Shuttle mechanisms

In the rhizosphere, microorganisms utilize either organic acids or phytosiderophores to transport iron or produce their own low-molecular-weight metal chelators, called siderophores. There are a wide variety of siderophores in nature and some of them have now been identified and chemically purified (54). Pre.sently, three general mechanisms are recognized for utilization of these compounds by microorganisms. These include a shuttle mechanism in which chelators deliver iron to a reductase on the cell surface, direct uptake of metallated siderophores with destructive hydrolysis of the chelator inside the cell, and direct uptake followed by reductive removal of iron and resecretion of the chelator (for reviews, see Refs. 29 and 54). 

One paradigm for membrane transport of iron is the binding of the receptor protein to an iron-free siderophore molecule, followed by exchange of iron from an external ferri-siderophore to the receptor bound iron-free siderophore, and subsequent transfer across the cellular membrane. This shuttle mechanism has been explored in the transport system of ferric pyoverdine in P. aeruginosa (215,216). It is unclear why the bacterial system behaves in this manner, but mutagenesis studies of the protein suggest that residues involved in the closure of the P-barrel will not interact in the same way with the iron-free siderophore as they do with the ferri-siderophore. A similar mechanism has been suggested for A hydrophila and E. coli (182). 

To support this hypothesis, the OBC sample can be fractionated by the TREF experiment. TREF fractionation of the OBC, followed by evaluation of the octene content by 13C NMR, reveals the data shown in Fig. 21. For a polymer blend, each molecule dissolves and elutes according to its comonomer content. The results invariably fall on the line in Fig. 21 labeled random copolymer line. The triangles reveal the comonomer content of the TREF fractions from an OBC. At any given temperature, the polymer eluting has much more comonomer than would be expected for a random distribution. The only explanation is that the comonomer is blocked, as expected from the chain shuttling mechanism. The extent of deviation can even be quantified, and a new method was recently invented to determine the block index for a given polyolefin [46],... 

During the past two decades, the redox shuttle mechanism has been influencing researchers as the most promising solution to the challenge of cathode overcharge, and among the limited number of publications, most of the additives were selected based on their redox potentials. 

Oxaloacetate is returned to the mitochondrion, by the shuttle mechanism described in Chapter 9 (Figure 9.17). 

The other type of chemical mechanism is more selective and is used when the solute is not soluble in the membrane phase, therefore requiring the addition of a selective reactant into the membrane to form a complex or an ion pair with the solute. The reaction product then diffuses across the membrane and at the second interface it reacts with a species added to phase 3 so that stripping also takes place by chemical reaction (Fig. 15.2b). This mechanism is called carrier-mediated membrane transfer. The reagent recovered from the reversed reaction then transfers back to the extraction interface. This is usually called the reagent shuttle mechanism. 

The schizokinen-mediated Fe " transport in Bacillus megaterium was studied by double labelling with e and (8). At 37°C, uptake of Fe and of are parallel during the first 30 sec, then that of e continues until it levels off after 2 min, while that of [ H]-schizokinen drops to a low constant level. At 0°C, uptake of both labels reaches this low level which is obviously due to the binding of the ferri-siderophore to the cell surface. At 37°C, transport into the cell, release of iron, and re-export of the ligand follow. Apparently a shuttle mechanism takes place, cf. the experimental results obtained with parabactin (Sect. 3.2) indicative of a taxi mechanism. 

E. There are two shuttle mechanisms, the malate-aspartate shutde and the glycerol 3-phosphate shuttle, that transport electrons to the inner mitochondrial matrix to be used in the electron transport chain. 

A. The electrons released in glycolysis and transported into the mitochondria by shuttle mechanisms (see Chapter 6) and those derived from the TCA cycle are transferred to oxygen and combined with protons to form HjO. 

A similar "shuttle mechanism of electron scavenging was also suggested [81 for the reaction of a hydrated electron, eaq, with p-nitrobenzoatopen-taamine Co(III),... 

The hydrocarbon profiles of the cuticles of egg-layers closely match the profiles of their eggs. This is most likely owing to a shuttle mechanism that transports hydrocarbons from the site of synthesis to the cuticle and the ovaries as has been shown in cockroaches (see Chapter 5). Differences in the cuticular profile between differently fertile individuals may consequently also be found on the eggs. Extracts from the hemolymph of egg-layers in the... 

A potential O-atom donor molecule, N2O, has been allowed to react with SiH+ to probe the formation of HSiO+, a higher energy isomer of SiOH+. The expectations were fulfilled (equation 13)43, as tested by reaction with a base capable of deprotonating HSiO+ but not SiOH+. The formal O-atom insertion product, SiOH+, was ascribed to a proton shuttle mechanism which is dominant when OX is SO2 and CO2 (equation 14). In fact, with these two neutrals the formation of HSiO+ and X as bare species is somewhat endothermic and the proton transfer process within the ion-neutral complex, driven by the stability of SiOH+, is allowed by the proton affinity (PA) of X which is intermediate between those of the Si and O sites of SiO. On the other hand, when O2 is used as the neutral reagent this pathway is not accessible and HSi02+ is the only observed product ion. 

Answer Pyruvate dehydrogenase is located in the mitochondrion, and glyceraldehyde 3-phosphate dehydrogenase in the cytosol. Because the mitochondrial and cytosolic pools of NAD are separated by the inner mitochondrial membrane, the enzymes do not compete for the same NAD pool. However, reducing equivalents are transferred from one nicotinamide coenzyme pool to the other via shuttle mechanisms (see Problem 21). 

Because reduced redox cofactors, NADH and FADH2, are produced in the mitochondria, there is no need for shuttle mechanisms to reoxidize them via oxidative phosphorylation. NADH is reduced directly by complex I. FADH2 is reduced by the electron transfer flavoprotein, which then reduces ubiquinone. See Chapter 17 for details. 

Although mitochondria contain both NAD+ and NADH, as does the cytoplasm, the mitochondrial and cytoplasmic pools are unable to exchange their contents directly, as the mitochondrial membranes are impermeable to the cytoplasmic compounds. The shuttle mechanisms allow the H on cytoplasmic NADH to be transported on other compounds into the mitochondria, where it is donated to NAD+ (to form mitochondrial NADH) or to FAD (to form mitochondrial FADH2). There are several shuttle mechanisms that are used by mammalian cells two of the most important are the malate-aspartate shuttle and the glycerol 3-phosphate shuttle. 

The reducing equivalents of cytosolic NADH are transferred into mitochondria via shuttle mechanisms, such as the one involving aspartate and malate, shown in Fig. 11-19 (page 333). The net effect of this shuttle is the transport of NADH into the mitochondrion. 

These two enzymes provide a shuttle mechanism for transport of reducing equivalents of NADH (generated during glycolysis in the cytosol) into the mitochondria. The cytosolic enzyme catalyzes the following reaction ... 

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