and hSGLT3, glutamine and glutamate, respectively. The presence of glutamine at residue 457 increased the apparent Na(+) and sugar affinities, whereas glutamate decreased the apparent Na(+) affinity. Moreover, glutamate transported more cations per sugar molecule than the wild type protein. We propose a model where cations are released intracellularly without the release of sugar from an intermediate state. This model explains the uncoupled charge:sugar transport phenotype observed in wild type and G457E-mSGLT3b compared with SGLT1 and the sugar-activated cation transport without sugar transport that occurs in hSGLT3.

Crystal Structure and Inhibition Studies of Transglutaminase from Streptomyces mobaraense

The crystal structure of the microbial transglutaminase (MTGase) zymogen from Streptomyces mobaraense has been determined at 1.9-Å resolution using the molecular replacement method based on the crystal structure of the mature MTGase. The overall structure of this zymogen is similar to that of the mature form, consisting of a single disk-like domain with a deep active cleft at the edge of the molecule. A major portion of the prosequence (45 additional amino acid residues at the N terminus of the mature transglutaminase) folds into an L-shaped structure, consisting of an extended N-terminal segment linked with a one-turn short helix and a long α-helix. Two key residues in the short helix of the prosequence, Tyr-12 and Tyr-16, are located on top of the catalytic triad (Cys-110, Asp-301, and His-320) to block access of the substrate acyl donors and acceptors. Biochemical characterization of the mature MTGase, using N-α-benzyloxycarbonyl-L-glutaminylglycine as a substrate, revealed apparent K(m) and k(cat)/K(m) values of 52.66 mM and 40.42 mM(-1) min(-1), respectively. Inhibition studies using the partial prosequence SYAETYR and homologous sequence SQAETYR showed a noncompetitive inhibition mechanism with IC(50) values of 0.75 and 0.65 mM, respectively, but no cross-linking product formation. Nevertheless, the prosequence homologous oligopeptide SQAETQR, with Tyr-12 and Tyr-16 each replaced with Gln, exhibited inhibitory activity with the formation of the SQAETQR-monodansylcadaverine fluorophore cross-linking product (SQAETQR-C-DNS). MALDI-TOF tandem MS analysis of SQAETQR-C-DNS revealed molecular masses corresponding to those of (N)SQAETQ(C)-C-DNS and C-DNS-(N)QR(C) sequences, suggesting the incorporation of C-DNS onto the C-terminal Gln residue of the prosequence homologous oligopeptide. These results support the putative functional roles of both Tyr residues in substrate binding and inhibition.

Peptide-Based Imaging Agents Targeting Phosphatidylserine for the Detection of Apoptosis

A 14-residue phosphatidylserine (PS)-binding peptide FNFRLKAGQKIRFG (PSBP-0) was scanned with Ala. In addition, a radiometal chelator (SAAC) was introduced at selected sites of the lead peptides. Substitution of the Gln(6) residue in PSBP-0 with Ala resulted in a significant increase in binding affinity to PS as determined by surface plasmon resonance sensorgrams. The binding affinity of the resulting peptide FNFRLKAGAKIRFG (PSBP-6, molecular mass = 1623 Da) to PS (K(d) ∼ 100 nM) increased 10-fold as compared to PSBP-0 (K(d) ∼ 1.38 μM). Introduction of SAAC-Re to the N terminus of PSBP-6 further increased the binding affinity of the resulting peptide SAAC(Re)-PSBP-6 (K(d) ∼ 26 nM). SAAC(Re)-PSBP-6 shows specific binding to apoptotic cells in cell-based assays. Biodistribution studies showed significantly higher uptake of SAAC((99 m)Tc)-PSBP-6 in B16/F10 melanoma treated with poly(L-glutamic acid)-paclitaxel than untreated tumors (4.06 ± 0.55% ID/g vs 1.61 ± 0.33% ID/g, P = 0.00011). SAAC((99 m)Tc)-PSBP-6 is a promising probe for noninvasive imaging of apoptotic cells.

Sequence Elucidation of an Unknown Cyclic Peptide of High Doping Potential by ETD and CID Tandem Mass Spectrometry

Identification of an unknown substance without any information remains a daunting challenge despite advances in chemistry and mass spectrometry. However, an unknown cyclic peptide in a sample with very limited volume seized at a Pennsylvania racetrack has been successfully identified. The unknown sample was determined by accurate mass measurements to contain a small unknown peptide as the major component. Collision-induced dissociation (CID) of the unknown peptide revealed the presence of Lys (not Gln, by accurate mass), Phe, and Arg residues, and absence of any y-type product ion. The latter, together with the tryptic digestion results of the unusual deamidation and absence of any tryptic cleavage, suggests a cyclic structure for the peptide. Electron-transfer dissociation (ETD) of the unknown peptide indicated the presence of Gln (not Lys, by the unusual deamidation), Phe, and Arg residues and their connectivity. After all the results were pieced together, a cyclic tetrapeptide, cyclo[Arg-Lys-N(C(6)H(9))Gln-Phe], is proposed for the unknown peptide. Observations of different amino acid residues from CID and ETD experiments for the peptide were interpreted by a fragmentation pathway proposed, as was preferential CID loss of a Lys residue from the peptide. ETD was used for the first time in sequencing of a cyclic peptide; product ions resulting from ETD of the peptide identified were categorized into two types and named pseudo-b and pseudo-z ions that are important for sequencing of cyclic peptides. The ETD product ions were interpreted by fragmentation pathways proposed. Additionally, multi-stage CID mass spectrometry cannot provide complete sequence information for cyclic peptides containing adjacent Arg and Lys residues. The identified cyclic peptide has not been documented in the literature, its pharmacological effects are unknown, but it might be a "designer" drug with athletic performance-enhancing effects.