From lectin structure to functional
glycomics: principles of the sugar
code
Alexander Hsieh
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Intro
 Lectins: carbohydrate-binding proteins which
lack enzymatic activity on their ligand; are
distinct from antibodies and free mono- and
oligosaccharide sensor/transport proteins.
 Sugar-binding capacity determined by dozens of
folds and wide range of binding site architecture
 Question: How do we explain target specificity of
endogenous lectins for certain cellular glycans?
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Glycosylation: Abundant and
Frequent
 Factors influencing protein functionality:
 Post-translational modification
 Glycan chain branching and length
 Ex: glycocalyx: the sugar coating that protects cells
in eukaryotes and bacteria; acts as
filtration/selective-binding device, key for
recognition
 Conjugation of sugars to proteins
 known total of 13 monosaccharides + 8 amino acids
= at least 41 types of glycosidic linkages
 Variety allows for glycan bioactive signaling ability
 Hold information coding exclusive to proteins
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N-glycosylation
 Typical entry point = Asn – X – Ser/Thr
 X = any amino acid except proline
 Common in proteins from the extracellular side of
the membrane and those on the route trafficking
from the ER to cell membrane/lysosomes
 Assume loop, turn or β-sheets
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N-glycosylation cont’d
 N-glycans are complex (100+ structures) and
have highly sophisticated enzymatic machinery
 Implies more than just passive effect (ex: solubility) of
protein glycosylation
 Suggest a coding system for carbohydrates (aka
“Sugar Code”)
 Coding capacity ideal for generating compact units
with explicit informational properties
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Carbohydrates: the sweet
side of biorecognition
 Much more diversely coded structures
(1.44x10^15) than oligonucleotides (4096) or
peptides (6.4x10^6)
 Factors contributing to structural diversity:
 2 anomeric configurations (α,β)
 Bonding at different linkage positions (1->1, 2, 3, 4, 6)
 Change in ring size (pyranose/furanose)
 Branching
 Site-specific substitutions (acetylation,
phosphorylation, sulfation)
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Carbohydrates cont’d
 Hydroxyl groups suited as donors/acceptors in Hbonding
 C-H bonds suited for van der Waals interaction
and stacking because of inherent polarization
 Epimerization – main carbohydrate constituents
differ only in relative position of 1 or 2 hydroxyl
 OH group location + coordinated hydrophobic
patches create characteristic profile of potential
contact (as recognized by amino acid side chains)
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Calcium as a factor
 Ca2+ ion = natural alternative for amino acid
side chains to sense spatial distribution of OH
groups
 Strategic Role of Ca2+ in Lectin Activity:
 Stabilizes lectin domain and organizes site for ligans
binding
 Oligomerizes subunits
 Facilitates direct contact to neutral and anionic
groups of ligans
 Neutralizes repulsive forces between the anionic
charge on the ligand and lectin