Valinomycin mobile carrier

A compound that binds to an ion in a manner which greatly facihtates the bound ion s permeabihty across a membrane. Naturally occurring ionophores include both mobile carriers (e.g., valinomycin and nigericin) and channel formers (e.g., gramicidin A). 

Since valinomycin transports K+ specifically, it must bind the ion rather than provide a channel, as does gramicidin. Since transport can occur only when the membrane is fluid and since valinomycin is a small molecule, this suggests that the valinomycin-K+ complex must move through the membrane i.e., it behaves as a mobile carrier. The hydrocarbon side chains of the cyclic structure can be imagined on the outside of the structure, thus making the latter compatible with the hydrocarbon portion of the lipid bilayer. The inside contains the 12 carbonyl groups of the ester and amide bonds, six of which coordinate the K+ in the space at the center of the structure (Fig. 6-19). 

Mueller and Rudin and Lev and Buzhinsky found, however, that valinomycin induced an increase in the ion permeability of black lipid membranes which obviously were free of all complicated biological transducers. An alternative mechanism was proposed to explain the activity of valinomycin in which it was suggested that valinomycin forms an ion conducting channel across the lipid bilayer This mechanism was rejected in favor of the currently accepted mobile carrier mechanism when it was demonstrated that both valinomycin and nigericin could transport ions across bulk phases too thick to accommodate stacked molecular channels The generic term ionophore, i.e. ion carrier, was chosen to describe these compounds by Pressman et al. in order to emphasize the dynamic aspects of the transport mechanism Although this term is widely accepted, Ovchinnikov... 

There are two kinds of ionophores charged ones, which are called liquid ion exchangers, and neutral carriers. Because they are mobile in both the free and in the complexed form, mobilities of all species are again part of the selectivity coefficient together with the ion-exchange equilibrium constant. The best-known neutral ionophore is valinomycin (Fig. 6.15a) which shows a 1,000 1 selectivity for K+ in preference to Na+ and no pH dependence. In its uncomplexed form, it is electrically neutral. A better-known representative is di(n-octyl phenyl) phosphonate (Fig. 6.15c), which shows good selectivity for calcium ion and is relatively pH insensitive. 

Valinomycin, an antibiotic, is a mobile K+ carrier with an interior rich in polar amino acid residues (providing a binding site for K+) and hydrophobic valine residues on the outside. The latter allow valinomycin, bearing K+ in its interior, to diffuse across membranes, thus rendering them permeable to K+. Free K+ is unable to diffuse at appreciable rates across membranes. 

In 1969, Wipf and Simon reported an outstanding potassium ionophore, valinomycin. Valinomycin is an example of a mobile ion carrier. It is a ring-shaped polypeptide that increases the permeability of a membrane to K . The ring has a hydrophobic exterior, made up of valine side chains, and a polar interior, where a single K can fit precisely (see Figure 8.30). In the electrode process, valinomycin transports potassium ions across the membrane by picking up on the solution side of the membrane and releasing it at the transducer surface. 

Thylakoid membrane as a Molecular Voltmeter and Ammeter. The antibiotic valinomycin was found in 1964 by Moore and Pressman to affect ion transport across the mitochondrial membrane. Subsequently, Pressman and coworkers proposed that valinomycin is an ionophore and acts as a mobile ion carrier across the lipid membrane [cf. sectionlll.B. above]. Since the light-induced FIAC in thylakoid membrane appears to be an electrochromic band shift in pigments produced by an electric field, Boeck and Witt °, Junge and Schmid used this unique effect to probe the effect of compounds such as the ionophore, valinmycin, on the voltage and current density generated across the membrane. 

At a more detailed level, it is still not certain how even a relatively simple selective membrane operates (e.g. PVC containing plasticizer and valino-mycin, selective to K" ). The idea that PVC plus plasticizer acts as an inert structureless membrane in which the sensor species (the neutral carrier valinomycin in this case) dissolves, has been abandoned. A more heterogeneous structure is proposed, in which free dissociated counter-ions contribute to the membrane s conductivity. The counter-ions are presumed to arise from the dissociation of impurities associated with the method of manufacture of the PVC. Both fixed and mobile charged sites have been identified, and may represent, via the process of dissociation, an important source of counter-ion charge carriers (9-11). It is known that water penetrates the PVC film, but the exact role of this water is a matter still open to debate. Even the method of entrance of the water molecules is not clear do they enter in a uniform manner,... 

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