Active transport of substrate

The biotin-dependent decarboxylases of anerobic microorganisms are transmembrane proteins. In addition to their roles in the metabolism of ox-aloacetate, methylmalonyl CoA, and glutaconyl CoA, they serve as energy transducers. They transport 2 mol of sodium out of the cell for each mole of substrate decarboxylated. The resultant sodium gradient is then used for active transport of substrates by sodium cotranspoit systems, or maybe used to drive ATP synthesis in a similar manner to the proton gradient in mammalian mitochondria (Buckel, 2001). 

The NBDs are responsible for the binding and hydrolysis of ATP, which is needed for the active transport of substrates. On each NBD sequence the characteristic ABC domain consisting of the Walker A and B region, as well as the "signature" or C motif, can be found (Fig. 3). One ATP molecule is supposed to be sandwiched between the Walker A and B of one NBD and the C motif of the other NBD (Fig. 3). As they are highly conserved, the NBDs show large sequence identity among ABC transporters. 

Shape persistence as a basis for controllable function is one of the main features of proteins that serve as mechanical support for cofactors (e.g., chromophores in light harvesting complexes), transmit mechanical force (e.g., in muscles), or function as nanoscopic pumps in active transport of substrates through cell membranes. Transfer of this concept to the realm of functional materials is a rather recent development and the term shape persistence for synthetic macromolecules is often used with the loose meaning of relatively rigid compared to most synthetic polymers. For linear polymers, shape persistence can be quantified by the persistence length Lp if one assumes that residual flexibility conforms to the worm-like chain (WLC) model. This assumption has been rarely tested and for many synthetic polymers Lp is either unknown or known with rather limited precision. 

The gradients of H, Na, and other cations and anions established by ATPases and other energy sources can be used for secondary active transport of various substrates. The best-understood systems use Na or gradients to transport amino acids and sugars in certain cells. Many of these systems operate as symports, with the ion and the transported amino acid or sugar moving in the same direction (that is, into the cell). In antiport processes, the ion and the other transported species move in opposite directions. (For example, the anion transporter of erythrocytes is an antiport.) Proton symport proteins are used by E. coU and other bacteria to accumulate lactose, arabinose, ribose, and a variety of amino acids. E. coli also possesses Na -symport systems for melibiose as well as for glutamate and other amino acids. 

ABC Transporters Use ATP to Drive the Active Transport of a Wide Variety of Substrates... 

Yet another way to alter the effective activity of an enzyme is to change the accessibility of its substrate. The hexokinase of muscle cannot act on glucose until the sugar enters the myocyte from the blood, and the rate at which it enters depends on the activity of glucose transporters in the plasma membrane. Within cells, membrane-bounded compartments segregate certain enzymes and enzyme systems, and the transport of substrate into these compartments may be the limiting factor in enzyme action. 

Active transport of a solute against a concentration gradient also can be driven by a flow of an ion down its concentration gradient. Table 17.6 lists some of the active-transport systems that operate in this way. In some cases, the ion moves across the membrane in the opposite direction to the primary substrate (antiport) in others, the two species move in the same direction (symport). Many eukaryotic cells take up neutral amino acids by coupling this uptake to the inward movement of Na+ (see fig. 17.26c). As we discussed previously, Na+ influx is downhill thermodynamically because the Na+-K+ pump keeps the intracellular concentration of Na+ lower than the extracellular concentration and sets up a favorable electric potential difference across the membrane. Another example is the /3-galactosidc transport system of E. coli, which couples uptake of lactose to the inward flow of protons (see fig. 17.26Proton influx is downhill because electron-transfer reactions (or,... 

Because of its cyclic nature, this process presents analogies with molecular catalysis it may be considered as physical catalysis operating a change in location, a translocation, on the substrate, like chemical catalysis operates a transformation into products. The carrier is the transport catalyst which strongly increases the rate of passage of the substrate with respect to free diffusion and shows enzyme-like features (saturation kinetics, competition and inhibition phenomena, etc.). The active species is the carrier-substrate supermolecule. The transport of substrate Sj may be coupled to the flow of a second species S2 in the same (symport) or opposite antiport) direction. 

Adenosine, in addition to serving as a substrate for the generation of cAMP plays a physiologic role as a platelet inhibitor and a vasodilator and may attenuate neutrophil-mediated damage to endothelial cells, Adenosine diphosphate (ADP)— a potent platelet agonist—is converted to adenosine, which is taken up rapidly by cells, especially erythrocytes and endothelial cells, A small proportion is metabolized to the aforementioned cyclic nucleotides. The remainder is broken down to inosine and subsequently to xanthine. Dipyridamole inhibits the active transport of adenosine into cells, but does not interfere with the passive diffusion. Since the platelet inhibitory effects of adenosine proceed via stimulation of adenylate cyclase, these effects can also be amplified by dipyridamole, In circulating blood, the largest amount of adenosine is found in red blood cells, This may, in part, help explain why dipyridamole is much more effective in whole blood than in plasma. 

L-a-methyldopa a substrate for the amino acid transporter. In Caco-2 cells, the active transport of this dmg by the amino acid transporter was seven times higher than transport by passive diffusion. Its absorption may be further increased by upregulating the amino acid transporter, as has been observed in the 20-70% stimulation of carrier-mediated amino acid transport by treatment of 0.2 mg/kg growth hormone. 

Kinetics of Immobilized Enzymes. Another major factor in the performance of immobilized enzymes is the effect of the matrix on mass transport of substrates and products. Hindered access to the active site of an immobilized enzyme can affect the kinetic parameters in several ways. The effective concentration of substrates and products is also affected by the chemistry of the matrix especially with regard to the respective partition coefficients between the bulk solution and the matrix. In order to understand the effects of immobilization upon the rate of an enzyme-catalyzed reaction one must first consider the relationship between the velocity of an enzyme-catalyzed reaction and the... 

There exist a large number of phenomenological laws for example, Fick s law relates to the flow of a substance and its concentration gradient, and the mass action law explores the reaction rate and chemical concentrations or affinities. When two or more of these phenomena occur simultaneously in a system, they may couple and induce new effects, such as facilitated and active transport in biological systems. In active transport, a substrate can flow against the direction imposed by its thermodynamic force. Without the coupling, such uphill transport would be in violation of the second law of thermodynamics. Therefore, dissipation due to either diffusion or chemical reaction can be negative only if these two processes couple and produce a positive total entropy production. 

There are several mechanisms for explaining how biological membranes can transport charged or uncharged substrates against their thermodynamic forces. It is widely accepted that cross-transports by a protein are discrete events. Biomembranes contain enzymes, pores, charges or membrane potentials, and catalytic activities associated with the transport of substrates. It is well established that the electrostatic interactions between the membrane and a charged... 

Enzyme latency is an experimental manifestation of compart-mentation, which means that the activity of certain intraluminal enzymes is increased remarkably when the membrane is perme-abilized either by detergents or by channel-forming antibiotics (e.g., alamethicin). It is based on the rate-limiting transport of substrates and/or cofactors across the intact ER membrane. Some activities in the ER are more than 90% latent i.e., they increase more than 10-fold during permeabilization (20, 21). 

Another fruitful area of research has been that of the sonochemical activation of immobilized enzymes where ultrasound appears to be particularly useful in increasing the transport of substrate to the enzyme. Using a-chymotrypsin (on agarose gel) and casein as substrate, a two-fold increase in activity was observed at 20 kHz [12]. Here the origin of the enhancement was thought to be associated with increased penetration of the casein into the support gel induced by cavitational effects close to the surface. However an increase in the activity of a-amylase (on porous polystyrene) was produced on irradiation with 7 MHz ultrasound [13]. This is a very significant result since at this high-frequency cavitation cannot occur and... 

One of the popular experimental systems to investigate the hepatic efflux process is canalicular membrane vesicle (CMV). It is difficult to evaluate the transport activity of efflux transporters in cell systems because substrates cannot easily access the intracellular compartment, so CMV system is often used to rapidly determine the ATP-dependent efflux transport of substrates across bile canalicular membrane. 

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