Membranes translocation carrier specificity

Selective permeability. Because of their hydrophobic nature, the hydrocarbon chains in lipid bilayers provide a virtually impenetrable barrier to the transport of ionic and polar substances. Specific membrane proteins regulate the movement of such substances into and out of cells. To cross a lipid bilayer, a polar substance must shed some or all of its hydration sphere and bind to a carrier protein for membrane translocation or pass through an aqueous protein channel. Both methods shield the hydrophilic molecule from the hydrophobic core... 

A classic temperature jump investigation from Eigen s laboratory resulted in suggestions that the specificity of carriers for membrane translocation of certain ions rested with the rate of dissociation of the complex formed. Several antibiotics act as specific carriers of alkali metal ions across membranes. The cyclic polyether nonactin, for instance, which can form complexes with either Na or K, will selectively transport only the latter. Diebler et al. (1969) found that the association rate constants for... 

Oligomycin is an antibiotic that inhibits respiration in intact mitochondria. Respiration is not inhibited in uncoupled mitochondria, i.e., those mitochondria in which 02 consumption occurs but in which no ATP is synthesized. Thus, oligomycin does not block respiratory carriers, in contrast to inhibitors such as rotenone and cyanide. Instead, oligomycin blocks proton translocation through the F component to the F( component, through a specific interaction with a subunit of the membrane-associated F . The subscript o in the term F was originally used to indicate the oligomycin-sensitive" complex. 

Chloroplasts are enclosed by two membranes. The outer membrane is freely permeable to small molecules (up to about 10 kDa) due to the presence of a porin and the inner membrane is the osmotic barrier and the site where specific transport occurs. The specificity of envelope permeability is strikingly highlighted by the contrast between Pi and PP the former being among the most rapidly translocated molecules and the latter among those to which the envelope is relatively impermeable. Carrier-mediated anion transport can be classified as ... 

The carrier protein facilitating Pj and phosphate ester transport is of particular interest in leaves in connection with carbon processing - i.e., the synthesis, transport and degradation of carbohydrate, all of which occur in the cytosol [51]. This metabolite carrier, called the phosphate translocator, is a polypeptide with a molecular mass of 29 kDa and is a major component of the inner envelope membrane [52,53]. The phosphate translocator mediates the counter-transport of 3-PGA, DHAP and Pj. The rate of Pj transport alone is three orders of magnitude lower than with simultaneous DHAP or 3-PGA counter-transport [54]. Consequently operation of the phosphate translocator keeps the total amount of esterified phosphate and Pj constant inside the chloroplast. Significantly, the carrier is specific for the divalent anion of phosphate. 

Another form of facilitated diffusion involves membrane proteins called carriers (sometimes referred to as passive transporters). In carrier-mediated transport, a specific solute binds to the carrier on one side of a membrane and causes a conformational change in the carrier. The solute is then translocated across the membrane and released. The red blood cell glucose transporter is the best-characterized example of passive transporters. It allows D-glucose to diffuse across the red blood cell membrane for use in glycolysis and the pentose phosphate pathway. Facilitated diffusion increases the rate at which certain solutes move down their concentration gradients. This process cannot cause a net increase in solute concentration on one side of the membrane. 

The cell controls the expression and activity of membrane proteins that increase the movement of specific molecules across the membrane. These proteins can be classified broadly according to the energy consumed as those governing facilitated diffusion or active transport. They may also be categorized as channels that form an aqueous pore in the membrane through which solutes can pass or carriers that selectively bind a molecule to effect its translocation. The term transporter is currently used to refer to both channels and carriers, although some authors use it synonymously with carrier. 

However, some of the properties of electron carriers (such as their observed redox potentials) do not fit in such a simple loop model. This has led Mitchell [11] to propose a modified mechanism, the so-called proton-motive Q cycle (Fig. 4B). In this model quinones function in two separate reactions in the QH2/QH- and the Q/QH- couple. These couples have different midpoint redox potentials and would operate at the reducing and the oxidizing site of cytochrome b, respectively. During these reactions proton translocation is supposed to occur by diffusion of the quinones in the fully oxidized (Q) and fully reduced (QH2) forms through the hydrophobic environment between their successive reaction sites at both sides of the membrane. Recently some experimental support for such a role of quinones has been obtained. Alternative models which will not be discussed here, have been postulated by Papa [12] and Williams [13]. Currently there is no conclusive support for a specific model. 

Most solutes are translocated across the cytoplasmic membrane by secondary transport either passively, without the involvement of specific membrane proteins or facilitated by specific carrier proteins. This latter mechanism is often termed active transport (see Fig. 2). 

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