Substrate transport

Neurotransmitter transport can be electrogenic if it results in the net translocation of electrical charge (e.g. if more cations than anions are transferred into the cell interior). Moreover, some transporters may direction-ally conduct ions in a manner akin to ligand-gated ion channels this ion flux is not coupled to substrate transport and requires a separate permeation pathway associated with the transporter molecule. In the case of the monoamine transporters (DAT, NET, SERT) the sodium current triggered by amphetamine, a monoamine and psychostimulant (see Fig. 4) is considered responsible for a high internal sodium concentration... 

Many inhibitors of substrate oxidations, substrate transport, electron transport, and ATP synthesis are known including many well-known toxins (see Sherratt, 1981 Harold, 1986 Nicholls and Ferguson, 1992). These are not discussed here except to mention specific uncouplers of oxidative phosphorylation. Classic uncouplers such as 2,4-dinitrophenol have protonated and unprotonated forms, both of which are lipid soluble and cross the inner mitochondrial membrane discharging the proton gradient. This prevents ATP synthesis and stimulates respiration. 

Kitayama, S., Shimada, S., Xu, H., Markham, L., Donovan, D.M., and Uhl, G.R., Dopamine transporter site-directed mutations differentially alter substrate transport and cocaine binding, Proc. Natl. Acad. Sci. U.S.A., 89, 7782, 1992. 

Bossuyt, X., M. Muller, and P. J. Meier. Multispecific amphipathic substrate transport by an organic anion transporter of human liver. J. Hepatol. 1996, 25, 733-738. 

The Local Environment Determines the Nature of Substrate-Transporter Interactions... 

The activity of Dsz enzymes in vitro was reported to be almost equal for substituted DBTs from CO to C6-DBTs [71]. This implies that the variation in activity found with whole cells is probably due to the different rates of substrate transport into the cells. 

Recent work at Diversa has provided some insights into the host effects on desulfurization. It was reported that in addition to the obvious substrate transport and growth differences between various strains of same genus, the intracellular redox potential can potentially influence the rate as well as extent of desulfurization [219], Additionally,... 

The specificity of several of the enzymes identified in the 4S pathway of different organisms has been studied. In case of the DBT desulfurizing enzymes, little difference is expected in the specificity of the enzymes, say DszA, from different Rhodococcus strains found to date. This is essentially because the DNA sequence for the enzymes investigated so far has been the same. The difference in the specificity observed with whole cell assays is essentially due to the differences in substrate intake via the cell membrane and not necessarily due to a difference in the intrinsic enzyme specificity. It has been found that while isolated enzyme DszC (from KA2-5-1) can desulfurize up to 4,6 dipropyl DBT, whole cells cannot, indicating substrate transport as limiting factor. 

Some of the recent work from Japan has identified strains capable of desulfurizing dipentyl DBTs and other larger alkyl DBTs. Research in the area of substrate transport and identification of any active transport proteins would be greatly helpful in developing effective biocatalysts for desulfurization of larger molecules. Second, strains capable of desulfurization of benzonaphthothiophenes have also been identified [11]. Desulfurization of whole crude oils will require desulfurization of not just benzonaphthothiophenes... 

Current information indicates that OATP-E [30] is ubiquitously expressed in tissues [30, 37]. Some substrates transported by OATP-E include estrone-sulfate [30], prostaglandin E2 [30] and taurocholate [37]. The capacity for T3 and T4 transport and the wide tissue distribution suggests that OATP-E is largely responsible for the peripheral uptake of thyroid hormone [37]. Further studies are required to assess whether OATP-E is an important determinant of drag distribution. 

Seal, R. P, Shigeri, Y Eliasof, S., Leighton, B. H., and Amara, S. G. (2001) Sulfhydryl modification of V449C in the glutamate transporter EAAT1 abolishes substrate transport but not the substrate-gated anion conductance. Proc. Natl. Acad. Sci. USA 98, 15324-15329. 

Sur, C., Schloss, R, and Betz, H. (1997) The rat serotonin transporter identification of cysteine residues important for substrate transport. Biochem. Biophys. Res. Commun. 241, 68-72. 

Sur, C., Betz, H., and Schloss, P. (1997) A single serine residue controls the cation dependence of substrate transport by the rat serotonin transporter. Proc. Natl. Acad. Sci. USA 94, 7639-7644. 

Bosma et al. [1] have proposed including the details of extracellular substrate transport in the calculation of whole-cell Michaelis-Menten kinetics. For the situation of a quasi-steady-state (i.e. when the transport flux and the rate of degradation of the substrate are equal) the consumption of substrate by a microorganism is represented as a function of the distant, and effectively unavailable, substrate concentration ca ... 

Attaching the catalyst molecules to the electrode surface presents an obvious advantage for synthetic and sensor applications. Catalysis can then be viewed as a supported molecular catalysis. It is the object of the next section. A distinction is made between monolayer and multilayer coatings. In the former, only chemical catalysis may take place, whereas both types of catalysis are possible with multilayer coatings, thanks to their three-dimensional structure. Besides substrate transport in the bathing solution, the catalytic responses are then under the control of three main phenomena electron hopping conduction, substrate diffusion, and catalytic reaction. While several systems have been described in which electron transport and catalysis are carried out by the same redox centers, particularly interesting systems are those in which these two functions are completed by two different molecular systems. 

Substrate transport through the film may be formally assimilated to membrane diffusion with a diffusion coefficient defined as12 Ds = Dch( 1 — 9)/pjort. In this equation, the effect of film structure on the transport process in taken into account in two ways. The factor 1—0 stands for the fact that in a plane parallel to the electrode surface and to the coating-solution interface, a fraction 9 of the surface area in made unavailable for linear diffusion (diffusion coefficient Dcj,) by the presence of the film. The tortuosity factor,, defined as the ratio between the average length of the channel and the film thickness, accounts for the fact that the substrate... 

P-gp substrate 9 substrates and 69 substrates and 88.4% substrates Transport and Caco-2 (transport) Pharmacophore (sub- [54]... 

Figure 17.3 Effects of endothelin-1 (ET-1) on NBDL-CSA (P-glycoprotein substrate) transport in isolated capillaries. (A) Concentration-dependent decrease in steady-state luminal NBDL-CSA accumulation caused by ET-1. (B) Effect of ET-1 on steady-state luminal NBDL-CSA accumulation. Capillaries were loaded to steady state in medium with 2 /xM NBDL-CSA. Then, 100 nM ET-1 was added to the medium (time 0 on graph) 90 min later, ET-1 was removed. Each point represents the value for 7-15 isolated capillaries from a single preparation (tissue from 3 to 10 rats). Variability is given by S.E. bars. Units are arbitrary fluorescence units. Statistical comparison , significantly smaller than control, P < 0.0001 (with permission from 75).

Along with electronic transport improvements must come attention to substrate transport in such porous structures. As discussed above, introduction of gas-phase diffusion or liquid-phase convection of reactants is a feasible approach to enabling high-current-density operation in electrodes of thicknesses exceeding 100 jxm. Such a solution is application specific, in the sense that neither gas-phase reactants nor convection can be introduced in a subclass of applications, such as devices implanted in human, animal, or plant tissue. In the context of physiologically implanted devices, the choice becomes either milliwatt to watt scale devices implanted in a blood vessel, where velocities of up to 10 cm/s can be present, or microwatt-scale devices implanted in tissue. Ex vivo applications are more flexible, partially because gas-phase oxygen from ambient air will almost always be utilized on the cathode side, but also because pumps can be used to provide convective flow of any substrate. However, power requirements for pump operation must be minimized to prevent substantial lowering of net power output. 

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