Monensin selective binding

Fig. 16. The antibiotic ligands (a) monensin, which binds Na+ selectively, and (b) valinomycin and (c) enniatin-B, which bind K+ selectively.

The antibiotics are compounds secreted by microbes that enhance the permeability of membranes to cations. One class functions by binding a metal to give a liposoluble complex that can then pass across the membrane. Examples are valinomycin, a cyclic peptide that binds K+ selectively, and monensin which binds Na+. These too are oxygen-donor ligands, and will be discussed in the following section. They function as antibiotics because they allow the concentrations of a cation across membranes to become equalized, and so cause the collapse of the membrane potential. 

The cyclic polyether nonactin selectively binds and transports ions, while the acyclic polyether monensin (see page 234 for the structure) selectively binds and transports Na ions. Use information in Section 8.9 to help you answer the following questions. 

The question of carrier design was first addressed for the transport of inorganic cations. In fact, selective alkali cation transport was one of the initial objectives of our work on cryptates [1.26a, 6.4]. Natural acyclic and macrocyclic ligands (such as monensin, valinomycin, enniatin, nonactin, etc.) were found early on to act as selective ion carriers, ionophores and have been extensively studied, in particular in view of their antibiotic properties [1.21, 6.5]. The discovery of the cation binding properties of crown ethers and of cryptates led to active investigations of the ionophoretic properties of these synthetic compounds [2.3c, 6.1,6.2,6.4-6.13], The first step resides in the ability of these substances to lipophilize cations by complexation and to extract them into an organic or membrane phase [6.14, 6.15]. 

In biological systems, ionophores selectively transport alkali- and alkaline-earth metal ions for example, vali-nomycin is a selective carrier for K+ and monensin for Na+, bnt they also can bind other cations for which they have not been designed by natnre and this is an area of active cnrrent research with natnral ionophores. 

Some natural products are crown ether derivatives. They can sequestrate alkali and alkaline earth ions. These polypeptides include valinomycin, monactin, and mo-nensin. They play an important biological role They bind K+ in preference to Na" " ions, except for monensin, where the tendency is reversed. They also contribute to the maintenance of the different concentrations of these ions inside and outside the cells and, consequently, to that of the potential differences across cell membranes. Beyond these biological aspects, valinomycin is the basis for an ion-selective electrode for K+. 

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