Flux control

Figure 6,8 Speed-torque characteristics by flux (/ ,) control (single phasor control)...

Flux (/, ) control Double-phasor (vector) or Direct torque control (single-phasor afield-oriented control (DTC)... 

Because Norske Shell was using these turboexpanders in a dewpoint control process, it was necessary to factor in other specific conditions. For instance, these mrboexpanders could be exposed to possibly violent process transient conditions. For this reason, the magnetic bearings required a flux control system to increase the response capability, which was developed by the original equipment manufacturer in concert with the magnetic bearing manufacturer. The load factor of... 

In order to develop a rational approach to improving rates of metabolite production, it is necessary to consider the fate of the nutrients that are required for its synthesis. However, overcoming the major flux control points within a metabolic pathway may not lead to metabolite overproduction if the energetic consequences of the alteration are unfavourable to the organism. 

Jens and Leppert (J4), and, more recently, Barnett (B2), have emphasized the need to distinguish between a temperature-controlled surface and a heat-flux-controlled surface when referring to boiling phenomena. Failure to observe the distinction has caused some confusion in the literature, particularly as further complications arise from differences that exist between pool boiling and forced-convection boiling. 

B. Heat-Flux-Controlled Systems I. Pool Boiling... 

Fig. 2. Boiling curve of a heat-flux-controlled surface with pool boiling.

Fig. 3. Boiling curves for Freon 12 in a heat-flux-controlled uniformly heated tube [from Stevens et al. (SI 3)]. L = 51 in., 4 = 0.633 in.,F = 155 psia, Ah = 13 Btu/lb.

From here on, unless otherwise stated, we shall be considering only forced-convection boiling with a heat-flux-controlled surface. 

Coating for neutron flux control in nuclear reactors. Wear parts, sandblast nozzles, seals.O ]... 

In both intermediate and maximum rates of respiration, control is distributed between several different steps, including the activity of the adenine nucleotide translocator (Groen et al., 1983). It is now recognized that the idea of a simple rate-limiting step for a metabolic pathway is simplistic and that control is shared by all steps although to different extents (Kacserand Bums, 1978 Fell, 1992). Each step in a pathway has a flux control coefficient (FCC) defined as ... 

Metabolic control analysis (MCA) assigns a flux control coefficient (FCC) to each step in the pathway and considers the sum of the coefficients. Competing pathway components may have negative FCCs. To measure FCCs, a variety of experimental techniques including radio isotopomers and pulse chase experiments are necessary in a tissue culture system. Perturbation of the system, for example, with over-expression of various genes can be applied iteratively to understand and optimize product accumulation. 

Adjunctive to flux control analysis, other components of metabolism that contribute to product accumulation are needed including (1) substrate/precursor pool sizes (metabolomics), (2) co-factor capacities (metabolomics), (3) gene expression profiles (transcriptomics and quanfifafive real-time PCR), (4) protein profiles (pro-... 

After measuring the fluxes through the metabolic network, it is necessary to determine the extent to which each pathway or enzyme controls the net fluxes. Metabolic control analysis (MCA) is a technique used to elucidate how flux control is distributed in a metabolic network, thereby providing the information for identification of potential targets for metabolic engineering [8],... 

Thomas S, Fell DA. The role of multiple enzyme activation in metabolic flux control. Advan Enzyme Regul 1998 38 65-85. 

Correspondingly, the normalized flux control coefficient CJ is defined as... 

J. L. Galazzo and J. E. Bailey, Fermentation pathway kinetics and metabolic flux control in suspended and immobilized Saccharomyces cerevisiae. Enzyme Microb. Technol. 12(3), 162 172 (1990). 

For a lucid account of the kinetics of multi-enzyme systems, the reader should consult Cornish-Bowden who defines such related parameters as flux control coefficients, summation relationships, and response coefficients. 

The formalized application of metabolic control analysis deals with several parameters (a) The flux control coefficient is defined as the fractional change in pathway flux... 

Quantitative data obtained as described in Figure 15-33 can be used to calculate a flux control coefficient,... 

C, for each enzyme in a pathway. This coefficient expresses the relative contribution of each enzyme to setting the rate at which metabolites flow through the pathway—that is, the flux, J. C can have any value from 0.0 (for an enzyme with no impact on the flux) to 1.0 (for an enzyme that wholly determines the flux). An enzyme can also have a negative flux control coefficient. In a branched pathway, an enzyme in one branch, by drawing intermediates away from the other branch, can have a negative impact on the flux through that other branch (Fig. 15-34). C is not a constant, and it is not... 

FIGURE 15-34 Flux control coefficient, C, in a branched metabolic pathway. In this simple pathway, the intermediate B has two alternative fates. To the extent that reaction B —> E draws B away from the pathway A —> D, it controls that pathway, which will result in a negative flux control coefficient for the enzyme that catalyzes step B —> E. Note that the sum of all four coefficients equals 1.0, as it must. 

When real data from the experiment on glycolysis in a rat liver extract (Fig. 15-33) were subjected to this kind of analysis, investigators found flux control coefficients (for enzymes at the concentrations found in the extract) of 0.79 for hexoldnase, 0.21 for PFK-1, and 0.0 for phosphohexose isomerase. It is not just fortuitous that these values add up to 1.0 we can show that for any complete pathway, the sum of the flux control coefficients must equal unity. 

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