Transport of ions across biological membranes

A further type of mediator includes substances with a relatively low molecular weight that characteristically facilitate the transport of ions across biological membranes and their models. These transport mechanisms can be divided into four groups ... 

Passive and active transport of ions across biological membranes... 

PASSIVE AND ACTIVE TRANSPORT OF IONS ACROSS BIOLOGICAL MEMBRANES 187... 

Transport of ions across the membranes of the cell and organelles is a prerequisite for many of life s processes. Transport often involves very precise selectivity for specific ions. Recently atomic-resolution structures have been determined for channels and pumps that are selective for sodium, potassium, calcium and chloride, for the most abundant ions in biology. These structures provide the desired understanding of the principle of selective ion-transport in terms of the architecture and detailed chemistry of conduction pathways [35]. 

Various ATPases that drive the transport of cations across biologic membranes have been described. The classes of ion-motive ATPases thus far described are listed in the following table and can be classified into multi-subunit and single-or two-subunit types, the F,Fo and V-ATPases representing the former and the P type the latter. The class of ATPases represented by the P glycoproteins or ABC transporters will not be discussed here. 

This section outlined some approaches used to study the mechanism of proteins that transport inorganic ions across biological membranes. In the next section the properties of some individual proteins will be discussed. 

Tetrahydropyran-2-methanol and tetrahydrofurfuryl alcohol are key chiral intermediates for the synthesis of polyether antibiotics. Such antibiotics are known as ionophores, which are able to transport metal ions across biological membranes. Some of their medicinal applications are in diuretics and analgesics [211]. Chiral (S)-tetrahydropyran-2-methanol 119 and 120 has been prepared from the hydrolysis of its butyrate ester 121 using porcine pancreatic lipase [212]. The unreacted (R) enantiomer of ester 122 was hydrolyzed by lipase from Candida rugosa. Both enantiomers were prepared in more than 97% e.e. (Fig. 42). 

Macrocyclic compounds with ion-chelating properties occur naturally and often function as ionophores, translocating ions across biological membranes many of these compounds are small cyclic polypeptides. Some natural carboxylic polyethers are selective for Li+ and are, therefore, ionophores for Li+. Monensin, shown in Figure Id, is a natural ionophore for Na+ but it will also complex with Li+ and it has been shown to mediate the transport of Li+ across phospholipid bilayers [21]. It has been proposed that synthetic Li+-specific ionophores have a potential role as adjuvants in lithium therapy, the aim being to reduce the amount of... 

Movement of polar compounds and ions across biological membranes requires protein transporters. Some transporters simply facilitate passive diffusion across the membrane from the side with higher concentration to the side with lower. Others bring about active movement of solutes against an electrochemical gradient such transport must be coupled to a source of metabolic energy. 

Polylactones (for an example, see Fig. 14.4) are synthetic analogues of naturally occurring ionophores such as enniatin (species that transport ions across biological membranes). Molecular mechanics calculations have been used to predict the stability and selectivity with respect to Li+, Na+, and K+ of a series of new polylactones12661. Metal-ligand interactions were again modeled using a combination of van der Waals and electrostatic terms. 

Fig. B.6.1. Schematic representation of the action of gramicidin A (a), an ionophore (b), and a ligand-controlled ion channel (c) in the transport of ions across a biological membrane.

Interest in these compounds (Fig. 1) derives from their ability to transport ions across biologic membranes, and some terrestrial polyethers have been used widely in veterinary medicine. Marine polyethers are responsible for numerous cases of human food poisoning and toxic algal tides, which cause massive fish kills. 

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