Lectins

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Lectins

• Venugopal Gudipati
• Chem 2810
• Submitted to Prof. Rob Coalson
• University of Pittsburgh
• December 6, 2002

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Lectins

• Lectins are glycoproteins or carbohydrate binding
  proteins having agglutinating property.
• Lectins were first found in plants and were
  latter isolated from animals as well.
• Interaction of lectins with carbohydrates is very
  specific.
• Lectin-carbohydrate interactions are comparable
  to enzyme-substrate, or antigen-antibody
  interactions.
• Their specificity to particular sugar is defined
  by the mono-or-oligosaccharide that inhibits the
  agglutination completely.

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Animal Lectins

• Animal lectins are complex, multidomain proteins.
• Their sugar binding activity results from a
  particular domain called Carbohydrate Recognition
  Domain (CRD).
• Animal lectins are broadly classified into the
  following categories
• a Calnexin and L-type lectin- both function as
  protein sorters in the endoplasmic reticulum.

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b C-type Lectins

• C-type Lectins Contain Ca2 that stabilizes the
  local conformation of the protein by making
  coordinate bonds to the surface of the protein
  and key hydroxyl groups of the sugar ring.
• Function Provide the sugar recognition activity
  in a variety of cell surfaces, initiate
  biological processes such as adhesion,
  endocytosis and complement fixation.
• Amino acid residues that coordinate the Ca2 also
  make hydrogen bonds to the sugar hydroxyl groups.
  The specific orientation of sugar binding is
  further directed by the hydrophobic interactions.

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C-type lectin domain
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Other Lectin categories
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Plant Lectins

• Lectins, with a molecular weight of
  60,000-100,000 MW, are present in various plants.
• Their functions in plants include, enzymatic
  activity, storage of proteins, defense mechanism,
  cell wall extension, mitogenic stimulation,
  transport of carbohydrates and packaging and
  mobilization of storage materials.
• Most important function of plant lectins include
  binding of nitrogen fixing bacteria to the legume
  roots. The symbiotic specificity of the Rhizobium
  to specific species of plant is governed by
  lectin interactions.
• Biological activity of many lectins can be
  attributed to metal ions which are important
  components of the native structure of most
  leguminous lectins.
• Many of the plant lectins are toxic in nature.
  They primarily effect gastrointestinal tract,
  cell membrane, and other organelles.

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Structural basis for the recognition of sugars

• Con A was the first lectin to be crystallized and
  analyzed by X-ray diffraction. X-ray analysis
  gave valuable insight of the folded conformation
  of lectins and their sugar binding specificity.
• Many of the legume lectins contain one or two
  chains, with a very similar three dimentional
  structure. In general, they contain a curved
  seven-stranded b-sheet and a flat six-stranded
  b-sheet interconnected by turns and loops to
  form a flat domed shape structure.
• Most legume lectins contain two protomers that
  interact with each other in a two fold symmetry
  (Figure 2).

• Dimerization creates a 12-stranded b-sandwich in
  which the two faces of monomers associate by
  their flat bottoms.
• Figure 2

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Structural basis for the recognition of sugars
cont,.

• Interaction of lectins with sugars, which is
  responsible for the carbohydrate-binding
  specificity of the lectins, depends on the
  presence of a monosaccharide binding site at the
  surface of lectin monomer.
• Example Co-crystallization of ConA with
  D-mannose and D-glucose has shown that amino acid
  residues at the top of the domain shaped lection
  monomer form a monosaccharide binding site
  responsible for the specific recognition of sugar
  by lectin.

• These residues (Asp81, Gly99, Asn125, Gly29,
  Ala30, and Glu31 in LoLI) form a network of seven
  hydrogen bonds with O3, O4, O5 and O6 of the
  simple sugar (Figure 3).
• Figure 3

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NMR investigation of protein-carbohydrate
interaction

• Protein-carbohydrate interactions have been
  frequently studied by X-ray crystal analysis of
  protein-carbohydrate complexes.
• NMR studies were less suitable for the studies
  with lectins of high molecular mass, but are
  generally employed in case of simple lectins,
  which provides valuable insight of the driving
  forces behind protein-carbohydrate interactions
  in solution.
• Hevein domain was studies by NMR investigations.
  Hevein inhibits the growth of chitin-containing
  fungi protecting plants from the attack of vide
  range of potential pathogens.

• Ligand binding studies of WGA-B (Wheat gram
  agglutinin) in presence of increasing amounts of
  chitotriose were carried out by monitoring 1D
  1H-NMR spectrum (Figure 4).
• The observed effects of chemical shifts and line
  broadening indicate that the interaction is fast
  in NMR time scale thus allowing the chemical
  shift values in determination of equilibrium
  constants.

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References

• Damme, E. J. M. V., Peumans, W. J., Barre, A.,
  Rouge. (1998) Plant Lectins A Composite of
  Several Distinct Families of Structurally and
  Evolutionally Related Proteins with Diverse
  Biological Roles. Crystal Reviews in Plant
  Sciences. 17, 575-692.
• Espinosa, J. F., Asensio, Garcia, J. L., Laynez,
  J., Bruix, M., Wright, C., Siebert, H-C., Gabius,
  H-J., Canada, F. J., Barbero, J. J. (2000) Eur.
  J. Biochem. 267, 3965-3978.
• http//ctld.glycob.ox.ac.uk/ctld/lectins.html
• http//www.ansci.cornell.edu/plants/toxicagents/le
  ctins/lectins.html