7 research outputs found
Gauge copies in the Landau-DeWitt gauge: a background invariant restriction
The Landau background gauge, also known as the Landau-DeWitt gauge, has found
renewed interest during the past decade given its usefulness in accessing the
confinement-deconfinement transition via the vacuum expectation value of the
Polyakov loop, describable via an appropriate background. In this Letter, we
revisit this gauge from the viewpoint of it displaying gauge (Gribov) copies.
We generalize the Gribov-Zwanziger effective action in a BRST and background
invariant way; this action leads to a restriction on the allowed gauge
fluctuations, thereby eliminating the infinitesimal background gauge copies.
The explicit background invariance of our action is in contrast with earlier
attempts to write down and use an effective Gribov-Zwanziger action. It allows
to address certain subtleties arising in these earlier works, such as a
spontaneous and thus spurious Lorentz symmetry breaking, something which is now
averted.Comment: 14 pages. v2: version to appear in Phys.Lett.B, with minor
modifications and extra reference
NMR and Molecular Recognition of NāGlycans: Remote Modifications of the Saccharide Chain Modulate Binding Features
Glycans
play a key role as recognition elements in the communication
of cells and other organisms. Thus, the analysis of carbohydrateāprotein
interactions has gained significant importance. In particular, nuclear
magnetic resonance (NMR) techniques are considered powerful tools
to detect relevant features in the interaction between sugars and
their natural receptors. Here, we present the results obtained in
the study on the molecular recognition of different mannose-containing
glycans by <i>Pisum sativum</i> agglutinin. NMR experiments
supported by Corcema-ST analysis, isothermal titration calorimetry
(ITC) experiments, and molecular dynamics (MD) protocols have been
successfully applied to unmask important binding features and especially
to determine how a remote branching substituent significantly alters
the binding mode of the sugar entity. These results highlight the
key influence of common structural modifications in natural glycans
on molecular recognition processes and underscore their importance
for the development of biomedical applications
Immobilization of Biantennary NāGlycans Leads to Branch Specific Epitope Recognition by LSECtin
The molecular recognition features of LSECtin toward
asymmetric
N-glycans have been scrutinized by NMR and compared to those occurring
in glycan microarrays. A pair of positional glycan isomers (LDN3 and
LDN6), a nonelongated GlcNAc4Man3 N-glycan (G0), and the minimum binding
epitope (the GlcNAcĪ²1-2Man disaccharide) have been used to shed
light on the preferred binding modes under both experimental conditions.
Strikingly, both asymmetric LDN3 and LDN6 N-glycans are recognized
by LSECtin with similar affinities in solution, in sharp contrast
to the results obtained when those glycans are presented on microarrays,
where only LDN6 was efficiently recognized by the lectin. Thus, different
results can be obtained using different experimental approaches, pointing
out the tremendous difficulty of translating in vitro results to the in vivo environment
Molecular Recognition of Complex-Type Biantennary <i>N</i>āGlycans by Protein Receptors: a Three-Dimensional View on Epitope Selection by NMR
The current surge in defining glycobiomarkers by applying
lectins
rekindles interest in definition of the sugar-binding sites of lectins
at high resolution. Natural complex-type <i>N</i>-glycans
can present more than one potential binding motif, posing the question
of the actual mode of interaction when interpreting, for example,
lectin array data. By strategically combining <i>N</i>-glycan
preparation with saturation-transfer difference NMR and modeling,
we illustrate that epitope recognition depends on the structural context
of both the sugar and the lectin (here, wheat germ agglutinin and
a single hevein domain) and cannot always be predicted from simplified
model systems studied in the solid state. We also monitor branch-end
substitutions by this strategy and describe a three-dimensional structure
that accounts for the accommodation of the Ī±2,6-sialylĀated
terminus of a biantennary <i>N</i>-glycan by viscumin. In
addition, we provide a structural
explanation for the role of terminal Ī±2,6-sialylĀation
in precluding the interaction of natural <i>N</i>-glycans
with lectin from Maackia amurensis.
The approach described is thus capable of pinpointing lectin-binding
motifs in natural <i>N</i>-glycans and providing detailed
structural explanations for lectin selectivity
Thermodynamic Switch in Binding of Adhesion/Growth Regulatory Human Galectinā3 to Tumor-Associated TF Antigen (CD176) and MUC1 Glycopeptides
A shift
to short-chain glycans is an observed change in mucin-type
O-glycosylation in premalignant and malignant epithelia. Given the
evidence that human galectin-3 can interact with mucins and also weakly
with free tumor-associated Thomsen-Friedenreich (TF) antigen (CD176),
the study of its interaction with MUC1 (glyco)Āpeptides is of biomedical
relevance. Glycosylated MUC1 fragments that carry the TF antigen attached
through either Thr or Ser side chains were synthesized using standard
Fmoc-based automated solid-phase peptide chemistry. The dissociation
constants (<i>K</i><sub>d</sub>) for interaction of galectin-3
and the glycosylated MUC1 fragments measured by isothermal titration
calorimetry decreased up to 10 times in comparison to that of the
free TF disaccharide. No binding was observed for the nonglycosylated
control version of the MUC1 peptide. The most notable feature of the
binding of MUC1 glycopeptides to galectin-3 was a shift from a favorable
enthalpy to an entropy-driven binding process. The comparatively diminished
enthalpy contribution to the free energy (Ī<i>G</i>) was compensated by a considerable gain in the entropic term. <sup>1</sup>Hā<sup>15</sup>N heteronuclear single-quantum coherence
spectroscopy nuclear magnetic resonance data reveal contact at the
canonical site mainly by the glycan moiety of the MUC1 glycopeptide.
Ligand-dependent differences in binding affinities were also confirmed
by a novel assay for screening of low-affinity glycanālectin
interactions based on AlphaScreen technology. Another key finding
is that the glycosylated MUC1 peptides exhibited activity in a concentration-dependent
manner in cell-based assays revealing selectivity among human galectins.
Thus, the presentation of this tumor-associated carbohydrate ligand
by the natural peptide scaffold enhances its affinity, highlighting
the significance of model studies of human lectins with synthetic
glycopeptides
Modulation of the Interaction between a Peptide Ligand and a G Protein-Coupled Receptor by Halogen Atoms
Systematic halogenation of two native
opioid peptides has shown
that halogen atoms can modulate peptideāreceptor interactions
in different manners. First, halogens may produce a steric hindrance
that reduces the binding of the peptide to the receptor. Second, chlorine,
bromine, or iodine may improve peptide binding if their positive Ļ-hole
forms a halogen bond interaction with negatively charged atoms of
the protein. Lastly, the negative electrostatic potential of fluorine
can interact with positively charged atoms of the protein to improve
peptide binding
Mechanism of Sulfur Transfer Across ProteināProtein Interfaces: The Cysteine Desulfurase Model System
CsdA cysteine desulfurase
(the sulfur donor) and the CsdE sulfur
acceptor are involved in biological sulfur trafficking and in ironāsulfur
cluster assembly in the model bacterium Escherichia
coli. CsdA and CsdE form a stable complex through
a polar interface that includes CsdA Cys328 and CsdE Cys61, the two
residues known to be involved in the sulfur transfer reaction. Although
mechanisms for the transfer of a sulfur moiety across proteināprotein
interfaces have been proposed based on the IscSāIscU and IscSāTusA
structures, the flexibility of the catalytic cysteine loops involved
has precluded a high resolution view of the active-site geometry and
chemical environment for sulfur transfer. Here, we have used a combination
of X-ray crystallography, solution NMR and SAXS, isothermal calorimetry,
and computational chemistry methods to unravel how CsdA provides a
specific recognition platform for CsdE and how their complex organizes
a composite functional reaction environment. The X-ray structures
of persulfurated (CsdA)<sub>2</sub> and persulfurated (CsdAāCsdE)<sub>2</sub> complexes reveal the crucial roles of the conserved active-site
cysteine loop and additional catalytic residues in supporting the
transpersulfuration reaction. A mechanistic view of sulfur transfer
across proteināprotein interfaces that underpins the requirement
for the conserved cysteine loop to provide electrostatic stabilization
for the in-transfer sulfur atom emerges from the analysis of the persulfurated
(CsdAāCsdE)<sub>2</sub> complex structure