Ion Pumps and Na+-K+ Pump Overview

Ion Pumps

• Pumps are transporters that use active transport.
• Active transport requires energy as it moves molecules against their concentration gradient.
• Pumps alternate access to the substrate binding pocket from one side of the membrane to the other.
• Energy is provided by primary or secondary transport.

Primary Transporters

• Primary transporters are driven by ATP hydrolysis.
• ATP hydrolysis is coupled to the movement of a substrate against its electrochemical gradient.
• One example of a primary transporter is the Na+-K+ pump.

The Na+-K+ Pump

• It is also called Na+-K+ ATPase.
• Moves Na+ out of the cell and K+ into the cell.
• This sets up Na+ and K+ gradients needed for action potentials.
• Requires a large proportion of a cell's energy.
• It is a P-type pump as it is phosphorylated during transport.
• The cycle takes about 10 milliseconds.
• It has 3 binding sites for Na+ and 2 binding sites for K+.

Clinical Insight: Digitalis

• Digitalis is derived from the foxglove plant.
• It inhibits the Na+-K+ pump by preventing its dephosphorylation.
• This leads to an increase in intracellular Ca2+ levels, forcing the heart to pump harder.
• It is used to treat congestive heart failure.

Ca2+ ATPase (SERCA)

• SERCA is another P-type ion pump
• Pumps Ca2+ into the sarcoplasmic reticulum, which is the endoplasmic reticulum of muscle cells.
• This is also driven by ATP hydrolysis.
• The protein’s structure has been solved through x-ray crystallography, revealing two major conformations - E1 and E2.

E1 conformation of Ca2+ ATPase

• Contains 10 transmembrane α-helices with 2 Ca2+ ions bound.
• Has an actuator domain (A domain), phosphorylation domain (P domain), and a nucleotide binding domain (N domain).
• The P domain is where aspartate 351 is phosphorylated during the reaction cycle.

E2 conformation of Ca2+ ATPase

• The structure is solved with aspartate 351 phosphorylated.
• The conformation has switched to E2.

ABC Transporters

• They contain ATP-binding cassettes (ABC) domains.
• Have two transmembrane domains and two ABC domains.
• Switching between conformations causes substrate transport.
• Substrate binding and ATP hydrolysis at the ABC domains cause conformational changes.

Clinical Insight: Multidrug Resistance in Cancer

• Cancer cells often develop resistance to anti-cancer drugs.
• This is caused by overexpression of an ABC transporter called MDR (multidrug resistance) or P-glycoprotein.
• P-glycoprotein pumps a wide range of substrates, including drugs, out of the cell.
• The structure of the mouse MDR protein was solved in 2009.
• This knowledge could help scientists find inhibitors to prevent drug resistance.

Clinical Insight: Cystic Fibrosis

• Cystic fibrosis is an autosomal recessive inherited disease.
• Causes frequent lung infections and digestive problems.
• Caused by defects in an ABC transporter called cystic fibrosis transmembrane conductance regulator (CFTR).
• CFTR acts as a chloride ion channel, regulated by ATP binding and hydrolysis.

Secondary Transporters

• Driven by the movement of an ion down its electrochemical gradient.
• This energy is used to move another molecule against its gradient.
• There are two types: antiporters and symporters.

Antiporters

• The ion and substrate molecule move in opposite directions.

Symporters

• The ion and substrate move in the same direction.

Na+-Glucose Symporter

• Glucose can be moved into cells against its concentration gradient.
• This is driven by the Na+ electrochemical gradient, set up by the Na+-K+ ATPase.

Glucose Transporter

• Facilitates the movement of glucose out of cells down its concentration gradient.
• The two glucose-transporting proteins (Sglt1, Glut1) are compartmentalized within the cell membrane by tight junctions.

Summary of Ion Channels and Pumps

• Ion channels allow passive transport of ions down their electrochemical gradient.
• The structure of K+ channels is well studied and explains their selectivity for K+.
• Ion channels can be voltage-gated or ligand-gated.
• Ligand-gated ion channels are important in action potentials.
• Pumps actively transport molecules against their electrochemical gradient.
• ATP-driven pumps (primary transporters) include P-type pumps and ABC transporters.
• Secondary transporters utilize the energy of an ion's electrochemical gradient to move other molecules against their concentration gradient.