Chemistry Reference
In-Depth Information
The central question of how ligand binding triggers channel opening has
been investigated during recent years at the single amino acid level by combining
protein engineering and single-channel electrophysiology. Key residue interactions
engaged in the molecular pathway coupling ligand binding to gating have been
identi
ed by producing mutations at the interfacial region between the extracellular
ligand-binding domain and the transmembrane channel domain [77, 78]. The
propagation of conformational changes between intermediate pre-opening states
was studied on a series of nAChR mutants affecting the binding site, the interfacial
region, and transmembrane segments lining the pore [79, 80]. A picture of the
sequential molecular events was suggested from the position of the gating transition
state along the reaction coordinate estimated at
the different sites in the
protein [81, 82].
7.5.3
Chemical Gating by Specific Ligand Binding inside Ion Channels
Diffusion of hydrophilic solutes across membranes of cells and organelles occurs
through protein channels comprising an aqueous pore. A variety of biological
processes rely on channel-based transport systems, such as the transfer of nucleic
acids and proteins across the nucleus membrane, the export of nucleotides across the
mitochondria membrane, and the uptake of nutrients across the outer membrane of
bacteria, to mention a few.
Selectivity of ions or metabolites is achieved through a network of attractive
interactions between the diffusing particles and protein residues lining the
pore. As a consequence of the presence of an internal binding site, both the
occupation probability and the mean residence time of a solute inside the channel
increase. Diffusion models have demonstrated that the net
flux, or translocation
probability via passive diffusion, of particles can be larger in the presence of a
potential well compared with that in its absence [83
87]. Moreover, two important
facts have been described. First, there is an optimal well depth that enhances
channel ef
ciency depending on the solute concentration [83, 85]. Second, the
asymmetric position of the binding site inside the channel can lead to asymmetric
transport [86].
Although the binding of solutes in the pore does not initiate subsequent molecular
events mediated by the channel protein itself, the above considerations show that
facilitated diffusion through membrane channels can be viewed as a biological
process resulting from speci
c interactions between a ligand (the permeating
molecule) and a membrane receptor (the channel).
In some cases, the interactions are so strong that the transient occupancy of a pore
by a diffusing molecule can be resolved in time at the single-molecule level by means
of electrophysiology techniques. A simple kinetic scheme of channel-facilitated
membrane transport assumes a two-state model, in which the channel is either
open or (partially) closed to ionic current by a diffusingmolecule. In the standard one-
site two-barrier model, the occupied channel corresponds to the bound state [88]; in
the diffusionmodel, the ligand is not necessarily bound to obstruct the pore (e.g. [84]).
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