Proteins conduction, mechanisms

Figure 7.2 G protein-mediated mechanisms of opioid cellular actions. Activation of the p receptor results in inhibition ofadenylyl cyclase (AC), the enzyme responsible for the formation ofcAMP, via the Gi protein, and increased potassium conductance and decreased calcium conductance, mediated via Go proteins.

The energy released by electron transfer can be used in the transport of protons through the membrane. One of the proton conduction mechanisms in proteins is through a chain of hydrogen bonds in the protein, i.e. a Grotthus mechanism (Section 2.9), similar to the mechanism of proton movement in ice. Protons are injected and removed by the various oxidation/reduction reactions which occur in the cell there is no excess of protons or electrons in the final balance, and the reaction cycle is self-sustaining. 

At the current time, a wealth of knowledge for protein glycosylation has been obtained from numerous studies conducted in eukaryotes rather than in bacteria. However, the simplicity of the bacterial systems has made them a great model for studying different basic aspects entailed in protein glycosylation. Moreover, recent studies identified some unique protein glycosylation mechanisms in bacteria such as Neisseria and Pseudomonas that have not been described for any other eukaryotes before. 

Proteins in the body liquids may be considered as a colloidal electrolyte solute in a water solvent. Contact with water is the natural state of a protein. In more or less dry form, a protein powder loses some of its electrolytic character it loses the charged double layer on the surface and behaves electrically very differently from protein with water. Such materials may well be mixed conductors—electronic in the dry state and ionic with water content. Keratin is a more or less dry protein found in the natural state of no longer living biological materials such as hair, nails, and the stratum corneum. The water content of such materials is dependent on the relative humidity of the ambient air. The question of ionic or electronic conductivity in proteins is important, and an electronic conduction mechanism must be considered in many cases. 

Preliminary experiments were conducted to see if cyclic AMP-mediated protein phosphorylation mechanisms might be involved in the variation (and regulation) of enzyme activity, as had been reported for lipase and cholesteryl ester hydrolase in other tissues (Khoo et al., 1976 Pittman-and Steinberg, 1977). These experiments did not provide evidence for the involvement of such mechanisms in the observed interanimal variation of RPH activity. This variation, additionally, was not related to the age of the animals, time of day of death, order of animal kill, or strain of rat. At present, an explanation for the observed... 

With help of the Wentzel-Kramers-Brillouin approximation the field dependent transmission coefficient was calculated for the vacuum potential barrier. Integrating over all energies for a free electron gas at room temperature results in an exponentially increasing I-V curve that reaches 1 nA at 0.7 kV. With a band gap well above 2 eV proteins are good insulators, their electric breakdown is believed to be a conduction mechanism, which occurs at a voltage of about 200 V. The necessary potential for tip-emission can be further decreased to about 100 V using nano-fibers. To create transparent... 

The elegant genetic studies by the group of Charles Yanofsky at Stanford University, conducted before the crystal structure was known, confirm this mechanism. The side chain of Ala 77, which is in the loop region of the helix-turn-helix motif, faces the cavity where tryptophan binds. When this side chain is replaced by the bulkier side chain of Val, the mutant repressor does not require tryptophan to be able to bind specifically to the operator DNA. The presence of a bulkier valine side chain at position 77 maintains the heads in an active conformation even in the absence of bound tryptophan. The crystal structure of this mutant repressor, in the absence of tryptophan, is basically the same as that of the wild-type repressor with tryptophan. This is an excellent example of how ligand-induced conformational changes can be mimicked by amino acid substitutions in the protein. 

As with conductivity measurements, methods and results of theoretical treatments of CT in DNA have varied significantly. Mechanisms invoking hopping, tunneling, superexchange, or even band delocalization have been proposed to explain CT processes in DNA (please refer to other reviews in this text). Significantly, many calculations predicted that the distance dependence of CT in DNA should be comparable to that observed in the a-systems of proteins [26]. This prediction has not been realized experimentally. The dichotomy between theory and experiment may be related to the fact that many early studies gave insufficient consideration to the unique properties of the DNA molecule. Consequently, CT models derived for typical conductors, or even those based on other biomolecules such as proteins, were not adequate for DNA. 

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