lecture 32-34 - glycan binding proteins - lectins Flashcards
what do lectins bind? with what affinity?
- tend to bind monosaccarides and oligosaccarides
- N-glycans, O-glycans, glycolipids
- few recognize sulfated GAGs
- low affinity
describe N-glycans
- oligosaccarides
- connected to proteins through N-acetyl glucosamine that is N-linked to an Asn res
describe O-glycans
-oligosaccarides
-connected to proteins through an O-linkage to Ser/Thr
(typically connected through N-acetyl glucosamine, but other sugars have been found as well)
glycolipids
- oligosaccarides
- O-linked to lipid chains through glucose or galactose
are lectins branched or unbranched structures? is this the same or the opposite of GAGs?
- branched
- opp of GAGs
where do lectins typically bind glycans?
- terminal grps (minimal binding epitopes=what they bind)
- di, tri, tetra-saccarides displayed at the ends of glycans
are lectins evolutionarily related?
yes
how are lectins classified into groups?
grouped into evolutionarily related families based on carbohydrate-recognition domains (CRDs)
what do Galectins (fam of lectins) bind?
Beta-galactose containing glycans
explain prototypical galectins
- single CRD
- can associate as homodimers
explain chimera galectins
- single CRD
- CRD has a terminal polypeptide tail (usually rich in pro, gly, tyr)
- tail can help form oligomers
explain tandem galectins
- 2 CRDs
- CRDs connected through polypep-linker
- each CRD can have diff galectin binding properties
what are C-type lectins?
- calcium dependent GBPs
- share primary and secondary homology in their CRDs
- are some exceptions to these
explain the CRDs for c-type lectins
- most have a single CRDs
- most are transmembrane proteins
what can c-type lectins do to affect bidning affinity? what is the effect?
- can cluster together
- increase binding affinity
what do p-type lectins bind?
monnose-6-phosphate on N-glycans
what are the actions of p-type lectins?
selective delivery of lysosomal enzymes to lysosomes
what family to I-type lectins belong to? why? what does this exclude?
- immunoglobulin super family (bc have similar folds)
- excludes antibodies and t-cell receptors
- most are transmembrane proteins
give an example of a major family of I-lectins and their recognition site
-siglecs (sialic acid binding immunoglobulin type lectins)
-have two types of the immunoglobulin folds:
Cset - constant like domain
Vset - variable like domain = carb recognition domain
how can the affinity of lectin binding be enhanced?
by multivalency (multiple copies of the carb epitope interact with multiple copies of the lectin/CRD
how can multivalency occur?
multivalency can occur w/in glycan/glycoprotein structures OR by clustering (of CRDs)
describe monovalent interacitons
- one protein binds one CRD from the lectin to form a glycan
- have part of the glycan unbinded
describe multivalent interactions
- multiple binding epitopes on a single glycan bind multiple CRDs
- forms a branched glycan
describe multiple glycan binding
- multiple glycans on a single protein scaffold bind multiple CRDs
- have extra branches