Product inhibition systems

The enzymatic saccharification of cellulose is a product inhibited system. The Linewearer Burk model is ... 

In this scheme, F symbolizes an essential metabolite, such as an amino acid or a nucleotide. In such systems, F, the essential end product, inhibits enzyme 1, xAie first step in the pathway. Therefore, when sufficient F is synthesized, it blocks further synthesis of itself. This phenomenon is called feedback inhibition or feedback regulation. 

The 2-ethoxyethanol was a by-product, as shown in Figure 5.13. The formation rate of 2-ethoxyethanol was the same as the conversion rate of the (S)- or (R)-ibuprofen ester one mole of 2-ethoxyethanol was formed when one mole of ester was catalysed. A known concentration of 2-ethoxyethanol was added in the organic phase before the start of the reaction for product inhibition. The plots of the kinetics for the free lipase system are presented in Figure 5.17 and immobilised enzyme (EMR) in Figure 5.18, respectively. The Kw value was 337.94 mmoFl 1 for the free lipase batch system and 354.20 mmoll 1 for immobilised... 

Fig. 5.17. Free lipase in batch system Product inhibition plots (Left Hanes-woolf Right Curve fit).

By manipulating the genetic machinery of the cell, it is possible to cause most cellular systems to produce virtually any biochemical material. Unfortunately, the growth of cellular systems (particularly in tissue cultures) is constrained by end-product inhibition and repression hence, it is difficult to produce end products in high concentration. Furthermore, cells are always grown in aqueous solution, so biochemicals produced by cellular routes must have intrinsically high value in order for the cost of recovery from dilute aqueous solution to be minimized. Thus, most biochemicals of commercial interest... 

It is possible that product inhibition by PAC or by-product inhibition by acetoin or acetaldehyde may play a more important role than benzaldehyde in influencing PAC production in this aqueous phase system. 

Formic acid is stable under the reaction conditions. Optimization of the reaction conditions led to a system that afforded 30 turnovers of 1 after 5 h (Table 2). Based on both IR and UV-vis, the Ce-POM retains its structure under the turnover conditions. The formic acid product inhibits the reaction, so procedures to remove it during reaction are needed to increase CH20 conversions. 

The process known as SPREAD (Surface Promoted Replication and Exponential Amplification of DNA Analogues) attempts to reach the target, striven for by many researchers, of an exponential proliferation of biomolecules in model systems. As already mentioned, product inhibition (e.g., by dimerisation of the new matrices to give C2) only allowed parabolic growth. In the SPREAD process, both solid phase chemistry and feeding have a positive effect on the synthesis. Thus, no separation processes are required, as excess reagents can be removed just by washing. The synthetic process consists of four steps ... 

Fig. 8.3 The proliferation curves of RNA strands (the Q beta system) for decreasing concentrations of added matrix molecules. If the number of matrix molecules is larger than that of the enzymes, a linear proliferation is observed (first curve). This slows down at high concentrations, due to product inhibition. RNA proliferation is exponential if the amount of enzyme is larger than that of the matrix. If no matrix is added, the system goes through an incubation phase and then forms an RNA sequence which is related to certain Q beta fragments (Eigen et al., 1982)...

Similarly, cyclodextrin accelerates the cleavage of pyrophosphates by about 200-fold. This enhancement is associated with a simultaneous transfer of a phenylphosphate group to the host by the vicinal action of the hydroxy groups [see Figure 5.4] (Hennrich Cramer, 1965). In this case the product monophenylphosphate also forms an inclusion complex and thus product inhibition occurs. Because of this, the system is not truly catalytic. 

Results have generally been disappointing. It can be difficult to remove the TSA from the polymer, but a more fundamental problem concerns the efficiency of the catalysis observed. The most efficient systems catalyze the hydrolysis of carboxylate and reactive phosphate esters with Michaelis-Menten kinetics and accelerations (koAJKM)/kunoJ approaching 103,1661 but the prospects for useful catalysis of more complex reactions look unpromising. Apart from the usual difficulties the active sites produced are relatively inflexible, and the balance between substrate binding and product inhibition is particularly acute. 

Dye setting, control of, 9 493-498 Dyesite content, of polymer fibers, 11 195 Dye solubility, modification of, 9 509 Dye stability, in color photography, 19 263 Dyestuff migration, 9 231-232 Dyestuff production trends in, 9 294t United States, 9 295 West European, 9 294t Dyestuffs industry, growth of, 9 238-239 Dye-transfer inhibition system, 10 284 Dye transfer system, in color photography, 19 242... 

MS is lower than that of M the system is in the regime of substrate saturation addition of more S does not lead to a rate increase. The behaviour of the reaction rate in case B is typical of enzymes and in biochemistry this is referred to as Michaelis-Menten kinetics. The success of the application of the Michaelis-Menten kinetics in biochemistry is based on the fact that indeed only two reactions are involved the complexation of the substrate in the pocket of the enzyme and the actual conversion of the substrate. Usually the exchange of the substrate in the binding pocket is very fast and thus we can ignore the term k2[H2] in the denominator. Complications arise if the product binds to the binding site of the enzyme, product inhibition, and more complex kinetics result. 

Figure 4.11 Effect of inhibition of enzyme 2 by cofactor A and of enzyme 1 by cofactor B (i.e., product inhibition) on the concentration of B in the basic system when operated as a fed-batch reactor. For the central and right panels the inhibition constants are indicated on top of each section. In the left panel, inhibition by products was not considered, and—indicates that the parameter is not applicable. Data presented in the left panel are taken from Figure 4.4. The values used for all other parameters ares given in Table 4.1, set I.

A major problem associated with the above-described systems is that of product inhibition of the catalyst and isomerization processes, when a certain yield of a-olefin has been attained. It would be desirable, therefore, to have a catalyst (or a combination of catalysts) capable of using and/or draining the newly formed double bonds in such a way that the olefin would cause neither of these problems. 

Herb-drug interactions are of growing concern due to the increased use and awareness of natural health products. They generally arise when natural health products inhibit CYP enzymes, altering the rate of metabolism for other drugs in the system. It is important to note, however, that interactions may also arise when... 

The rate-limiting step in the synthesis of the catecholamines from tyrosine is tyrosine hydroxylase, so that any drug or substance which can reduce the activity of this enzyme, for example by reducing the concentration of the tetrahydropteridine cofactor, will reduce the rate of synthesis of the catecholamines. Under normal conditions tyrosine hydroxylase is maximally active, which implies that the rate of synthesis of the catecholamines is not in any way dependent on the dietary precursor tyrosine. Catecholamine synthesis may be reduced by end product inhibition. This is a process whereby catecholamine present in the synaptic cleft, for example as a result of excessive nerve stimulation, will reduce the affinity of the pteridine cofactor for tyrosine hydroxylase and thereby reduce synthesis of the transmitter. The experimental drug alpha-methyl-para-tyrosine inhibits the rate-limiting step by acting as a false substrate for the enzyme, the net result being a reduction in the catecholamine concentrations in both the central and peripheral nervous systems. 

Rate experiments that are typically carried out in the presence of different concentrations of an alternative product (or product analog) while using the normal substrates . This approach can be particularly useful when the normal product cannot be used because it is unstable, insoluble, or ineffective (the latter indicated by a very high Ki value). Moreover, the normal product may be consumed as an essential substrate in a coupled assay system for the primary enzyme. Fromm and Zewe used the alternative product inhibition approach in their study of hexokinase. Wratten and Cleland later applied this procedure to exclude the Theorell-Chance mechanism for liver alcohol dehydrogenase. See Abortive Complexes... 

The assay protocol should measure true initial rates (See Initial Rate Condition). For most systems, this represents a time period in which less than ten percent of the substrate concentration has undergone conversion. However, if a reaction is not significantly favored thermodynamically or if product inhibition is particularly potent, then a much smaller percentage of substrate conversion may be needed such that true initial rate conditions are obtained. Addition of an auxiliary enzyme system may prove necessary to avoid product accumulation. See Coupled Enzyme Assays... 

Fromm and Rudolph have discussed the practical limitations on interpreting product inhibition experiments. The table below illustrates the distinctive kinetic patterns observed with bisubstrate enzymes in the absence or presence of abortive complex formation. It should also be noted that the random mechanisms in this table (and in similar tables in other texts) are usually for rapid equilibrium random mechanism schemes. Steady-state random mechanisms will contain squared terms in the product concentrations in the overall rate expression. The presence of these terms would predict nonhnearity in product inhibition studies. This nonlin-earity might not be obvious under standard initial rate protocols, but products that would be competitive in rapid equilibrium systems might appear to be noncompetitive in steady-state random schemes , depending on the relative magnitude of those squared terms. See Abortive Complex... 

Pharmacology These agents are synthetic adrenocortical steroids with basic glucocorticoid actions and effects. Glucocorticoids may decrease number and activity of inflammatory cells, enhance effect of beta-adrenergic drugs on cyclic AMP production, inhibit bronchoconstrictor mechanisms, or produce direct smooth muscle relaxation. Inhaler use provides effective local steroid activity with minimal systemic effect. 

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