Potassium K Ion Channels

1 Introduction. Studies on ion channels, potassium ion channels in particular, carried out by Roderick MacKinnon and his research group, resulted in his receiving the 2003 Nobel Prize in chemistry for structural and mechanistic studies of ion channels. Thanks to this contribution and research continuing in the MacKinnon and many other laboratories, it is now possible to 

Physically, ion channels are tiny pores that stud the surface of all cells. The ion channels are important for, among other things, the function of muscles and the nervous system. These channels allow the passage of potassium, calcium, sodium, and chloride ions. Through a balance of electrical forces and chemical bonds, ion channels are specific for one ion for instance, a potassium ion channel will reject a sodium ion trying to enter its channel. An excellent visualization of the overall process is found at the website http //www. rockefeller.edu/pubinfo/howkion.html. It will be helpful to look at this website before going any further in the discussion. 

Potassium channels are part of a complex system that helps maintain the normal ionic balance across the cell membrane. In excitable cells, like those in nerves and muscles, the channels also help reestablish the electrical difference between the inside and outside of the cells after excitation. In the case of neuron firing, potassium ions, and thus positive charge, builds up inside the 

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Figure 12.11 Schematic diagram of the ion pore of the K+ channel. From the cytosolic side the pore begins as a water-filled channel that opens up into a water-filled cavity near the middle of the membrane. A narrow passage, the selectivity filter, links this cavity to the external solution. Three potassium ions (purple spheres) bind in the pore. The pore helices (red) are oriented such that their carboxyl end (with a negative dipole moment) is oriented towards the center of the cavity to provide a compensating dipole charge to the K ions. (Adapted from D.A. Doyle et al.. Science 280 69-77, 1998.)...

Potassium channels are a diverse and ubiquitous family of membrane proteins present in both excitable and nonexcitable cells that selectively conduct K+ ions across the cell membrane along its electrochemical gradient at a rate of 106-108 ions/s. 

Fromm and Spanswick [79] found that electrical stimulation of a plant is followed by ion shifts which are most striking in the phloem cells. While their content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly (Table 1). These displacements lead to the conclusion that Ca + influx as well as and CP efflux are involved in the propagation of action potentials. The main difference between propagation of action potentials in animals and plants is that in an axon there is the K /Na transmembrane transport but in phloem cells the K /Ca channels are involved in this process [Fig. 22(b)]. 

Potassium secretion is enhanced by aldosterone. As the concentration of K+ ions in the extracellular fluid increases, the secretion of aldosterone from the adrenal cortex also increases. The mechanism of action of aldosterone involves an increase in the activity of the Na+, K+ pump in the basolateral membrane. Furthermore, aldosterone enhances formation of K+ channels in the luminal membrane. 

Cloned Human Potassium Channels. Assessment of effects on cloned HERG K channels stablely expressed in a cell line by measurement of whole cell K current (lKr) using voltage clamp. Other cloned human ion channels (e.g., KvLQTl/minK-IKs currents) are also possible. 

BaS04 used as contrast agent for radiography and MRI and in bone cements used for the fixation of joint prostheses, Ba inhibits K flux through potassium ion channels Radiation therapy for carcinomas... 

Cell membranes contain selective ion channels that are highly discriminatory for potassium ions, sodium ions, calcium ions, and the proton. For instance, the highly selective potassium channels of nerves show selectivity for ions as Li < Na K > Rb > Cs, and calcium channels show selectivity as Mg Ca > Sr > Ba. Ion selection operates on the basis of size and repulsion, not... 

Facilitated diffusion within organisms takes place when carriers or proteins residing within membranes—ion channels, for instance—organize the movement of ions from one location to another. This diffusion type is a kinetic, not thermodynamic, effect in which a for the transfer is lowered and the rate of diffusion is accelerated. Facilitated diffusion channels organize ion movements in both directions, and the process can be inhibited both competitively and noncompetitively. It is known that most cells maintain open channels for K+ most of the time and closed channels for other ions. Potassium-ion-dependent enzymes include NaVK+ ATPases (to be discussed in Section 5.4.1), pyruvate kinases, and dioldehydratases (not to be discussed further). 

The Shaker potassium channel did not yield crystals for the first potassium ion channel X-ray crystallographic structure. Rather, it was the prokaryote bacterium Streptomyces lividans, abbreviated as the KcsA K+ channel, that first produced crystals suitable for crystallography. This structure, published in 1998 by MacKinnon s group in Science magazine, was received with great praise from the scientific community. It was known at that time that the amino acid sequence of KcsA was similar to that of other K channels, including vertebrate and invertebrate voltage-dependent K (Kv) channels (such as the... 

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