Describe the mechanism of P-type ATPases; select one P-type ATPase and examine how its physiological role has been investigated.
The P-type ATPases are a large family of membrane enzymes, with 476 different subtypes categorised in the Swiss-Prot protein information database. The P-type or E1-E2 ATPases were first discovered and categorised by Jens Christian Skou, a Danish physician-turned-physiologist in 1957. Skou discovered the Na+/K+ ATPase, and later shared half of the 1997 Nobel Prize for Chemistry in reward for his work (Skou 1997).
As mentioned above, the P-type ATPases have a broad range of interventions, for example, the use of the Digitalis toxin in the treatment of heart failure (Rang et al. 2007). There are 5 broad classes of P-type ATPase, and numerable sub-classes. Human ATP-ases are primarily grouped in classed I, II and IV, with group III consisting of bacterial enzymes, and group V largely unclassified (Stokes and Green 2003). An especially important role of an ATPase in humans is the transport of sodium and potassium ions across the cell membrane. It is this Na+/K+ ATPase that J.C Skou discovered, and worked on for most of his academic career (Skou 1997).
The fundamental basis of the P-type ATPase's ability to function is its capacity to form 2 conformational states, E1 and E2. Both of these states are ion-binding, one allowing intramembrane ion binding, and the other with an extramembrane ion binding site. The Na+/K+ ATPase is an anti-porter, transporting Na+ ions out of the cell, and K+ ions into the cell, at a 3:2 ratio (Na:K), against the concentration gradient (Lehninger et al. 2000). The process of transporting ions across the membrane is a 4 step mechanism, as shown in figure 1. Firstly, 3 Na+ ions bond to the intracellular bonding sites. ATP is then broken down to ADP, releasing a phosphate molecule which then binds to the ATPase, causing a conformational change of the enzyme, reducing the enzymes affinity to Na+ ions, causing the Na ions to be released extracellularly, and opening the K+ binding domain. 2 K+ ions then bind to their high-affinity binding sites, and the enzyme is dephosphorylated. This reduces the ATPase's K+ affinity, causing the release of K+ into the cell (Lehninger et al. 2000). It is the above process that is responsible for approximately 25% of the bodies daily ATP breakdown and consequent energy usage (Lehninger et al. 2000). The sodium and potassium ions, and the large concentration gradients that exist, consist a vital physiological feature of the cell, and one that is implicated in myriad cellular processes.
The main cellular process, as mentioned above, for which the Na/K-ATPase is responsible, is the maintenance of a membrane potential via the movements of its bonding ions. It is the maintenance, and the presence of this potential that allow the existence of the cell. The presence of a pre-existing potential allows transport of glucose and other molecules, such as Cl- ions...