Membrane-binding transport proteins

Dutta-Roy, A.K. (2000) Cellular uptake of long chain fatty acids role of membrane associated fatty acid binding/transport proteins. Cellular and Molecular Life Sciences (in press). 

F. 10.12. Active transport by Na, K -ATPase. Three sodium ions bind to the transporter protein on the cytoplasmic side of the membrane. When ATP is hydrolyzed to ADP, the carrier protein is phosphorylated and undergoes a change in conformation that causes the sodium ions to be released into the extracellular fluid. Two potassium ions then bind on the extracellular side. Dephosphorylation of the carrier protein produces another conformational change, and the potassium ions are released on the inside of the cell membrane. The transporter protein then resumes its original conformation, ready to bind more sodium ions. 

FIGURE 10.18 A model for the structure of the a-factor transport protein in the yeast plasma membrane. Gene duplication has yielded a protein with two identical halves, each half containing six transmembrane helical segments and an ATP-binding site. Like the yeast a-factor transporter, the multidrug transporter is postulated to have 12 transmembrane helices and 2 ATP-binding sites. 

SNAPs is an acronym for soluble NSF attachment proteins. They were originally discovered as cofactors for NSF that mediate the membrane binding of NSF in in vitro transport assays. Several isoforms of SNAPs exist in mammalian cells. SNAPs are also highly conserved proteins. Crystallographic studies indicated that the proteins form a very stiff and twisted sheet that is formed by a series of antiparallel and tightly packed helices connected by short loops. 

The free fatty acid uptake by tissues is related directly to the plasma free fatty acid concentration, which in turn is determined by the rate of lipolysis in adipose tissue. After dissociation of the fatty acid-albumin complex at the plasma membrane, fatty acids bind to a membrane tty acid transport protein that acts as a transmembrane cotransporter with Na. On entering the cytosol, free fatty acids are bound by intracellular fatty acid-binding proteins. The role of these proteins in intracellular transport is thought to be similar to that of serum albumin in extracellular transport of long-chain fatty acids. 

Figure 50-4. Absorption of iron. is converted to Fe + by ferric reductase, and Fe " is transported into the enterocyte by the apicai membrane iron transporter DMTl. Fieme is transported into the enterocyte by a separate heme transporter (HT), and heme oxidase (FiO) reieases Fe from the heme. Some of the intraceiiuiar Fe + is converted to Fe + and bound by ferritin. The remainder binds to the basoiaterai Fe + transporter (FP) and is transported into the biood-stream, aided by hephaestin (FiP). in piasma, Fe + is bound to the iron transport protein transferrin (TF). (Reproduced, with permission, from Ganong WF Review of Medical Physiology, 21 st ed. McGraw-Hill, 2003.)...

Recent evidence indicates that the 5-HT transporter is subject to post-translational regulatory changes in much the same way as neurotransmitter receptors (Blakeley et al. 1998). Protein kinase A and protein kinase C (PKC), at least, are known to be involved in this process. Phosphorylation of the transporter by PKC reduces the Fmax for 5-HT uptake and leads to sequestration of the transporter into the cell, suggesting that this enzyme has a key role in its intracellular trafficking. Since this phosphorylation is reduced when substrates that are themselves transported across the membrane bind to the transporter (e.g. 5-HT and fi -amphetamine), it seems that the transport of 5-HT is itself linked with the phosphorylation process. Possibly, this process serves as a homeostatic mechanism which ensures that the supply of functional transporters matches the demand for transmitter uptake. By contrast, ligands that are not transported (e.g. cocaine and the selective serotonin reuptake inhibitors (SSRIs)) prevent the inhibition of phosphorylation by transported ligands. Thus, such inhibitors would reduce 5-HT uptake both by their direct inhibition of the transporter and by disinhibition of its phosphorylation (Ramamoorthy and Blakely 1999). 

Binding to transport proteins may be of particular interest, since binding not only assays the affinities of the binding site on the transporter protein but also the translocation equilibria [67], In terms of enzyme catalysis, a transport protein transforms a substrate, a molecule located at one side of the membrane, into a product, the same molecule at the other side of the membrane, without chemical modification. Substrate must bind to a particular conformation of the enzyme with the binding sites accessible only from, for example, the outside. Similarly, the release of the product has to occur from a conformation which opens the binding site to the inside only this implies at least one transition step between the two types of conformations (see Fig. 

Note that in equilibria (2) the subscripts per and cyt are omitted where substrate S is concerned. This is obvious when the binding is measured to a solubilized transport protein, but also in the case where the enzyme is embedded in the membrane of closed vesicular structures, internal and external substrate will have equal concentrations at equilibrium (see Eig. 5). Consequently, the binding is independent of the orientation of the enzyme in the membrane. 

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