Feedback inhibitors

A second class of herbicides primarily affects ( -carotene desaturase. These herbicides are apparent feedback inhibitors of PD as well. This class of compounds includes dihydropyrones like LS 80707 [90936-96-2] (56) and 6-methylpyridines (57,58). The third class consists of the ben2oylcyclohexane-diones, eg, 2-(4-chloro-2-nitroben2oyl)-5,5-dimethyl-cyclohexane-I,3-dione. This class of atypical bleaching herbicides induces phytoene accumulation when appHed either pre- or post-emergence. However, it does not inhibit phytoene desaturase activity in vitro (59). Amitrole also has been considered a bleaching herbicide, though its main mode of action is inhibition of amino acid synthesis. 

Figure 6.24 The function of the enzyme phosphofructokinase. (a) Phosphofructokinase is a key enzyme in the gycolytic pathway, the breakdown of glucose to pyruvate. One of the end products in this pathway, phosphoenolpyruvate, is an allosteric feedback inhibitor to this enzyme and ADP is an activator, (b) Phosphofructokinase catalyzes the phosphorylation by ATP of fructose-6-phosphate to give fructose-1,6-bisphosphate. (c) Phosphoglycolate, which has a structure similar to phosphoenolpyruvate, is also an inhibitor of the enzyme.

The leucme-reversible toxicity of 5,5, -trifluoroleudne (10) to microorganisms, such as Escherichia coli, has been attributed to incorporation of JO into protem [32] and to the action of 10 as a false feedback inhibitor of a-isopropylmalate... 

Inhibition of a regulatory enzyme by a feedback inhibitor does not conform to any normal inhibition pattern, and the feedback inhibitor F bears little structural similarity to A, the substrate for the regulatory enzyme. F apparently acts at a binding site distinct from the substrate-binding site. The term allosteric is apt, because F is sterically dissimilar and, moreover, acts at a site other than the site for S. Its effect is called allosteric Inhibition. 

Selection of these regulatory mutants is often done by using toxic analogues of amino adds for example p-fluoro-DL-phenylalanine is an analogue of phenylalanine. Mutants that have no feedback inhibition or repression to the amino add are also resistant to the analogue amino add. They are therefore selected for and can be used to overproduce the amino add. Some amino add analogues function as false co-repressors, false feedback inhibitors or inhibit the incorporation of foe amino acid into foe protein. 

In a branched biosynthetic pathway, the initial reactions participate in the synthesis of sevetal products. Figure 9—4 shows a hypothetical btanched biosynthetic pathway in which cutved attows lead from feedback inhibitors to the enTymes whose activity they inhibit. The sequences S3 —> A, S4 —> B, S4 —> C, and S3 — > D each represent hneat teaction sequences that are feedback-inhibited by theit end products. The pathways of nucleotide biosynthesis (Chaptet 34) provide specific examples. 

The lack of structural similarity between a feedback inhibitor and the substrate for the enzyme whose activity it regulates suggests that these effectors are not isosteric with a substrate but allosteric ( occupy another space ). Jacques Monod therefore proposed the existence of allosteric sites that are physically distinct from the catalytic site. Allosteric enzymes thus are those whose activity at the active site may be modulated by the presence of effectors at an allosteric site. This hypothesis has been confirmed by many lines of evidence, including x-ray crystallography and site-directed mutagenesis, demonstrating the existence of spatially distinct active and allosteric sites on a variety of enzymes. 

Reversible inhibitors are more subtle and act usefully to control the rate of particular enzymes. Often, reversible inhibitors are substrates found at or near the end of a pathway. These compounds act in a negative feedback manner to slow down the activity of an enzyme at or near the beginning of the same pathway. Occasionally, feedback inhibitors may be substrates found within a pathway which is functionally related to the one in which the target enzyme can be found. Furthermore, the products of an enzyme-catalysed reaction are often inhibitory to the enzyme that generated them (Figure 3.2). This is should not be surprising from a structural point of view because the product must fit into the active site of the enzyme and so block the binding of substrate. 

Answer D. IMP is a feedback inhibitor of PRPP amidophosphoribosyl transferase, the first reaction in the biosynthesis of purines. IMP is formed by the HPRT reaction in the salvage of hypoxanthine. 

Studies on the mechanism of action of 6-mercaptopurine are complicated by the fact that its anabolic product, thioinosinic acid, is further metabolized by oxidation to 6-thioxanthylic acid [219] and by methylation to 6-(methylthio)purine ribonucleotide [206, 296]. the effects of which could be even more important than those of thioinosinic acid itself, since the methylthio compound is about 20 times as potent as a feedback inhibitor [289]. 

A number of other purines, purine ring analogues, and their ribonucleosides have been evaluated as feedback inhibitors [13, 173, 294, 297, 297a] by a modification [294] of the method of LePage and co-workers [291,298,299,300]. This method utilizes azaserine to isolate the first few steps of the de novo pathway in whole cells by the specific blockade of the conversion of formylglycinamide... 

A comparison of EDso values for feedback inhibition and for growth inhibition in H.Ep.-2 cells in culture is shown in Table 2.3. It is readily apparent that for most of the purine analogues listed, the correlation between feedback inhibition and cytotoxicity is good. The few discrepancies may be due to the fact that these particular compounds are not feedback inhibitors, but metabolism (or lack of it) of the analogue in question may be important. In comparing these ED50 values, the difference in experimental conditions for cytotoxicity determination (long... 

In addition to the analogues listed in Table 2.3, cordycepin [302]. 3 -amino-3 -deoxyadenosine [173], and formycin [303] can inhibit the de novo pathway by blocking the phosphoribosylpyrophosphate amidotransferase. Thus, a number ofpurine analogues—after anabolism to nucleoside phosphates—can act as feedback inhibitors, and this inhibition may be the primary cause of their cytotoxicity. 

Cumulative Feedback Inhibition In cumulative feedback inhibition, the end products can inhibit the reaction of the target enzyme separately. Many textbooks erroneously indicate that the cumulative feedback inhibition of E. coli glutamine synthetase involves separate regulatory sites for each feedback inhibitor. See Cumulative Feedback Inhibition... 

FEEDBACK INHIBITION CUMULATIVE FEEDBACK INHIBITION Feedback inhibitor,... 

Postsynaptic Hj- and Hj-receptors are responsible for a variety of processes in the CNS. Hi-receptors mediate the maintenance of wakeful states, while Hj- and Hj-receptors participate in the regulation of blood pressure, body temperature, fluid homeostasis, and pain sensation. Presynaptic Hj-receptors serve as feedback inhibitors of the release of histamine, norepinephrine, and other neurotransmitters. 

Molecules with structures as diverse as carbamoyl-phosphate, tryptophan, and cytidine triphosphate are feedback inhibitors of the E. coli glutamine synthase. The feedback inhibition is cumulative, with each metabolite exerting a partial inhibition on the enzyme. Why would complete inhibition of the glutamine synthase by a single metabolite be metaboli-cally unsound ... 

S-(2-Aminoethyl)-L-cysteine (AEC), H2N-CH2-CH2-S-CH2-CH(NH2)-COOH, a lysine analog, acts as a false feedback inhibitor on aspartokinase, which produces aspartylphosphate from aspartate. The inhibitor simulates, for aspartokinase, the absence of lysine and threonine, and as a consequence the AEC insensitive mutant is no longer inhibited by lysine and threonine. The result was a yield increase from 0 to 16 g L 1. 

ThiolMP and ThioGMP are feedback inhibitors of phosphoribosylpyrophosphate amido-transferase, which is the first, and rate-limiting step in the synthesis of purine. In addition, these analogs inhibit the de novo biosynthesis of purine and block the conversion of inosinic acid to adenylic acid or guanylic acid. The triphosphate nucleotides are incorporated into DNA, and this results in delayed toxicity after several cell divisions. 

---