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
describe single glycan w multiple binding epitopes
causes clustering of CRD containing proteins
describe multiple glycans w single binding epitope
- causes clustering of CRD containing proteins
- MOST COMMON
- have extra brances
CRD recognition sites are selective for what?
specific glycan structures or epitopes
what blocks/prevents epitope binding?
- adding sugar residues to epitopes (Fuc, NeuSAC)
- secondary interactions can block OR enhance binding
describe the CRD site for C-type lectins
recall: bind Ca and gylcans, CRD has:
- 2 (long) loops - where bind Ca (typically bind 2-4)
- 2-stranded antiparallel beta-sheet connected by 2-alpha-helicies and a 3-stranded antiparallel beta sheer
- 2 conserved disulfide bonds (alpha1-beta5 and beta3-beta5)
describe the interactions that occur at the CRD for c-type lectins
- the Ca that binds glycans/sugar is located btwn B4 and the loop
- commonly found surrounding aa = asn, asp, glu
- carbonyl side chains (c=o) from both the CRD and the sugar coordiate their interaction
- H-bonding also occurs (H from aa to O from sugar) ( OH* from sugar to N/O from aa)
describe the CRD site for I-type lectins (siglecs specifically)
- recall: CRD = vset domain & likes to bind sialic acids
- CRD is on one face of the beta sandwich (typically btwn sheets A and G)
- usually a larger, flat surface w two types of residues:
- residues at the top = typically hydrophobic (tyr, trp, phe)
- other = always ARG (basic/positively charged) = very important for recognition!!
describe the struture of a siglec (what are the key motifs?)
6 carbon ring (chair)
- c4 = glycerol side chain
- c3 =actually an O
- c2 = carboxylate and the -O-sugar
- c6 = OH
- c5 = acetymide (NHAc)
which parts of the siglec interact with the aromatic groups in the CRD? what type of interaction is this?
- methyl group of the NHAc and the glycerol side chain
- hydrophobic interactions
how is the glycerol side chain able to interact with the aromatic group despite the fact that it has many hydroxyl groups?
its flexible - able to create a hydrophobic (to interact w hydrophobic aromatics) and a hydrophilic face
why is the arg residue so important for I-type CRD interactions? what type of interaction is this?
- forms a salt bridge btwn the guanalyle group from the Arg (pos) and the carboxylate group from the siglec (neg)
- have ionic (salt bridge) and hydrogen bonding
is the arg residue highly conserved throughout I-type CRD? why or why not?
- yes
- responsible for recognizing and binding siglecs
what two modifications to the siglec would decrease binding to its CRD?
(1) add OH to the NHAc group (makes more hydrophilic - less hydrophobic interactions)
(2) cleave glycerol side chain (also decreases hydrophobic interactions)
