Cell Biology

... from active transport to vesicles


Active transport of some substances against concentration gradients employs the ATPase-derived energy stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to drive transporter membrane proteins (pumps). In a symport, the ATPase transported molecule and the coupled, co-transported ion move in the same direction. Conversely, the ATPase transported molecule and the coupled, co-transported ion move in the opposite direction in an antiport.

The Na+K+ATPase is a highly-conserved integral membrane protein that is expressed in virtually all cells of higher organisms. Depending on cell type, cell surfaces possess between 800,000 and 30 million pumps. It has been estimated that roughly 70% of neural ATP and 25% of all cytoplasmic ATP is hydrolyzed by sodium pumps in humans.

Sodium pumps generate both an electrical and chemical gradient across the plasma membrane. Such gradients are critical for:
a) maintaining resting electrical potential gradients in nerve and muscle, facilitated transport of glucose, amino acids and other nutrients into the cell,
b) directional fluid and electrolyte movement,
c) generating osmotic gradients that facilitate passive absorption of water

The Na+-K+-ATPase comprises two subunits: the α-subunit (~113 kD) contains a phosphorylation site and binds ATP, sodium ions, and potassium ions. The smaller β-subunit (~35 kDa glycoprotein) appears to be critical in facilitating the plasma membrane localization and activation of the α-subunit. Several isoforms of both α- and β-subunits have been identified.

Based on primary amino acid sequence, the pump is believed to possess 8 or 10 transmembrane domains. Cation transport occurs in a cycle of conformational changes apparently triggered by phosphorylation of the pump. As currently understood:
1. The pump (having bound ATP) binds 3 intracellular Na+ ions.
2. Hydrolysis of ATP leads to phosphorylation of a cytoplasmic loop of the pump, with release of ADP.
3. The pump undergoes a conformational change that exposes the Na+ ions to the extracellular space, where they are released.
4. The pump then binds 2 extracellular K+ ions, inducing dephosphorylation of the α-subunit.
5. ATP binds and the pump reorients, releasing the 2 K+ ions to the cell's interior.
The pump is ready to cycle again.

animation NaK-ATPase :

A functional Na+K+ ATPasepump requires synthesis and assembly of both α- and β-subunits, allowing for regulation of pump expression by a variety of hormones (aldosterone, thyroid hormone, insulin, catecholamine neurotransmitters). Rapid changes in pump activity appear to reflect modulation of kinetic properties, which are induced by intracellular signalling pathways. Phosphorylation of the α-subunit enhances pump activity, and several hormones stimulate kinase or phosphatase activities within the cell. Some cells contain an intracellular pool of pumps that can be rapidly recruited to a functional state within the plasma membrane.
Chronic or sustained changes in pump activity within cells is typically related to increases in transcription rate or mRNA stability.

V-ATPases are multi-subunit proton pumps, that extrude H+ ions from the cytoplasm while hydrolysing ATP. ATP binds in the fold between an α- and a β- subunit. The energy released by the phosphate bond is employed to distort/rotate the headgroup, forcing the protons into the Vo proteolipid channel. As protons leave the ATPase on the extracytoplasmic face, the remaining ADP dissociates from the ATPase, enabling the pump to start another cycle. The whole process is repeated about 100 times per second.[s] animation V-ATPase :

Sarco(endo)plasmic reticulum calcium-ATPases (SERCAs) pump calcium from the cytoplasm of mammalian cells into organellar structures such as the sarcoplasmic reticulum (muscle) or the endoplasmic reticulum. SERCAs exhibit a threshold of activation of the order of 100-200 nM of calcium, such that they set the resting level of cytoplasmic calcium. The SERCA1 isoform is expressed at high levels in fast-twitch muscle, and it is highly concentrated in the sarcoplasmic reticulum. The key structural features of SERCA1 include a transmembrane domain (10 transmembrane alpha helices), a stalk sector (helical extensions of transmembrane helices), and cytoplasmic β-strand, phosphorylation and nucleotide binding domains attached to the stalk domain at a distance of 60 Å from the transmembrane domain. It is proposed that binding of calcium to high-affinity sites in the transmembrane domain trigger phosphorylation of SERCA1 by ATP. Phosphorylation then initiates a series of conformational changes that distort the transmembrane helices such that access of calcium shifts from the cytoplasm to the lumen while calcium binding sites are destroyed, thus releasing calcium to the lumen. [s] animation - sarco-endoplasmic reticulum Ca pump :

Harvard University provides a wonderful video explaining operation of the F1-F0 ATPase.

active transport : GPCRs : GPCR families : hormones : ion channels : neurotransmission : receptor proteins :

Animation Movies of ATP Synthase : image - carrier proteins : animation - carrier proteins : image - ion channel proteins : animation - facilitated diffusion : animation - Na glucose symport : animation - Na K ATPase : microtubule motors : Virtual Cell Textbook - Cell Biology :

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