Neurons in our nervous system use electrical impulses to transmit information
throughout our bodies.
This process is mediated by membrane proteins called voltage gated ion channels.
Voltage-gated ion channels open and close in reaction to changes in membrane voltage
or membrane potential.
A cell membrane potential is generated by concentration gradients of positively charged
sodium and potassium ions across the membrane.
When the neuron is at rest, the ion-channels are closed,
and the overall concentrations of ions and proteins create a higher potential on the outside
and a lower potential on the inside.
This state is referred to as resting potential and the membrane is said to be polarized.
To transmit a signal, the neuron creates an action potential
which travels down the axon.
The action potential is mediated by the opening of the
voltage gated sodium channels, which allow sodium to enter the neuron.
This depolarizes the membrane in the local area.
Neurons then repolarize the membrane in several ways.
First, voltage-gated sodium channels close and become inactivated.
Then, to counteract the positive charge of the sodium ions,
potassium channels start to open allowing potassium ions to exit
lowering the voltage across the membrane.
In this way the signal travels along the neuron.
Sodium channels open and trigger each other to generate action potentials
and potassium channels prime the neuron for the next signal by allowing potassium to exit.
After the signal is finished, the locations of ions have been reversed.
Sodium concentration is high on the inside and potassium concentration is high on the outside
The sodium-potassium pump restores the sodium and potassium gradients
making the cell ready to send another signal.
With each pumping cycle, this transport protein moves 2 potassium ions back into the cell
and 3 sodium ions out of the cell.
Because the pump moves these ions against their concentration gradients
it requires the energy of one ATP molecule for each cycle.
This molecular machine comprises three chains: alpha, beta and gamma.
Within the alpha chain, three domains, A, P, and N, form the core engine of the pump.
The pumping cycle occurs in several steps.
With ATP bound in the N site and the transmembrane region open into the cell,
the pump binds three sodium ions from the cytoplasm.
This causes the N, P, and A sites to shift closer together.
With the help of magnesium co-factor, the ADP is cleaved off and the P-site is phosphorylated
and ADP is cleaved off.
This leads a conformational change that exposes the the sodium ions to the outside.
The sodium ions can then be released, one after another, into the extracellular space.
In this ion-empty state, the pump allows the entry of 2 potassium ions.
This causes dephosphorylation of the pump
followed by closing of the extracellular gate and allows the N domain to accept the ATP again.
As a result of ATP binding, the intracellular gate opens, the potassium ions are released into the cytoplasm,
and the cycle can start again.
The sodium-potassium pump is a key protein working to maintain the sodium-potassium gradients
and the intra- and extracellular potential differences necessary for neuronal signaling.
Roughly a third of the ATP made daily by our cells is spent to power this molecular machine.