Extracytoplasmic

The H,K-ATPase, expressed in the parietal cells of the stomach, transports H+ ion from cytoplasm to lumen in exchange for extracytoplasmic K+ ion in an electroneutral exchange using the energy of ATP hydrolysis. 

Substituted benzimidazole inhibitors show slightly different effects depending on the inhibitor structure. Omeprazole binds to cysteines in the extracytoplasmic regions of M5/M6 (cys-813) and M7/M8 (cys-892). 

Proton Pump Inhibitors and Acid Pump Antagonists. Figure 2 Chemical mechanism of irreversible PPIs. PPIs are accumulated in acidic lumen and converted to active sulfenic acid and/or sulfenamide by acid catalysis. These active forms bind to extracytoplasmic cysteines of the gastric H.K-ATPase [3]. 

Table 2. Estimated cytoplasmic and extracytoplasmic K, Na and Cl concentrations in some eukaryotes...

Figure 46-5 shows a variety of ways in which proteins are distributed in the plasma membrane, in particular, the amino terminals of certain proteins (eg, the LDL receptor) can be seen to be on the extracytoplasmic face, whereas for other proteins (eg, the asialoglycoprotein receptor) the carboxyl terminals are on this face. To explain these dispositions, one must consider the initial biosynthetic events at the ER membrane. The LDL receptor enters the ER membrane in a manner analogous to a secretory protein (Figure 46-4) it partly traverses... 

DTSSP reportedly has been used to crosslink the extracytoplasmic domain of the anion exchange channel in human erythrocytes (Staros and Kakkad, 1983), for the characterization... 

The Cryptosporidium parasite attaches to the host s intestinal epithelium, becomes intracellular but remains extracytoplasmic. In vitro studies suggest that attachment is mediated by a Cryptosporidium parvum sporozoite ligand and an intestinal epithelial cell surface protein interaction [83, 84],... 

Bitopic proteins with a single transmembrane helix are more common. If oriented with the N-terminus extra-cytoplasmic, they are classified as type I or, if cytoplasmic, type II (Fig. 2-4). Bitopic membrane proteins are often involved in signal transduction, as exemplified by receptor-activated tyrosine kinases (Ch. 24) agonist occupation of an extracytoplasmic receptor domain can transmit structural changes via a single transmembrane helix to activate the latent kinase activity in a cytoplasmic domain. 

Most ZIP proteins have eight predicted transmembrane domains (Figure 7.10) and similar predicted topologies with both the N- and C-termini located on the extracytoplasmic face of the membrane with a His-rich domain frequently in the long cytoplasmic loop between transmembrane domains 3 and 4. In contrast, most CDF transporters have six predicted transmembrane domains and a His-rich domain in the loop between domains 4 and 5, but here the N- and C-termini are on the cytoplasmic side of the membrane. 

The last class of three major membrane anchors is caused by the modification by a glycophospholipid, glycosylphosphatidylinositol (GPI) (Udenfriend and Kodukula, 1995a Takeda and Kinoshita, 1995). They are observed in many eukaryotes, especially in protozoa and yeasts. Unlike other classes, the GPI-anchored proteins are exposed at the (extracytoplasmic) surface of the plasma membrane. Thus, we can predict the localization at the plasma membrane from the presence of a GPI anchor, although some of them are further incorporated into the cell wall in S. cerevisiae (as described in Section III,K,1). 

Gangliosides are a class of glycosphingolipid characterized by the presence of one or more sialic acid residues. They are located in the cell membranes, particularly in the plasma membrane, of almost all cell types. They are anchored in the membranes through their ceramide moiety and have the polar sugar-containing group in the extracytoplasmic side. They are particularly abundant in the nervous system where their expression pattern is modulated during development. 

Lukes J (1992) Life cycle of Goussia pannonica (Molnar 1989) (Apicomplexa, Eimerior-ina), an extracytoplasmic coccidium from the white bream Blicca bjoerkna. Proto-zool 39 484-494... 

DTSSP reportedly has been used to cross-link the extracytoplasmic domain of the... 

The first essential coupling rule states that the phosphorylation from ATP requires prior binding at the cytoplasmically orientated high-affinity uptake sites of those ions that are to be transported away from the cytoplasm (henceforth designated a-type ions, see Table 2). As long as the extracy toplasmic discharge sites have not yet been vacated by release of the translocated a-type ions, the phosphorylation reaction is fully reversible. After extracytoplasmic dissociation of the a-type ion, reversal of pump phosphorylation with resulting ATP synthesis can still... 

Figure 6. Time course of change in catalytic specificity (upper panel) and Ca2+ dissociation from extracytoplasmic low affinity sites (lower panel) following phosphorylation of the SR Ca2+-ATPase with ATP. The amount of ADP-insensitive phosphoen-zyme (E2P) was measured in two ways (I) [y-32P]ATP was included in the reaction mixture and the radioactivity incorporated into the enzyme was determined after acid quenching at various time intervals. To remove the ADP-sensitive phosphoenzyme so that only ADP-insensitive phosphoenzyme was measured, ADP was added 4 sec before the quench (upper panel, right scale) (2) by the enhancement of fluorescence from a trinitrophenyl-derivative of ADP bound in the catalytic site in exchange with ADP after the phosphorylation (upper panel, left scale). The change in Ca2+ binding was measured indirectly by use of murexide as an indicator of free Ca2+ in the medium. The data show that Ca2+ dissociates simultaneously with formation of E2P. The data points were taken from Andersen et al., 1985.

For the SR Ca2+-ATPases, it is commonly assumed that the extracytoplasmic sites on E2P from which the two translocated calcium ions have been released, either remain empty during recirculation to the cytoplasmic side or bind and countertransport 2-3 protons (Levy et al., 1990). In the former case, there would be a clear difference from the sodium pump, since the hydrolysis of the empty E2P intermediate would occur at a much higher rate in Ca2+-ATPase. It has yet to be established... 

Figure 7. Outline of the current concepts in cation transport by the Na+-K+-pump. Three Na+ and two K+ are thought to be transported consecutively by protein sites containing two negative carboxylate groups. The ions become occluded during the transport process by the simultaneous closure of cytoplasmic and extracytoplasmic gates. A low conductance access channel connects the sites to the extracytoplasmic surface, resulting in an ion well with voltage sensitivity of extracytoplasmic ion binding and dissociation. Reproduced with permission from Glynn, 1993.

Figure 11. Proposed reaction cycles of the Na+-K+-ATPase (A) and the SR Ca2+-AT-Pase (B) involving transitions between different conformational states of the enzymes (see text for further explanation). The cytoplasmic side of the membrane is upward and the extracytoplasmic side downward. Brackets indicate that all the cation binding sites reside in an occluded state. A tentative H+-countertransport limb is shown for the SR Ca2+-ATPase (most likely n = 2). s indicates a relatively slow reaction step. ATP boxes indicate steps accelerated by ATP not being hydrolyzed. Mg2+ serving as a cofactor in phosphorylation and dephosphorylation is not shown.

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