Repression, of enzyme

Lipogenesis is regulated at the acetyl-CoA carboxylase step by allosteric modifiers, phosphorylation/de-phosphorylation, and induction and repression of enzyme synthesis. Citrate activates the enzyme, and long-chain acyl-CoA inhibits its activity. Insulin activates acetyl-CoA carboxylase whereas glucagon and epinephrine have opposite actions. 

The nature of the molecular defect Is unclear and presumably lies In the repression mechanism for the gene controlling formation of the enzyme protein. Exposure to any of the drugs listed In Table V results In further marked de-repressIon of enzyme synthesis and severe porphyria. 

DRUGS INDUCING MARKED DE-REPRESSION OF ENZYME SYNTHESIS... 

Some enzymes and carriers are synthesized only in response to the presence of the sugar, or of a structurally similar compound these enzymes and carriers are said to be inducible. Contrariwise, enzyme synthesis may be repressed by an increase in the concentration of ATP, or of some other metabolite. Induction and repression of enzymes and carriers provide two important kinds of control in metabolic regulation. 

The regulatory mechanisms described above modify the activity of existing enzyme molecules. However, cells can also regulate the amount of enzyme present—usually by altering the rate of enzyme synthesis. The increased (induction) or decreased (repression) of enzyme synthesis leads to an alteration in the total population of active sites. [Note The efficiency of existing enzyme molecules is not affected.] Enzymes subject to regulation of synthesis are often those that are needed at only one stage of development or under selected... 

The flow of intermediates through metabolic pathways is controlled by 1bir mechanisms 1) the availability of substrates 2) allosteric activation and inhibition of enzymes 3) covalent modification of enzymes and 4) induction-repression of enzyme synthesis. This scheme may at first seem unnecessarily redundant however, each mechanism operates on a different timescale (Figure 24.1), and allows the body to adapt to a wde variety of physiologic situations. In the fed state, these regulatory mechanisms ensure that available nutrients are captured as glycogen, triacylglycerol, and protein. 

Reoviruses 248. See also front cover Repair systems 16 Replication cycle. See Cell cycle Repression of enzyme synthesis 536, 538, 539 Repressor(s) 76, 239... 

Repression of enzymes, inhibition of enzyme synthesis by a product (or products) of the metabolic pathway in which the enzyme operates. A means of control on metabolism. 

Toda, K. (1981). Induction and repression of enzymes in microbial culture. Journal of Chemical Technology and Biotechnology, 31, 775-790. 

Overexpression/Repression of Enzymes Regulating Carbon Flux.9... 

OVEREXPRESSION/REPRESSION OF ENZYMES REGULATING CARBON FLUX... 

FIGURE 3.9.2 Inhibition of enzyme activity can be due to a high concentration of the product, or through direct inhibition of the product on the substrates, or through product repression of enzyme synthesis. 

A number of publications have appeared on the dynamics of enzyme synthesis in a variety of situations. Most of the models are based on more or less sophisticated versions of the operon model of Jacob and Monod. The role of m-RNA and its stability were modeled by Terui (1972). Repressor and inducer control was treated by Knorre (1968), Imanaka et al. (1972 1973), van Dedem and Moo-Young (1973), and Suga et al. (1975). Allowance for dual control and catabolite repression was made by Toda (1976). [See also the kinetic treatment by Yagil and Yagil (1971), Imanaka and Aiba (1977), and Bajpai and Ghose (1978)]. A simple structured model was developed by Roels (1978) showing a combination of the features of the models published. More recently Toda (1981) reviewed the effects of induction and repression of enzymes in microbial cultures and their modeling. 

Table 2. Differences between feedback inhibition and repression of enzyme synthesis.

Induction (derepression) and repression of enzyme synthesis by transcriptional regulation on the DNA template, or the RNA polymerase. 

The synthesis of the E. coli aspartate carbamyltransferase is under repressor control (28) , this is one of the classical examples of metabolite control by repression of enzyme S3mthesis. 

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