Membrane inner-channel

Exactly how this transporter carries noradrenaline across the neuronal membrane is not known but one popular model proposes that it can exist in two interchangeable states. Binding of Na+ and noradrenaline to a domain on its extracellular surface could trigger a conformation change that results in the sequential opening of outer and inner channel gates on the transporter. This process enables the translocation of noradrenaline from the extracellular space towards the neuronal cytosol. 

Figure 1 A schematic representation of the Ca + transporters of animal cells. Plasma membrane (PM) channels are gated by potential, ligands or by the emptying of Ca + stores. Channels in the ER/SR are opened by lnsP3 or cADPr (the cADPr channel is sensitive to ryanodine and is thus called RyR). ATPase pumps are found in the PM (PMCA), in the ER/SR (SERCA), and in the Golgi apparatus (SPCA). The nuclear envelope, which is an extension of ER, contains the same transporters of the latter. NCXs are located in the PM (NCX) and in the inner mitochondria membrane (mNCX). A uniporter (U) driven by the internal negative potential (-180 mV) transports Ca + into mitochondria. Ca +-binding proteins are represented with a sphere containing the four EF-hands Ca +.binding sites.

By employing patch-clamp techniques, several types of K+ channels have been identified (for a review see Henquin et al., 1992) (1) ATP-sensitive K+ channels which are closed when ATP increases at the inner surface of the membrane - these channels are also closed by sulphonylureas and opened... 

Protein Import into the chloroplast stroma occurs through inner-membrane and outer-membrane translocation channels that are analogous in function to mitochondrial channels but composed of proteins unrelated in sequence to the corresponding mitochondrial proteins. 

Chemiosmotic coupling is the mechanism most widely used to explain the manner in which electron transport and oxidative phosphorylation are coupled to one another. In this mechanism, the proton gradient is directly linked to the phosphorylation process. The way in which the proton gradient leads to the production of ATP depends on ion channels through the inner mitochondrial membrane these channels are a feature of the structure of ATP synthase. Protons flow back into the matrix through proton channels in the Fq part of the ATP synthase. The flow of protons is accompanied by formation of ATP, which occurs in the Fj unit. 

Fig 5.2 is a schematic diagram of a tubular type gas-diffusion separator. The separator is column shaped, with two concentric channels. The tubular membrane forms the inner channel, usually reserved for the donor stream, and extends beyond the outer channel to be connected to the suppK and waste conduits. The terminals of the outer channel are extended to inlet and outlet ports on the column which are furnished with connectors for the acceptor stream conduits. 

However, the tube-in-tube reactor is not only used for gas—hquid reactions. The group of Kappe used the gas-permeable membrane to prepare the highly toxic and explosive diazomethane (CH2N2) in a safe manner. The diazomethane was formed from N-methyl-JV-nitroso-p-toluenesulfona-mide (Diazald) and KOH in the inner channel of the microreactor and subsequently diffused through the hydrophobic membrane where it formed the desired products 38, 40, and 42 in the outer chamber. The potential... 

Beavis, A. D. Davatol-Hag, H. The mitochondrial inner membrane anion channel is inhibited by DIDS. J. Bioenerg. Biomembr. 1996, 28, 207-214. 

Figure 12.9 Schematic diagram of the stmc-ture of a potassium channel viewed perpendicular to the plane of the membrane. The molecule is tetrameric with a hole in the middle that forms the ion pore (purple). Each subunit forms two transmembrane helices, the inner and the outer helix. The pore heJix and loop regions build up the ion pore in combination with the inner helix. (Adapted from S.A. Doyle et al., Science 280 69-77, 1998.)...

The C-terminal transmembrane helix, the inner helix, faces the central pore while the N-terminal helix, the outer helix, faces the lipid membrane. The four inner helices of the molecule are tilted and kinked so that the subunits open like petals of a flower towards the outside of the cell (Figure 12.10). The open petals house the region of the polypeptide chain between the two transmembrane helices. This segment of about 30 residues contains an additional helix, the pore helix, and loop regions which form the outer part of the ion channel. One of these loop regions with its counterparts from the three other subunits forms the narrow selectivity filter that is responsible for ion selectivity. The central and inner parts of the ion channel are lined by residues from the four inner helices. 

All K channels are tetrameric molecules. There are two closely related varieties of subunits for K channels, those containing two membrane-spanning helices and those containing six. However, residues that build up the ion channel. Including the pore helix and the inner helix, show a strong sequence similarity among all K+ channels. Consequently, the structural features and the mechanism for ion selectivity and conductance described for the bacterial K+ channel in all probability also apply for K+ channels in plant and animal cells. 

Mitochondria are surrounded by a simple outer membrane and a more complex inner membrane (Figure 21.1). The space between the inner and outer membranes is referred to as the intermembrane space. Several enzymes that utilize ATP (such as creatine kinase and adenylate kinase) are found in the intermembrane space. The smooth outer membrane is about 30 to 40% lipid and 60 to 70% protein, and has a relatively high concentration of phos-phatidylinositol. The outer membrane contains significant amounts of porin —a transmembrane protein, rich in /3-sheets, that forms large channels across the membrane, permitting free diffusion of molecules with molecular weights of about 10,000 or less. Apparently, the outer membrane functions mainly to... 

Inward Rectifier K+ Channels. Figure 5 Proposed three-dimensional arrangement of the transmembrane region of Kir channels. Two of four subunits are indicated. The pore consists of a selectivity filter close to the outside part of the membrane, a central inner vestibule, and a cytoplasmic entrance. Spermine may block in the inner cavity or in the selectivity filter. Large intracellular vestibule where polyamines may also block the channel is not shown. 

Mitochondrial permeability transition involves the opening of a larger channel in the inner mitochondrial membrane leading to free radical generation, release of calcium into the cytosol and caspase activation. These alterations in mitochondrial permeability lead eventually to disruption of the respiratory chain and dqDletion of ATP. This in turn leads to release of soluble intramito-chondrial membrane proteins such as cytochrome C and apoptosis-inducing factor, which results in apoptosis. 

While it has been known for many years that the N-terminal presequence is sufficient to promote mitochondrial targeting and assembly, the subsequent interaction of the precursor molecule with the outer mitochondrial membrane and the uptake of the protein is still an area of active research. There seems little doubt, however, that there are proteins on the outer mitochondrial membrane which are required for the import process. The function of these proteins is uncertain, but they may act as receptors with the subsequent transfer through the membrane at proteinous pores located at contact sites between the inner and outer membranes. Several proteins have been identified which seem to play an important role as either receptor proteins or part of the import channel (Pfanner et al., 1991). Again, not all proteins seem to depend on this mechanism. Cytochrome c, which is loosely associated with the outer aspect of the inner mitochondrial membrane, can cross... 

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