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Examples of Transport: Metal Ions


These ions are both transported by the Na/K ATPase. This protein keeps the K+in and Na+out high compared to their respective concentrations on the other side of the membrane. The protein exists in two essential conformations, E1 and E2, depending on the phosphorylation state of the protein. ATP and 3 Na+ bind to the cytoplasmic domain of the enzyme in the E1 conformation. In the presence of Na ions, the bound ATP is cleaved in a nucleophilic atack by an Asp side chain of the protein. (Hence, the protein is a Na+-activated ATPase. The phosphorylated enzyme changes conformation to the E2 form in which Na+ ions are now on the outside of the cell membrane, from which they dissociate. The phosphorylated protein in conformation E2 now binds 2 K+ ions on the outside, which activates hydrolysis of the Asp-PO3 mixed anhydride link. The dephosphorylated protein is more stable in the E1 conformation to which it changes as it bring K+ ions into the cell. Thisis an example of an electrogenic antiporter. Transport proteins that use this mechanism of transport are designated as P types, since ATP cleavage is required and PO43- is covalenty transferred to an Asp residue from the ATP. P-Type transporters are inhibited by vanadate (VO43-), a transition state analog of phosphate.Note:Transport mediated by P type membrane proteins can, in the lab, be used to drive ATP synthesis.  

Detailed kinetic analysis of ATP and vanadate interactions show there are a low affinity and high affinity site for each on Na/K ATPase.The high affinity vandate site appears to be the same as the low affinity ATP site, which suggest that vandate binds tightly to the E2 form of the enzyme.The low affinity vandate site appears to be the same site (based on competition assays) as the ATP site, which is probably the E1 form. Hence vandate binds preferentially to the E2 form would inhibit the transition to the E1 form.Vanadate also inhibits phosphatases, enymes that cleaves phosphorylated Ser, Thr, and Tyr - phosphoesters in proteins.This supports the notion that vanadate binds preferentially to the E2 form, which has a phosphoanhydride link (Asp-O-phosphate) that is hydrolyzed, promoting the conversion of E2 back to E1.  Vanadate is probably at transition state analog inhibitor in that it can readily adopt a trigonal bipyramidal structure, mimicking the transition state for cleavage of the tetrahedral anhydride bonds of ATP and Asp-O-PO4.




In addition to the above mechanism, K ions can be transported with protons in an electroneutral antiport mechanism by a K+/H+-ATPase found in stomach cells, which gives rise to a low pH in the lumen of the stomach.

Figure: K/H ATPase



Calcium levels are very low in cells. Transient increases are more likely to be detected in a signal transduction pathways than a transient decrease in high basal or constituitive cytoplasmic levels. Ca2+-ATPase, homologous to the Na/K-ATPase, removes Ca2+ from the cytoplasm to either the outside of the cells or into internal organelles. In addition a Na+-Ca2+ exchange protein (an antiporter) transports calcium ions out of the cell using a sodium-motive potential.
Transport of calcium ions



Figure: Transport of calcium ions

All of above ATPases are examples of P-type ion transporters.

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 ofacidic organelles (like lysosomes) and acidic vesicles within neurons, where neurotransmitters are stored.