BIOCHEMISTRY - DR. JAKUBOWSKI
Learning Goals/Objectives for Chapter 9A:
Cystic Fibrosis Transmembrane Conductance Regulator (CFTR) - This is a member of a family of an ATP-Binding Cassette or ABC transporter proteins. The membrane protein has 12 transmembrane helices. In contrast to other ion transporters which transport a discrete number of ions (3 sodium and 2 potassium ions, for example), this changes conformation to form an open pore through which chloride ions flow. This protein is defective in Cystic Fibrosis.
Multidrug Resistance Transporter - MDR - This is another example of an ATP-Binding Cassette or ABC transporter. It acts in a somewhat promiscuous fashion in pumping nonpolar toxic molecules out of the cell. This would seem quite beneficial to the organism, unless the toxic molecule is a chemotherapeutic drug used to kill a tumor cell.
Phospholipid Flippase or Transbilayer amphipath transporter (TAT) - This is a member of the P-Type ATPase family which instead of moving ions across the membrane flips amino lipids (like PE) across leaflets in the bilayer. In an early chapter we noted that flip-flop diffusion in liposomes was slow compared to that in cells, suggesting that the flip-flop diffusion was catalyzed in the cell. Catalysis requires ATP cleavage and produces two conformations of the protein. During the conformational change of the protein, a phospholipid appears to bind to the protein and is flipped to the other side of the membrane.
The disposition of phosphatidylserine, a
negatively charged phospholipid, between membrane leaflets is
especially interesting and important. Almost all the PS is localized
in the inner leaflet. Cells in which PS is found in the outer leaflet
are target for program cell death (apoptosis). PS in the outer leaflet
can also promote blood clotting as clotting factors are recruited to the
surface. It appears that a P-type ATPase is required. Using gene
silencing by RNA interference in C. Elegans, Darland-Ranson found that
onespecific P-type ATPase, TAT-1 out of 6 found in the organisms had PS
flippase activity, which would retain PS in the inner leaflet. Cells
with PS in the outer leaflet were often targets of phagocytosis, suggesting
the phagocytes have receptors that recognize PS. Cells with PS
receptors may also bind and internalize virus, which have membrane leaflets
acquired from infected cells as the virus buds off from the cells.
Such cells might have PS in their outer leaflets since the infected cells
may be in the process of dying through apoptosis, which would increase PS in
the outer leaflet.
Experimental Study of Flipping: Labeling new PL ; Assay for Flip-flop Diffusion
There are also other types. F-type are similar to the F0F1ATPases and can transport protons against a concentration gradient powered by ATP breakdown. Notice that this is the opposite role for this enzyme that we discussed in mitochondrial oxidative phosphorylation. V-type (vacuolar) are found in the membranes of acidic organelles (like lysosomes) and acidic vesicles within neurons, where neurotransmitters are stored.
As mentioned earlier, one of the biggest problems in medical drug development is the productions of drugs that can diffuse across the cell membrane. This requires that the drug be sufficiently nonpolar while at the same time being sufficiently polar to have reasonable aqueous solubility, allowing blood transport. Another approach to getting drugs across the membrane is to modify them to bind to transporters whose normal function is to move solutes against a concentration gradient across a lipid bilyaer. The extent of modification of the drug depends on how close the structure of the drug is in comparison to the normal ligand for the transporter. This approach has been used by the biotech company XenoPort, to develop drugs that can be more readily absorbed by the small intestine, which has many active transporters designed to move nutrients into cells.
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