Question

Q2. The Na+/K+ pump and voltage gated K+ channels both transport K+ across the membrane. A) Compare and contrast these two mechanisms for K+ transport indicating important functional and structural differences. B) Describe the process for putting a voltage gated K+ channel in the membrane starting at the ribosome and ending at the plasma membrane.

Answer 1

Q2.

A) Na⁺/K⁺ pump:

Na⁺/K⁺-ATPase (sodium potassium adenosine triphosphatase, also known as the Na⁺/K⁺ pump or sodium- potassium pump is an enzyme (an electrogenic transmembrane ATPase) found in the membrane of all animal cells.

The Na+/K+ -ATPase enzyme is active (i.e. it uses energy from ATP). For every ATP molecule that the pump uses, three sodium ions are exported and two potassium ions are imported; there is hence a net export of a single positive charge per pump cycle.

Mechanism:-

• The pump has a higher affinity for Na+ ions than K+ ions, thus after binding ATP, binds 3 intracellular Na+ ions.
• ATP is hydrolyzed, leading to phosphorylation of the pump at a highly conserved aspartate residue and subsequent release of ADP. This process leads to a conformational change in the pump.
• The conformational change exposes the Na+ ions to the outside. The phosphorylated form of the pump has a low affinity for Na+ ions, so they are released; by contrast it has high affinity for the K+ ions.
• The pump binds 2 extracellular K+
  ions. This causes the dephosphorylation of the pump, reverting it to its previous conformational state, thus releasing the K+ ions into the cell.
• The unphosphorylated form of the pump has a higher affinity for Na+
  ions. ATP binds, and the process starts again.

Function:-

The Na+/K+-ATPase helps maintain resting potential, affects transport, and regulates cellular volume. It also functions as a signal transducer/integrator to regulate the MAPK pathway, reactive oxygen species (ROS), as well as intracellular calcium. In fact, all cells expend a large fraction of the ATP they produce (typically 30% and up to 70% in nerve cells) to maintain their required cytosolic Na and K concentrations. For neurons, the Na+/K+-ATPase can be responsible for up to 3/4 of the cell's energy expenditure. In many types of tissue, ATP consumption by the Na+/K+-ATPases have been related to glycolysis. Recently, glycolysis has also been shown to be of particular importance for Na+/K+-ATPases in skeletal muscles, where inhibition of glycogen breakdown (a substrate for glycolysis) leads to reduced Na+/K+-ATPase activity and lower force production.

Voltage-gated potassium channel:

Voltage-gated potassium channels (VGKCs) are transmembrane channels specific for potassium and sensitive to voltage changes in the cell's membrane potential. During action potentials, they play a crucial role in returning the depolarized cell to a resting state.

Structure:-

Typically, vertebrate voltage-gated K⁺ channels are tetramers of four identical subunits arranged as a ring, each contributing to the wall of the trans-membrane K⁺ pore. Each subunit is composed of six membrane spanning hydrophobic α-helical sequences, as well as a voltage sensor in S4. The intracellular side of the membrane contains both amino and carboxy termini.

The structure of the mammalian voltage-gated K⁺ channel has been used to explain its ability to respond to the voltage across the membrane. Upon opening of the channel, conformational changes in the voltage-sensor domains (VSD) result in the transfer of 12-13 elementary charges across the membrane electric field. This charge transfer is measured as a transient capacitive current that precedes opening of the channel. Several charged residues of the VSD, in particular four arginine residues located regularly at every third position on the S4 segment, are known to move across the transmembrane field and contribute to the gating charge. The position of these arginines, known as gating arginines, are highly conserved in all voltage-gated potassium, sodium, or calcium channels. However, the extent of their movement and their displacement across the transmembrane potential has been subject to extensive debate. Specific domains of the channel subunits have been identified that are responsible for voltage-sensing and converting between the open and closed conformations of the channel. There are at least two closed conformations. In the first, the channel can open if the membrane potential becomes more positive. This type of gating is mediated by a voltage-sensing domain that consists of the S4 alpha helix that contains 6–7 positive charges. Changes in membrane potential cause this alpha helix to move in the lipid bilayer. This movement in turn results in a conformational change in the adjacent S5–S6 helices that form the channel pore and cause this pore to open or close. In the second, "N-type" inactivation, voltage-gated K⁺ channels inactivate after opening, entering a distinctive, closed conformation. In this inactivated conformation, the channel cannot open, even if the transmembrane voltage is favorable. The amino terminal domain of the K⁺ channel or an auxiliary protein can mediate "N-type" inactivation. The mechanism of this type of inactivation has been described as a "ball and chain" model, where the N-terminus of the protein forms a ball that is tethered to the rest of the protein through a loop (the chain). The tethered ball blocks the inner porehole, preventing ion movement through the channel.

Function:- Voltage-gated potassium channels are activated by depolarization, and the outward movement of potassium ions through them repolarizes the membrane potential to end action potentials, hyperpolarizes the membrane potential immediately following action potentials, and plays a key role in setting the resting membrane potential. In this way, potassium channels control electrical signaling and regulate ion flux and calcium transients in nonexcitable cells.