Transfer through membranes carrier proteins

Fig. 13.1.1. Schematic overview of mitochondrial oxidative phosphorylation. A part of the mitochondrion is represented, showing the outer mitochondrial membrane (OMM), inner mitochondrial membrane (IMM) and crista (an invagination of the inner membrane). Substrates for oxidation enter the mitochondrion through specific carrier proteins, e.g., the pyruvate transporter, (PyrT). Reducing equivalents from fatty acyl CoA dehydrogenases, pyruvate dehydrogenase and the TCA cycle are delivered to the electron transport chain through NADH, succinate ubiquinol oxidoreductase (SQO), electron transfer flavoprotein (ETF) and its ubiquinol-...

The movement of solutes from the external environment into the cell is usually achieved using cell membrane-spanning proteins that facilitate solute transfer. These are necessary, since most solutes (e.g. sugars, amino acids, salts) will not readily diffuse through the hydrophobic cell membrane. Movement of solutes into the epithelial cell can involve a variety of protein carriers or channels including (see Figure 1) ... 

Figure 3. Step 1 in the modified MacFarland bioactivity model Diffusion or carriage by plasma protein from the entry point to the anterior membrane surface (ams) transfer from the first aqueous phase (0 ) to the ams passage through the membrane by diffusion or by lipid soluble membrane carrier molecule bound to the posterior membrane surface (pms) transfer from the pms to the second aqueous phase (02).

There are, however, various types of active transport systems, involving protein carriers and known as uniports, symports, and antiports as indicated in Figure 3.7. Thus, symports and antiports involve the transport of two different molecules in either the same or a different direction. Uniports are carrier proteins, which actively or passively (see section "Facilitated Diffusion") transport one molecule through the membrane. Active transport requires a source of energy, usually ATP, which is hydrolyzed by the carrier protein, or the cotransport of ions such as Na+ or H+ down their electrochemical gradients. The transport proteins usually seem to traverse the lipid bilayer and appear to function like membrane-bound enzymes. Thus, the protein carrier has a specific binding site for the solute or solutes to be transferred. For example, with the Na+/K+ ATPase antiport, the solute (Na+) binds to the carrier on one side of... 

The rate limiting step in fatty acid synthesis is catalyzed by acetyl-CoA carboxylase to produce malonyl-CoA at the expense of one ATP.31 Malonate and acetate are transferred from CoA to acyl carrier protein in the cytosolic fatty acid synthetase complex, where chain extension leads to the production of palmitate. Palmitate can then be transferred back to CoA, and the chain can be extended two carbons at a time through the action of a fatty acid elongase system located in the endoplasmic reticulum. The >-hydroxylation that produces the >-hydroxyacids of the acylceramides is thought to be mediated by a cytochrome p450 just when the fatty acid is long enough to span the endoplasmic reticular membrane. 

The overall process of photosynthetic electron transfer is promoted by an array of catalytic proteins, only a few of which are real photochemical enzymes. It is now realized that these proteins form a number of well-defined complexes, partially independent from each other, but nevertheless interacting through redox carriers, freely diffusable either in the membrane lipids or at the membrane-water interface. The concept of membrane photosynthetic complex is experimentally justified by the possibility of isolating specific multiprotein associations following micellization of the membrane with mild detergents. In general these associations are characterized by well-defined catalytic activities, which are lost, however, if the complex is dissociated into the individual polypeptides by more drastic detergent treatments. 

In the respiratory chain, electrons from the powerful reducing agents NADH and FADH2 pass through four membrane-bound protein complexes and two mobile electron carriers before reducing O2 to H2O. We shall see that the electron transfer reactions drive the synthesis of ATP at three of the membrane protein complexes. 

The main barrier for solutes is the cytoplasmic membrane which consists of a liquid-crystalline bilayer of phospholipids in which proteins are embedded. The selective movement of solutes through this membrane is catalyzed by specific carrier proteins. In addition several other cellular functions are located in the cytoplasmic membrane such as the energy. transducing cyclic and linear electron transfer systems and the Ca + Mg -dependent ATPase complex. 

In plants, the photosynthesis reaction takes place in specialized organelles termed chloroplasts. The chloroplasts are bounded in a two-membrane envelope with an additional third internal membrane called thylakoid membrane. This thylakoid membrane is a highly folded structure, which encloses a distinct compartment called thylakoid lumen. The chlorophyll found in chloroplasts is bound to the protein in the thylakoid membrane. The major photosensitive molecules in plants are the chlorophylls chlorophyll a and chlorophyll b. They are coupled through electron transfer chains to other molecules that act as electron carriers. Structures of chlorophyll a, chlorophyll b, and pheophytin a are shown in Figure 7.9. 

The second proton transfer mechanism involves protonation of carboxyl or histidyl groups associated with electron carriers in the membrane and release of protons from these sites through proposed channels when the electron carrier is oxidized. This is essentially a proton channel system with movement through the channel gated by the oxidation-reduction state of the prosthetic group on the electron transport protein. The classical example of this is seen in cytochrome c oxidase (Figure 3). 

PSI and PSII. PSII contains the site of water cleavage, and utilizes the electrons extracted from water to reduce plastoquinone to plastoquinol. The latter diffuses through the membrane until it is reoxidized by another membrane protein, the cytochrome bf complex, which transfers the electrons to a water-soluble electron carrier (plastocyanin or cytochrome c6). This carrier in turn is oxidized by PSI, which delivers the electrons via ferredoxin to the enzymes that produce NADPH (Figure 11.6) [12],... 

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