Pathway-substrates

The use of recombinant inhibitors becomes easy when the protein is attached to green fluorescent protein (GFP), which eases the interpretation of the data [e.g., in the case of the Epsl5 mutant GFP-EA95/295 (epidermal growth factor receptor (EGFR) pathway substrate clone 15 mutant), a regulatory protein of clathrin assembly (57,58)]. 

Another protein called EGFR pathway substrate clone 15 (Epsl5) has been shown to be critical for endocytosis as well. Antibodies against Epsl5 prevent EGFR internalization. Recently, a family of... 

An enzyme in a biochemical pathway may have two substrates, one of which is a cofactor. The other which is usually the substrate involved in the primary flax through the pathway is termed a pathway-substrate in this book. 

Consider an enzyme at the beginning of a pathway whose pathway-substrate concentration is much less than that required to saturate the enzyme (see Figure 3.7), e.g. similar to or lower than that of the K. As the catalysis proceeds, the concentration of substrate falls so that the activity of the enzyme decreases more and more. Consequently, the activity of such an enzyme cannot maintain a constant flux through a pathway, so that a steady state cannot be achieved. 

The concentrations of the substrates in the liver of the rat indicates that there is no reaction in the cycle that approaches saturation with the pathway substrate i.e. these is to flux-generating step, so that the cycle cannot be described as a biochemical pathway, at least as defined in Chapter 3. The physiological pathway can be considered to start in either of two processes ... 

Figure 6.3 Biosynthesis of cucujolide II, showing several possible pathways. Substrates with names entirely in capital letters have been tested for incorporation into cucujolide II (Vanderwel et al., 1990, 1992b). Precursors that were incorporated are shown by solid arrows those that were not have a cross above the arrow. The actual biosynthetic pathway is shown in heavy arrows, and likely proceeds through activated (e.g. CoA) derivatives of the fatty-acyl intermediates.

The redox chemistry of dioxygen and its reduction products is heavily dependent on mechanistic pathway, substrate, and solution acidity. For those circumstances that are limited by direct electron transfer, the redox mechanisms... 

Intracellular Signal Transduction the Protein Cascades of the MAP Kinase Pathways Substrates of ERKs... 

Phytoene desaturase and lycopene cyclase New carotenoid pathway (substrate and reaction specificity) Production of torulene in E.coli DNA family shuffling+ screening E. coli [209]... 

Represslble scavenging pathway Substrate seldom present in high concentrations... 

Inducible detoxification pathway Substrate frequently present in high concentration... 

Wojcik C, Rowicka M, Kudlicki A et al. (2006) Valosin-containing protein (p97) is a regulator of ER stress and of the degradation of N-end rule and ubiquitin-fusion degradation pathway substrates in mammalian cells. Mol Biol Cell 17(11), 4606-4618. 

Here Ei - E4 denote the enzyme-catalyzed reactions and A - D represent the intermediates of the pathway (often referred to as the pathway substrates ). For reasons given later it is useful to regard such a sequence of reactions as transmitting a chemical (or metabolic) flux, with each intermediate carrying the flux between the component reactions. 

Flux generation is needed because the steady-state is characterized by a defined rate of operation of the whole pathway in the above hypothetical system this means that reactions Ex to E4 all proceed at the same rate. For this to be possible the first reaction of the sequence (Ej) must not respond to changes in its pathway substrate concentration, [A]. If it did, the rate of Ej would decline continuously as A was consumed by the pathway, so that a constant rate of Ej and hence a steady-state would be impossible. In metabolic systems, saturation of the first reaction with its pathway substrate (denoted by the symbol, / ) produces the required insensitivity to concentration of this substrate. Reactions such as E, which are saturated with their pathway substrate, can be regarded as generating the steady-state flux and have therefore been termed fluxgenerating steps — see Newsholme and Crabtree (31). 

Another example in which conclusions based solely on intrinsic sensitivities may give misleading results is the sensitivity conferred by a substrate cycle. The intrinsic sensitivity for effectors of the forward reaction in the cycle is (1 -I- C/J), where C is the rate of cycling and J is the net flux across the cycle (Table I). However, this sensitivity only equals the net sensitivity if all other potential effectors of the cycle, particularly the pathway - substrate and product, remain at constant concentrations, or if oppositions due to changes in these concentrations are minimized. Changes in the concentrations of substrates and products will produce internal oppositions (see Section III,C) that result in a lower net sensitivity than that predicted from the intrinsic sensitivity. Moreover, whereas the intrinsic sensitivity increases without limit as C/J increases, the net sensitivity may reach a limiting value (9). This system has been analyzed previously (9) but in view of some criticisms of the role of substrate cycles (42) it is analyzed more extensively here. The system to be analyzed is the same as that in the previous section (Fig. 2), except that reaction E2 is replaced by a substrate cycle ... 

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