Lecture 5 Flashcards
Na+/K+ ATPase exchange pump (Sodium Potassium Pump)
Maintains Na+ and K+ gradients across the cell membrane
- Pumps 3 Na+ out of the cell for every 2 K+ pumped in
Consumes energy (ATP–>ADP) - Active Transport
Structure of the Na+/K+ ATPase exchange pump
Dimeric structure of one alpha (α) subunit and one beta (β) subunit
Human Sodium Potassium Pumps
Humans have 4 α genes: • ATP1A1 (α1): Ubiquitous expression • ATP1A2 (α2): Muscle and nervous tissue • ATP1A3 (α3): Neurons • ATP1A4 (α4): Testis
Humans have 4 β genes:
• ATP1B1 (β1): Ubiquitous expression
• ATP1B2 (β2): Muscle and brain
• ATP1B3 (β3): lung, testis, skeletal muscle, and
liver
• ATP1B4 (β4): Divergent function in placental mammals (i.e. nuclear); still associates with α subunit in fish, avian, and amphibian species
**Don’t need to memorize gene numbers or types, just know that there are several genes with different expression patterns*
Sodium Potassium Pump and RMP
- The sodium-potassium exchange pump is “electrogenic”
- Every transport cycle results in the net extrusion of 1 positive charge
- Thus, the pump contributes to the negative RMP of the cell
- This contribution is -6 to -11 mV
R.C. Thomas’ experiment on the Electrogenic Nature of the Sodium Potassium Pump
• Impaled a single large snail neuron with 5
electrodes!
• The Li+ and Na+ electrodes inject those ions into the cell, if one injects the other removes cations to compensate
• Injection of Na+, but not Li+, causes the membrane potential to drop
• Blocking the pump with ouabain blocks the Na+ effect • External K+ is also required
Oubain
Na+/K+ ATPase blocker
α3 subunit is 1000-fold more sensitive
Rapid Onset Dystonia Parkinsonism (ROPD)
Mutations in α3 subunit
can be artificially created by injecting Oubain into the brain
can also be triggered by stress
Why Ca2+ is good
- Binds oxygen atoms
- Causes conformational changes in proteins (good for signalling or activating mechanical processes)
- Vesicle exocytosis, muscle contraction, activating other ion channels)
- Due to toxicity, kept at very low levels in cells, making it ideal as a transient signalling molecule
Why Ca2+ is bad
• It precipitates phosphates (CaPO4), which can accumulate and become toxic
to cells
• Cannot be chemically altered for neutralization
[Ca2+]in vs. [Ca2+]out
[Ca2+]in
Ca2+ Pumps
2 types of Ca2+ pumps
• Plasma membrane calcium ATPase (PMCA)
• Sarcoplasmic/endoplasmic reticulum calcium ATPase (SERCA)
PMCA Pump
1 Ca2+ ion pumped out of the cell per cycle (hydrolysis
of a single ATP molecule)
Human PMCA Pumps
Humans have 4 α genes
• ATP2B1 (PMCA1): Brain/Ubiquitous – lethal if mutated
• ATP2B2 (PMCA2): Brain and muscle – hearing loss and balance
• ATP2B3 (PMCA3): Brain and muscle
• ATP2B3 (PMCA3): Broad distribution – male infertility
* Don’t need to memorize gene numbers or types*
SERCA Pump
2 Ca2+ ions pumped into the SR/ER per cycle
hydrolysis of a single ATP molecule
Human SERCA Pumps
Humans have 3 α genes
• ATP2A1 (SERCA1): Muscle contraction
• ATP2A2 (SERCA2): Muscle contraction
• ATP2A3 (SERCA3): Non-muscle, but expressed in
cardiomyocytes
Don’t need to memorize subunit genes or types
Speed of Ca2+ Pumps
SERCA (2 Ca2+) and PMCA (1 Ca2+) are sluggish at removing Ca2+
Speed of Ca2+ Exchangers
NCX and NCKX exchangers remove Ca2+ much more quickly
NCX and NCKX exchangers
• Exchangers do not hydrolyze ATP as energy source for moving ions against
their gradients
• Exchangers consume energy from existing ion concentration gradients to move other ions “uphill” against their gradients
NCX: • Na+ Ca2+ exchanger (a.k.a. sodium-calcium antiporter)
NCX uses the Na+ gradient
• 1 Ca2+ out for 3 Na+ in
• Most widely distributed sodium-calcium exchanger
NCX can operate in reverse
- Both Na+ and Ca2+ want to get into the cell
- Whichever ion experiences the strongest inward full wins
- In order for Na+ to win, it’s charge x driving force must be greater than that for Ca2+
- Otherwise Ca2+ wins and it gets to go in the cell
NCKX: Na+-Ca2+-K+ exchanger
Even better at removing cytosolic Ca2+!!
- Uses sodium and potassium gradients to remove Ca2+
- 4 Na+ in and 1K+ out for 1 Ca2+ out (+74 mV for Ca2+ to move inward!)
Reverses at +75mV
NCX Structure
9 transmembrane segments
NCKX Structure
11 transmembrane segments
N-terminus is cleaved
Cl- transport in Immature Neurons
more [Cl-]in than [Cl-]out
Cl- transport in Mature Neurons
more [Cl-]out than [Cl-]in
How do you end up with more [Cl-]in?
Cotransporters use the Na+ gradient to
move Cl- into the cell
NCC (Na+/Cl- cotransporter)
transports 1 Na+and 1 Cl- into the cell
Electrically neutral
NKCC (Na+/K+/Cl- cotransporter)
transport 1 Na+, 1 K+ and 2 Cl- into the cell
Electrically neutral
NCC in Mammals
have 1 NCC gene
NKCC in Mammals
have 2 NKCC genes
How do you end up with less [Cl-]in?
Cotransporters that use the K+ gradient move Cl- out of the cell
KCC: (K+/Cl- cotransporter)
transport 1 K+ and 1 Cl- out of the cell
Electrically Neutral
If you want to inhibit (hyperpolarize) a neuron via Cl- channel activation, you need KCC to produce [Cl-]in
KCC in Mammals
4 KCC genes
• SLC12A4(KCC1) • SLC12A5(KCC2) • SLC12A6(KCC3) • SLC12A6(KCC4)
Don’t need to memorize subunit numbers or types
Cl- co-transporters and pH regulation
- Na+-dependent Cl-/HCO3- exchange system uses the Na+ gradient to move bicarbonate into the cell and protons out
- HCO3- is part of a physiological buffering system crucial in the nervous system, where there is little tolerance for fluctuation in pH
CO2 + H2O ↔ H2CO3 ↔ H+ + HCO3-
- ↑ in cytoplasmic H+ promotes H+ efflux and HCO3- influx
- ↑ in HCO3- then shifts the equitation to the left, further neutralizing cytoplasmic pH
