Enzymes repressed

Synthesis of the enzyme repressed by high glucose and ammonium levels. 

The determination of sites of active centers is effected not only by splitting of enzymes, but also by treating them wilh lemporary or permanent inhibitors, and determining their points of attachment. Other methods of studying enzymes are by means of enzyme induction and enzyme repression. 

Be able to define constitutive enzymes, inducible enzymes, enzyme repression, coordinate repression, and derepression. 

An effect opposite to induction is enzyme repression. Wild-type E. coli cells can be grown on a medium containing an ammonium salt as the sole source of nitrogen. Under these conditions, all nitrogenous compounds must be formed from ammonium ion and a carbon source, and such cells have all the enzymes necessary for the synthesis of the 20 amino acids requisite for protein synthesis. If, for example, the amino acid tryptophan is added to the culture medium, all the enzymes required for the biosynthesis of tryptophan disappear from the cell. In this case, the synthesis of five enzymes catalyzing five consecutive steps in tryptophan biosynthesis is repressed. This is a termed coordinate repression. It will become clear that induction and repression are manifestations of the same phenomenon. 

These results provide additional evidence that enzyme repression is an important mechanism in B(a)P-induced neurotoxicity and likely results from oxidative stress in the nervous system. Inhibition of Na /K -ATPase, an important enzyme in muscle contraction and nerve excitability, in addition to decreased motor conduction velocities may explain the suppression of motor activity observed in B(a)P intoxicated rats (Kim et al, 2000 Saunders et al, 2001). Furthermore, there is also strong experimental evidence showing that oxidative stress and lipid peroxidative products can cause decreases in dopamine and inhibit Na /K -ATPase activity as well (Madrigal et al, 2003). 

At the cellular level, mechanisms that specifically regulate the dis-similatory activities of heterotrophic microorganisms include enzyme repression and enzyme inhibition. In enzyme repression, a substrate or a metabolic intermediate related to the substrate represses further synthesis of an unrelated dissimilatory enzyme, whereas the term inhibition applies if a metabolic intermediate inhibits an existing unrelated dissimilatory enzyme (16). 

When a P. aeruginosa mutant (PALS 128) was grown under iron rich conditions, the specific activity of the SA-forming enzymes was below the limits of detection [79]. Liu et al. [88], suggest that entC gene expression may be limited at the translational level as well, even when the operon is induced under iron deficiency. This may be understandable because chorismic acid is an essential metabolite for Phe, Trp, Tyr, folate and ubiquinone synthesis. In B. subtilis it was shown that the accumulation of 2,3-DHBA(Glycine) was influenced by the levels of aromatic amino acids and anthranilic acid. Anthranilic acid inhibited the synthesis of DHBA from chorismic acid [117]. It seemed that the reduction in phenolic acid accumulation caused by aromatic amino acids is a consequence of enzyme repression [121]. The synthesis of 2,3-DHBA in B. subtilis is also reduced by other phenolic acids, such as m-substimted benzoic acids. Inhibition of accumulation of phenolic acid by other phenolic acids, would indicate a fairly specific effect on phenolic acid synthesis, but not on the accumulation of coproporphyrin that also accumulates in iron-deficient cultures oiB. subtilis [121]. 

Ans. Regulatory enzymes are present in each cell to prevent translation of most of the DNA messages in that particular cell. For example, heart cells do not need and must not synthesize the proteins found in brain cells or liver cells and so on. Repressor enzymes repress the synthesis of all proteins except those needed for the heart cell. Simultaneously, inducer enzymes induce the synthesis of proteins needed for the heart cell. Only about 2 percent of all the DNA in any cell is used for protein synthesis. 

Metabolic pathways are also controlled via enzyme repression and derepression (Chaps. 7 and 8) that occur in response to variations of metabolite levels. However, the time scale of this control is hours to days, in contrast to less than a few seconds for direct binding effects. 

Figure 2, The biosynthesis of lysine, methionine, threonine, and isoleucine in E. coli and S. marcescens. Solid arrows, steps catalyzed by enzymes repressed by lysine. Broken arrows, steps catalyzed by enzymes repressed by methionine. Open arrows, steps catalyzed by enzymes repressed by threonine plus isoleucine. Open, dashed arrows, steps catalyzed by enzymes controlled as described in Figure 1. Structural genes indicated in italics. Dashed lines indicate reactions controlled by endproduct inhibition. Reproduced, with permission, from Ref. 57. Copyright 1975, American...

Enzyme repression Mechanism by which the presence of a particular metabolite represses the genes coding for enzymes used in its synthesis. 

Attenuation a regulatory mechanism employed by the bacterial cell. Whereas enzyme repression al-... 

Derepression the release of an operon from repression of transcription. In prokaryotic cells it occurs by inactivation of a repressor, either by removal of a corepressor (see Enzyme repression) or by binding of an inducer (see Enzyme induction). D. in eukaryotic cells involves regulatory proteins and effectors, such as hormones (see Gene activation). 

End product repression inhibition of the synthesis of the enzraes of a reaction sequence (see Enzyme repression) by the end product of that reaction sequence. 

Enzyme repression blockage of the synthesis of enzymes of a biosynthetic pathway by the end product of the same pathway. This type of regulation is found in prokaryotes, in particular for operons of amino acid biosynthesis. If an amino acid is available in the growth medium, the synthesis of all the enzymes in the operon is turned off, but if it is in short supply, the operon is derepressed (see Derepression). See also Attenuation. 

The operon model was developed by Jacob and Monod, and it has only been demonstrated in prokar-yotie systems. It is the basis for the explanation of Enzyme induction (see) and Enzyme repression (see). See also Attenuation. 

In an inducible enzyme system, the R. is inactive in the presence of the effector (inducer) binding to the inducer apparently changes the conformation of the R., so that it no longer binds to the operator (see Enzyme induction). Synthesis of mRNA can therefore proceed only when the inducer is present. In enzyme repression, the situation is revets R. is activated by a corepressor (the endproduct of a biosynthetic pathway, e.g. an amino acid) so that it can bind to the operator. In this case, synthesis of mRNA proceeds only in the absence of corepressor (see Enzyme repression). See also Derepression. 

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