Transmembrane anion transport

Haynes CJE, Gale PA (2011) Transmembrane anion transport by synthetic systems. Chem Commun 47 8203... 

Transmembrane anion transport mediated by halogen-bond donors. Nat Commun 3 905... 

One of the major integral proteins of the erythrocyte membrane is the anion channel, or band-3 protein, which moves Cl- and HC03 anions across the membrane. The anion transporter has two identical subunits with molecular weights of about 95,000, and each subunit probably has 10 or 11 transmembrane helices. The band-3 protein is attached to the spectrin cytoskeleton through a smaller protein, anky-rin. The cytosolic domain of the anion transporter also binds the glycolytic enzyme glyceraldehyde-3-phosphate dehydrogenase. 

McNally BA et al (2008) Structure-activity relationships in cholapod anion carriers enhanced transmembrane chloride transport through substituent tuning. Chem Eur J 14 9599-9606... 

Eeng B, Shu Y, Giacomini KM. Role of aromatic transmembrane residues of the organic anion transporter, rOAT3, in substrate recognition. Biochemistry 2002 41 8941-8947. 

Coenzyme Q is a quinone derivative with a long isoprenoid tail. The number of five-carbon isoprene units in coenzyme Q depends on the species. The most common form in mammals contains 10 isoprene units (coenzyme Qio) For simplicity, the subscript will be omitted from this abbreviation because all varieties function in an identical manner. Quinones can exist in three oxidation states (Figure 18.10). In the fully oxidized state (Q), coenzyme Q has two keto groups. The addition of one electron and one proton results in the semiquinone form (QH ). The semiquinone form is relatively easily deprotonated to form a semiquinone radical anion (Q ). The addition of a second electron and proton generates ubiquinol (QH2), the fiilly reduced form of coenzyme Q, which holds its protons more tightly. Thus, ybr quinones, electron-transfer reactions are coupled to proton binding and release, a property that is key to transmembrane proton transport. 

OAT/OCT Most solute transporters, including the members of the 5LC22-family, share a predicted membrane topology with 12 transmembrane domains. This family comprises organic cation transporters (OCTs), as well as transporters for the zwitterion carnitine (OGTNs) and organic anion transporters (OATs). 

Ankyrin, anchorin, syndein a membrane protein found in erythrocytes and brain, which binds Spectrin (see) to the membrane. Spectrin-binding activity was first isolated as a proteolytic fragment of M, 72,000 the M, of the entire protein is 215,000. There are about 10 copies of A. per erythrocyte ghost, i.e. the number required to bind all the spectrin dimers in a 1 1 complex. In the erythrocyte, A. links spectrin to the cytoplasmic part of Band 3 (a transmembrane protein), to the anion-transport protein, and to microtubules. A. is also able to bind unpolymerized tubulin. [V. Bennett, Annu. Rev. Biochem. 54 (1985) 273-304 V. Bennet Ankyrins adaptors between diverse plasma membrane proteins and the cytoplasm J. Biol. 

There are far less examples of naturally occurring anion transporters. Dnramycin 3 is a naturally occurring lantibi-otic that has been reported to form ion channels displaying good Cl anion selectivity (Figure 5). Moreover, duramycin has recently been tested in clinical trials for treatment of cystic fibrosis, a disease cansed by defects in the protein that facilitates transmembrane transport of Cl and HC03 anions. 

Both secondary active transport and positive cooperativity effects enhance carrier-mediated solute flux, in contrast to negative cooperativity and inhibition phenomena, which depress this flux. Most secondary active transport in intestinal epithelia is driven by transmembrane ion gradients in which an inorganic cation is cotransported with the solute (usually a nutrient or inorganic anion). Carriers which translocate more than one solute species in the same direction across the membrane are referred to as cotransporters. Carriers which translocate different solutes in opposite directions across the membrane are called countertransporters or exchangers (Figs. 10 and 11). 

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