Question

Describe how the protein structure of a voltage-dependent sodium channel generates its functional properties in as much detail as possible. Be sure to include: conductance of sodium ions across a phospholipid bilayer, selectivity for sodium, voltage-dependent activation (m3h) and inactivation.

Answer 1

sodium channels are integral membrane proteins that conduct sodium ions across the phospholipid bilayer. they are triggered to open by a voltage change. in cells such as neurons, myocytes, these channels are responsible for rising phase of action potentials.

Protein Structure: This channel is made up of large alpha subunits associated with beta subunits, alpha subunit forming the core of the channel. the alpha subunit has four domains which are repaeted namely S I to S IV each subunit consisting of six transmembrane segments. The 4th transmembrane segment acts as a voltage sensor and is rich in positively charged amino acids that provides sensitivity to these channels. when the transmembrane voltage changes, this segment moves towards the extracelular region of the cell membrane making the channel permeable to sodium ions. sodium ions are conducted through a pore formed by segments S5 &S6 of the four domains.

Voltage dependent activation :In response to an action potential, the activation gates open allowing Na⁺ to flow into the cell through the channels. entrance of sodium ions into the cell leads to incease in membrane potential causing closing of inactivation gates (I) (part of domains III & IV) which stops further flow of Na ions. No entry of Na ions leads to drop in membrane potential decreasing back to its resting potential.

Selectivity for sodium: these channels are highly selective for sodium ions as the pore of sodium ions have a selectivity filter composed of negatively charged amino acids residues, which only attract posituvely charged ions and prevents entry of negative ions such Cl^- ions. the larger K⁺ ion does not fits and pore and hence prevented from transport.