Translocation rates

Methods used to demonstrate the existence of membrane phospholipid asymmetry, such as chemical labelling and susceptibility to hydrolysis or modification by phospholipases and other enzymes, are rmsuitable for dynamic studies because the rates of chemical and biochemical reactions are of a different order compared to the transmembrane translocahon of the phospholipids. Indirect methods have therefore been developed to measure the translocation rate which are consequent on the loss of membrane phospholipid asymmetry. Thus time scales appropriate to rates of lipid scrambling under resting conditions or when the forces preserving the asymmetric phospholipid distribution are disturbed can be monitored. Generally the methods rely on detecting the appearance of phosphatidylserine on the surface of cells. Methods of demonstrating Upid translocation in mammalian cells has been the subject of a recent review (Bevers etal., 1999). 

Considering the highly processive mechanism of the protein degradation by the proteasome, a question naturally arises what is a mechanism behind such translocation rates Let us discuss one of the possible translocation mechanisms. In [52] we assume that the proteasome has a fluctuationally driven transport mechanism and we show that such a mechanism generally results in a nonmonotonous translocation rate. Since the proteasome has a symmetric structure, three ingredients are required for fluctuationally driven translocation the anisotropy of the proteasome-protein interaction potential, thermal noise in the interaction centers, and the energy input. Under the assumption that the protein potential is asymmetric and periodic, and that the energy input is modeled with a periodic force or colored noise, one can even obtain nonmonotonous translocation rates analytically [52]. Here we... 

The protein strand can be cleaved if it lies close to the cleavage center or it could be transported forward by one amino acid. We assume that the probability of transport depends only on the length of the strand inside the proteasome. The probability of transport is, therefore, given by a translocation rate function, v(x + D) where x + D is the length of the strand inside the proteasome, in terms of amino acids. The probability of cleavage is assumed to be a constant, denoted by y. We also assume that the degradation of proteins by the proteasome is a highly processive mechanism [2], i.e., in other words, the protein is not released by the proteasome until it is completely processed. This leads to the possibility of the proteasome... 

Musser, S.M. and Theg, S.M. (2000) Proton transfer limits protein translocation rate by the thylakoid DpH/Tat machinery, Biochemistry, 39, 8228-8233. 

That active transport occurs within mycelia is well established, and invoked when measured rates of translocation are too rapid to be accounted for by diffusion alone. In many cases, diffusion-driven translocation alone can be adequate to account for observed translocation rates, but several models of fungal growth confirm the necessity for active transport in order to account for observed translocation rates, hyphal extension rates or mycelial growth rates (Davidson Olsson, 2000 Boswell et ah, 2002, 2003). 

Fig. 3.2. Translocation of from inoculum source to wood bait sink by basidiomycetes. (a) Experimental design, (b) Effect of inoculum volume upon translocation rates. Derived from Wells and Boddy (1995b).

In addition to the detoxification processes described, other factors play important parts in determining the physiological selectivity. For example, the translocation rate of the. r-triazine taken up by a plant varies considerably, depending on the plant species. When translocation is rapid, the quantity of triazine ne ed to cause wilting more easily reaches the chloroplasts. 

MS ]" are the concentrations of carrier ion complex on the left and right sides of the membrane and F is the Faraday. The complex translocation rate constants depend on voltage approximately as... 

Just after a voltage is applied to the membrane, the initial values of carrier complex surface concentrations determine the current. But because the translocation rates change on application of the voltage, the surface concentrations of carrier complex change with time, and the current relaxes to a new initial value. The rate constants of the various reactions will determine both the final value of the current (the amplitude of the relaxation) and the time course by which the current attains its final value. 

Several limiting cases are of particular interest. If the surface reaction rates and the translocation rate of free carrier of complex are very fast compared to translocation, the concentrations of complex at the surfaces will remain unchanged when voltage is applied. Under these conditions, the current will be proportional to the hyperbolic sine of the voltage ... 

PAHs are taken up by both leaves and roots, translocated to other parts of the plant, and may also be concentrated more in certain plant parts than others. Sorghum irrigated with PAH contaminated water has been reported to show an accumulation of fluoranthene and pyrene. The uptake and translocation rates vary with different plant species. The rate (amount per unit time) of PAH uptake by... 

The capacity of release of PL molecules from sonicated vesicles was measured by means of the translocation rate of PL molecules across a dialysis membrane. DPPC vesicles, about 2 ymol Pi, were added to the upper cell of the dialysis apparatus. The amount of PL translocated was measured by determining the inorganic phosphate content in the lower cell and it was found to increase linearly with the time. Fig. 1 shows the PL translocation rate from vesicles with different chain length when variable amounts of myristic acid were added to the upper cell of the dialysis apparatus. Temperature was 50°C. The translocation rate abruptly increased at a "critical" myristic acid concentration, dependent on the phospholipid chain length. Similar results were obtained when myristic acid was cosonicated with PL molecules to form mixed vesicles. The abrupt increase occurred with vesicles either in the solid or in the liquid--crystalline state. However the "critical" value of myristic acid... 

Fig. 1 - Phospholipid translocation rate in vesicles with different phosphatidylcholine chain length.

Fig. 3B shows that a marked increase of the light scattering response was induced at a "critical" fatty acid concentration. However this value was higher than that inducing the abrupt increase of PL translocation rate. When measured at various PL concentrations, the rate... 

Fig. 3 - A. Light scattering increase induced by the temperature in DPPC vesicles. B. DPPC vesicles light scattering increase and phospholipid translocation rate o—o at various myristic acid concentrations.

The jump of MS over the barrier may be characterized by a translocation rate constant KmsI Kms then gives direcdy the jump frequency of the complex. 

Figure 4 Transport of cation M by the carrier S, where Ks and Kms are the translocation rate constants, Kr is the rate constant of the association reaction, and Kd is the dissociation rate constant.

To simplify the mathematical analysis of the system, the transport of S, too, is treated as a rate process that may be characterized by a translocation rate constant, Ks- It may be shown that both descriptions, diffusion or rate process, are correlated by the relation... 

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