Transport . 97 proteins

Illustrations B-D show transporters whose structure has been determined experimentally or established on analogy with other known structures. They all belong to group III of the a-helical transmembrane proteins (see p. 214). 

Some cells couple the pure transport forms discussed on p. 218—i.e., passive transport (1) and active transport (2)—and use this mechanism to take up metabolites. In secondary active transport (3), which is used for example by epithelial cells in the small intestine and kidney to take up glucose and amino acids, there is a symport (S) located on the luminal side of the membrane, which takes up the metabolite M together with an Na ion. An ATP-dependent Na transporter (Na /lC ATPase see p. 350) on the other side keeps the intracellular Na+ concentration low and thus indirectly drives the uptake of M. Finally, a uniport (U) releases M into the blood. 

Glut-1 consists of a single peptide chain that spans the membrane with 12 a-helices of different lengths. The glucose is bound by the peptide loops that project on each side of the membrane. 

Aquaporins help water to pass through biological membranes. They form hydrophilic pores that allow H2O molecules, but not hydrated ions or larger molecules, to pass through. Aquaporins are particularly important in the kidney, where they promote the reuptake of water (see p. 328). Aquaporin-2 in the renal collecting ducts is regulated by antidiuretic hormone (ADH, vasopressin), which via cAMP leads to shifting of the channels from the ER into the plasma membrane. 

Aquaporin-1, shown here, occurs in the proximal tubule and in Henle s loop. It contains eight transmembrane helices with different lengths and orientations. The yellow-colored residues form a narrowing that only H2O molecules can overcome. 

The general features of cell membrane structure have been known since Evert Gorter (1881-1954) and his student E Grendel first postu- 

In collaboration with crystallographers, MacKinnon obtained an X-ray crystallographic structure at 3.2 A resolution in 1998, with 2.0 A resolution achieved in 2001. The details indicate that the conserved sequence is part of a loop that can be either external to the pore or extend into the pore to form the selectivity filter. This structure helps explain an interesting evolutionary anomaly. Charybdotoxin, a small protein in scorpion venom, inhibits the K channel in skeleton muscle cells. MacKinnon discovered that it also inhibits the fruit fly K channel protein and even the K channel in E. coli bacteria. He posed the rhetorical question  

In contrast to the structure of the K channel, the Cl channel maintains a large cavity but the ion is surrounded by the positive amino ends (-NH3 ) of peptide chains. The pore channel/filter places amide nitrogen atoms (-CO-NH-) rather than carbonyl oxygen atoms in contact with Cl . The result is to create a low-energy pathway for an anion rather than for a cation through the Cl membrane channel protein. MacKinnon s group and others are currently involved in studies of the structure and function of the gating mechanism that opens and closes the channel at the intracellular end. 

In 1991, Agre s research group established that the new protein was a transmembrane protein and cloned the c-DNA (complementary strand 

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CCK is found in the digestive tract and the central and peripheral nervous systems. In the brain, CCK coexists with DA. In the peripheral nervous system, the two principal physiological actions of CCK are stimulation of gaU. bladder contraction and pancreatic enzyme secretion. CCK also stimulates glucose and amino acid transport, protein and DNA synthesis, and pancreatic hormone secretion. In the CNS, CCK induces hypothermia, analgesia, hyperglycemia, stimulation of pituitary hormone release, and a decrease in exploratory behavior. The CCK family of neuropeptides has been impHcated in anxiety and panic disorders, psychoses, satiety, and gastric acid and pancreatic enzyme secretions. 

Food vitamin B 2 appears to bind to a saUvary transport protein referred to as the R-protein, R-binder, or haptocorrin. In the stomach, R-protein and the intrinsic factor competitively bind the vitamin. Release from the R-protein occurs in the small intestine by the action of pancreatic proteases, leading to specific binding to the intrinsic factor. The resultant complex is transported to the ileum where it is bound to a cell surface receptor and enters the intestinal cell. The vitamin is then freed from the intrinsic factor and bound to transcobalamin II in the enterocyte. The resulting complex enters the portal circulation. 

Transport. Transcobalamin II dehvers the absorbed vitamin 3 2 to cells and is the primary plasma vitamin B22-binding transport protein. It is found in plasma, spinal fluid, semen, and extracellular fluid. Many cells, including the bone marrow, reticulocytes, and the placenta, contain surface receptor sites for the transcobalamin II—cobalamin complex. 

Substances, including metabolites, that bind to transport proteins and/or receptors in the body. 

Antiparallel beta (P) structures comprise the second large group of protein domain structures. Functionally, this group is the most diverse it includes enzymes, transport proteins, antibodies, cell surface proteins, and virus coat proteins. The cores of these domains are built up by p strands that can vary in number from four or five to over ten. The P strands are arranged in a predominantly antiparallel fashion and usually in such a way that they form two P sheets that are joined together and packed against each other. 

Transport proteins Elemoglobin Serum albumin Glucose transporter... 

FIGURE 5.13 Two basic types of biological transport are (a) transport within or between different cells or tissues and (b) transport into or out of cells. Proteins function in both of these phenomena. For example, the protein hemoglobin transports oxygen from the lungs to actively respiring tissues. Transport proteins of the other type are localized in cellular membranes, where they function in the uptake of specific nutrients, such as glucose (shown here) and amino acids, or the export of metabolites and waste products. 

The electron transport protein, cytochrome c, found in the mitochondria of all eukaryotic organisms, provides the best-studied example of homology. The polypeptide chain of cytochrome c from most species contains slightly more than 100 amino acids and has a molecular weight of about 12.5 kD. Amino acid sequencing of cytochrome c from more than 40 different species has revealed that there are 28 positions in the polypeptide chain where the same amino acid residues are always found (Figure 5.27). These invariant residues apparently serve roles crucial to the biological function of this protein, and thus substitutions of other amino acids at these positions cannot be tolerated. 

Many proteins found in nature are glycoproteins because they contain covalently linked oligo- and polysaccharide groups. The list of known glycoproteins includes structural proteins, enzymes, membrane receptors, transport proteins, and immunoglobulins, among others. In most cases, the precise function of the bound carbohydrate moiety is not understood. 

Some transport proteins merely provide a path for the transported species, whereas others couple an enzymatic reaction with the transport event. In all cases, transport behavior depends on the interactions of the transport protein not only with solvent water but with the lipid milieu of the membrane as well. The dynamic and asymmetric nature of the membrane and its components (Chapter 9) plays an important part in the function of these transport systems. 

FIGURE 10.5 A model for the arrangement of the glucose transport protein in the erythrocyte membrane. Hydropathy analysis is consistent with 12 transmembrane helical segments. 

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. 

Half-saturation constant (concentration for 50% saturation of the transport protein). 

Gene activated Lipoprotein lipase fatty acid transporter protein adipocyte fatty acid binding protein acyl-CoA synthetase malic enzyme GLUT-4 glucose transporter phosphoenolpyruvate carboxykinase... 

Unfortunately, the pharmacology of chloride channels is poorly developed. Specific and highly useful inhibitors or modulators (e.g. strychnine, picrotoxin, diazepams) are only available for ligand-gated chloride channels (but these are covered in a different chapter). There are several chloride channel inhibitors such as the stilbene-disulfonates DIDS and SITS, 9-antracene-carboxylic acid (9-AC), arylaminobenzoates such as DPC and NPPB, niflumic acids and derivates, sulfony-lureas, and zinc and cadmium. All of these inhibitors, however, are not veiy specific. Several of these inhibitors (e.g. DIDS) inhibit many chloride channels only partially even at millimolar concentrations and have effects on other types of transport proteins. 

Cyclic Adenosine Monophosphate Table Appendix Membrane Transport Proteins Cyclic Guanosine Monophosphate Non-Selective Cation Channels... 

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