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Ca-ATPase, a P-Type ATPase

1. Distinguish protein-mediated transport from diffusion.Edit

Diffusion is always a passive process, however transport can be both active (requiring energy) or passive. For any given substance protein-mediated transport should be faster than diffusion. Protein-mediated transport can be achieved through multiple methods, including channels, facilitated transport, and primary and seconday active transport. Transporters display an affinity for a certain stereochemistry of molecules (only biologically active molecules will be transported), display saturation kinetics, and are subject to competitive and non-competitive inhibition.

Distinguish transporters from channels.Edit

Channels are open all the time from both sides of the membrane. Transporters, on the other hand, have binding sites that face one side of the membrane at a time (alternating access). Channels transport much faster than transporters and do not saturate, and are also 'gated' and merely oscillate between open and closed states.

2. Distinguish facilitated transport from active transport.

Facilitated transport requires no energy source and therefore cannot work against a concentration gradient or electrochemical potential, whereas both types of active transport can. Also, active transport proteins can only run in one direction, whereas facilitated transport can occur in either. Facilitated transport is simply a way to get molecules through the plasma membrane that might otherwise be to large or hydrophillic.

3. Distinguish primary from secondary active transport.

Primary active transport uses ATP or another ATP-like high energy molecule as an energy source. Secondary active transport uses the concentration gradient of a secondary molecule as the energy source. A common example of secondary active transport would be using Na ion gradients to transport a sugar or amino acid from the intestinal lumen into epithelial cells. Secondary active transporters can work either as symporters (both molecules move in the same direction) or antiporters (opposite directions).

4. List the classes of ion-transporting ATPases and their major functions. Give examples of each class.

P-Type Transport ATPases P-type ATPases use a single active subunit (~100 kD) to actively transport a molecule across a membrane. In most cases this molecule is a cation. The key mechanism of action is an aspartate residue that induces the transporter to switch conformations when it is phosphorylated. In different conformations, labeled E1 and E2, the binding site has different affinities for the molecule, allowing the transporter to “pick up” and then “release.”

P-type ATPases are some of the most common workhorses on various membranes, including Ca-ATPase on the sarcoplasmic reticulum of muscle cells, Na, K-ATPases on neurons, and H, K-ATPase in parietal cells of the stomach.

F-Type Transport ATPases

Primarily found in the mitochondrial inner membrane in humans, ATP Synthase functions in the reverse direction of a conventional ATPase. Using two subdomains, F0 and F1, ATP Synthase uses a H+ concentration gradient to drive ATP production. Because this process theoretically can (and is in bacteria) be reversed to use ATP to create a H+ gradient, we consider it a transporter.

V-Type ATPases

Pump H+ into the lumens of lysosomes and neurosecretory vesicles. Once the concentration gradient is established another antiporter can be used to uptake norepinephrine or other molecules.

ATP Binding Cassette (ABC) Transporters

Made of a single ~140 kD subunit, ABC transporters display less specificity than other types. ABC transporters are widely involved in disease processes such as cystic fibrosis, tumor resistance, and multidrug resistance.

5. Define transcellular and paracellular transport across an epithelium.

Paracellular Transport: Refers to the transport of substances between cells of an epithelium. Normal capillaries have both para and transcellular transport, however the tight junctions of capillaries forming the blood-brain barrier allow for only paracellular transport to occur.

Transcellular Transport: Refers to the transport of substances through the cells of an epithelium, from basal to apical membrane or vice-versa.