Cys-Loop Receptors Flashcards
(10 cards)
Cys-loop receptor structure
- first class of LGICs to be cloned (nAChR)
- 4TM + cys-loop
- 5 subunits surround the pore
- pentameric
- alpha helices kink to form gate
- if shorten sequence between TM3 and 4, still functions properly - suggests they are modular
Cys-loop receptor subtypes
- nAChR and 5-HT3R are cation-selection
- GABA-AR and glycineRs are anion-selective
Cys-loop receptor subunits
- multiple subunit isoforms and splice variants within each family
5-HT3Rs
- 5 subunits
- doesn’t work without at least one a subunit as this is the major 5-HT binding site
- not all subunit combinations exist in vivo
nAChRs:
- 17 subunits
- only 10 combinations found in vivo
Bacterial receptors
ELIC
- extracellular domain similar to nAChR
- no cys-loop
- no intracellular loop
- closed channel (X-ray structures)
- gated by GABA
GLIC
- no cys-loop
- open channel (X-ray structures)
similar to vertebrate cys-loop receptors
Cation-pi interactions
- lots of aromatic residues in the binding pocket
- cation-pi are stabilising interactions between carbon and the face of a simple aromatic
- strength between H bond and ion pair
- good evidence for cation-pi in nAChR binding
Cation-pi interactions - fluorination experiments
- fluorination of tryptophan residue
- unnatural amino acid mutagenesis
- more F = reduced potency of ACh binding
- confirms cation-pi interactions are important for binding
Smoking
- nicotine binds high affinity nAChRs in the brain
- would be fatal if did the same to nAChRs in muscle
- why doesn’t it? Amino acids forming the aromatic box are the same!
- in brain, but not muscle, cation-pi formed with nicotine
- both brain and muscle nAChRs form cation-pi with ACh
Modulatory sites
- competitive antagonists bind same size as agonists
- PAMs and NAMs bind modulatory site
- in pore or in TMD cavities
Binding site
- about half the way up the extracellular domain
- nAChR = alpha critical for binding
AChBP
AChBP = ACh binding protein
- protein produced by glial cells in molluses
- inhibits ACh
- sequence identity to nAChR is strong
- can bind nAChR agonists and antagonists
- soluvel
- twisted beta sandwich (X-ray crystallography)
