Access channel

Krainev AG, Weiner LM, Kondrashin SK, Kanaeva IP, Bach-manova GI. 1991. Substrate access channel geometry of soluble and membrane-bound cytochromes P450 as studied by interactions with type II substrate analogues. Arch Biochem Biophys 288 17-21. 

An optimum relationship between the DL and the flow field channels is a key factor in the overall improvement of fhe fuel cell s performance at both high and low current densities. Currently, flow field designs are typically serpentine, interdigitated, or parallel [207,264]. The FF plate performs several functions If is a current collector, provides mechanical support for the electrodes, provides access channels for the reactants to their respective electrode surfaces and for the removal of producf water, and it prevents mixing of oxidant, fuel, and coolant fluids. 

Ferritin is composed by the arrangement of 24 protein subunits, which results in a hollow shell of 8 nm inner diameter and 13 nm outer diameter (Fig. 13). Ferritin from vertebrates have two types of subunits heavy (H) and light (L). The subunit composition of human ferritins depends on the origin of the protein H2L22 for liver ferritin, H20L4 for muscle ferritin, etc. Access channels are formed by the intersection of subunits. The 8 channels located at the intersection of three subunits are hydrophilic while the 6 channels located at the intersection of 4 subunits are hydrophobic. The empty protein is called apoferritin (30). 

At this time, the proposal of additional access channels is quite conjectural. It seems likely that there is a channel or access route to the proximal side of the heme in order to provide access for the hydrogen peroxide or water needed for heme oxidation and His-Tyr bond formation. Furthermore, the electron density of compoimd I from PMC (97) reveals the presence of an anionic species that is not present in the native enz5une. However, the rapid influx-efflux rates up to 10 per sec needed for such a species to be a component of compoimd I would pose interesting constraints on a channel, and there does not seem to be a likely candidate in the region. Similarly, the potential channel leading to the cavity at the molecular center is not an ideal candidate for substrate or product movement because of its relationship to the active site residues. However, if the lateral channel is truly blocked by NADPH in small-subunit enzymes, this route may provide an alternative access or exhaust route. Both of these latter two channels require further investigation before a clear role can be ascribed to them. 

Fig. 5. Stereo view showing the substrate access channel in eNOS. The heme is lightly shaded and the substrate, L-Arg, is darkly shaded. The channel is deep yet solvent accessible for ready entry of substrate and exit of product. Part of the access channel is shaped by the second molecule (shaded) in the dimer. There appears to be no requirement for major structural changes upon substrate binding or release.

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS CHALCONE ISOM ERASE CHALCONE SYNTHASE CHANGE IN MECHANISM CHANNEL ACCESS Channeling,... 

Quantum system objectives. Typical objectives involve steering the quantum system out the desired channel versus that of other accessible channels. The detection of these outcomes is often simple and does not require extensive offline computations to deduce the relative success of an applied control field e(t). 

Fillingame, R. H., Angevine, C. M., and Dmitriev, O. Y. (2002). Coupling proton movements to c-ring rotation in F(1)F(0) ATP synthase Aqueous access channels and helix rotations at the a-c interface. Biochim. Biophys. Acta. 1555, 29-36. 

In their modeling study of DMS oxidation, Yin, et al. consider both addition and abstraction channels to produce SO2 and MSA (2). They do not consider other energetically accessible channels such as... 

In one report, open access channel electrophoresis was achieved on a glass channel plate, with no cover plate [792]. 

In another report, an open-access channel (i.e., no cover plate) has been used for CE separation before the MALDI-MS analysis [792]. 

Ludemann SK, Lounnas V, Wade RC. How do substrates enter and products exit the buried active site of cytochrome P450cam 1. Random expulsion molecular dynamics investigation of ligand access channels and mechanisms. J Mol Biol 2000 303 797-811. 

Winn PJ, Ludemann SK, Gauges R, et al. Comparison of the dynamics of substrate access channels in three cytochrome P450s reveals different opening mechanisms and a novel functional role for a buried arginine. Proc Natl Acad Sci U S A 2002 99 5361-5366. 

Figure 3. Model showing how a protein conformational change altering the conductances in access channels may allow the electric field to move past the bound ion (closed circle), which remains stationary. In either conformation the electrical potential gradient is located corresponding to a narrow access channel of low conductance. /(x) indicates electrical potential.

Figure 7. Outline of the current concepts in cation transport by the Na+-K+-pump. Three Na+ and two K+ are thought to be transported consecutively by protein sites containing two negative carboxylate groups. The ions become occluded during the transport process by the simultaneous closure of cytoplasmic and extracytoplasmic gates. A low conductance access channel connects the sites to the extracytoplasmic surface, resulting in an ion well with voltage sensitivity of extracytoplasmic ion binding and dissociation. Reproduced with permission from Glynn, 1993.

Fig. 5.12 The HRP substrate access channel. The aromatic residues flanking the access channel are highlighted. The arrow, located in the access channel, points to the 5-mew-carbon atom...

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