Question

Role of a ligand gated ion channel?

Answer 1

Fundamental signaling molecules in neurons are Ligand-gated ion channels(membrane proteins) which are localized in the plasmalemma and on intracellular organelles. They can be gated by intracellular as well as extracellular ligands. Fast excitation and inhibition in the nervous system arbitrate by the neurotransmitter-gated ion. Studies by means of the method of voltage and patch-clamp recording have examined the three fundamental features of an ion channel: gating, conductance and selective permeability. Generally, ligand-gated ion channels are described with the help of kinetic models concerning binding of two or more ligand molecules which bring on a conformational transformation in the protein. Consequently a central, water-filled pore opens which conducts ions at very high speed of up to 10⁷ ions per second. when the channel closes or when it enters a non-conducting state channel activity is terminated . acetylcholine and glutamate receptors are cation channels in vertebrates, while γ-aminobutyric acid (GABA) and glycine receptors are anion channels. Receptors like the ACh, GABA and glycine constitute the Cys-loop receptor super family which most probably have evolved from a common ancestral gene. A separate protein family which is comprised of Glutamate receptors. Homology is shared by the ligand-binding domain of glutamate channels with bacterial amino acid binding proteins, while homology by pore is shared with the pore of voltage-dependent ion channels.

All of the neurotransmitter-gated channels look like to be made of a hetero-oligomeric complex which is intimately associated with subunits adjacent to a central ion pore. For all receptor type there are a number of probable subunit combinations which can shape functional channels. AChR subunits is having four transmembrane domains as the second transmembrane domain lining the pore The residues of charged amino acid close to the mouth of the pore are vital in shaping the conductance and selectivity of the individual channel types. The three transmembrane glutamate subunit domains and a re-entrant loop domain forms a part of the pore. The availability of recent crystal structures of a homomeric Cys-loop receptor as well as glutamate receptor allows atomic level resolution of the conformational movements that is concerned in binding and gating of these channels.