Biosynthetic pathways feedback inhibition

Feedback control. The binding of metaboUtes, i.e. substrates/products or effectors to enzymes is an important mode of regulation. Feedback inhibition (negative feedback control) is an important example, in which the first committed step in a biosynthetic pathway is inhibited by the ultimate end product of the pathway (Stadtman, 1966). Table 11.14 summarizes different modes of negative feedback controls that have been evolved to accommodate the regulation of divergent metabolic pathways. 

The small overproduction of amino adds by wild type strains in culture media is the result of regulatory mechanisms in the biosynthetic pathway. These regulatory mechanisms are feedback inhibition and repression. 

Feedback inhibition refers to inhibition of an enzyme in a biosynthetic pathway by an end product of that pathway. For example, for the biosynthesis of D from A catalyzed by enzymes EnZj through Enz3,... 

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. 

Figure 9-4. Sites of feedback inhibition in a branched biosynthetic pathway. Si-Sj are intermediates in the biosynthesis of end products A-D. Straight arrows represent enzymes catalyzing the indicated conversions. Curved arrows represent feedback loops and indicate sites of feedback inhibition by specific end products.

Figure 9-5. Multiple feedback inhibition in a branched biosynthetic pathway. Superimposed on simple feedback loops (dashed, curved arrows) are multiple feedback loops (solid, curved arrows) that regulate enzymes common to biosynthesis of several end products.

Product Z binds to the first enzyme and forms a complex that is catalytically inactive. That shuts down the biosynthetic pathway. Should the amount of product Z diminish, the inhibition will be relieved and the pathway will again become active. This is an example of feedback inhibition of a metabolic pathway and is... 

At each level of a hormonal cascade, feedback inhibition of earlier steps in the cascade is possible an unnecessarily elevated level of the ultimate hormone or of one of the intermediate hormones inhibits the release of earlier hormones in the cascade. These feedback mechanisms accomplish the same end as those that limit the output of a biosynthetic pathway (compare Fig. 23-11 with Fig. 6-28) a product is synthesized (or released) only until the necessary concentration is reached. 

FIGURE 23-11 Cascade of hormone release following central nervous system input to the hypothalamus. In each endocrine tissue along the pathway, a stimulus from the level above is received, amplified, and transduced into the release of the next hormone in the cascade. The cascade is sensitive to regulation at several levels through feedback inhibition by the ultimate hormone. The product therefore regulates its own production, as in feedback inhibition of biosynthetic pathways within a single cell. 

Metabolic control can be understood to some extent by focusing attention on those enzymes that catalyze rate-limiting steps in a reaction sequence. Such pacemaker enzymes1-4 are often involved in reactions that determine the overall respiration rate of a cell, reactions that initiate major metabolic sequences, or reactions that initiate branch pathways in metabolism. Often the first step in a unique biosynthetic pathway for a compound acts as the pacemaker reaction. Such a reaction may be described as the committed step of the pathway. It usually proceeds with a large decrease in Gibbs energy and tends to be tightly controlled. Both the rate of synthesis of the enzyme protein and the activity of the enzyme, once it is formed, may be inhibited by feedback inhibition which occurs when an end product of a biosynthetic pathway accumulates... 

Probably the most common and widespread control mechanisms in cells are allosteric inhibition and allosteric activation. These mechanisms are incorporated into metabolic pathways in many ways, the most frequent being feedback inhibition. This occurs when an end product of a metabolic sequence accumulates and turns off one or more enzymes needed for its own formation. It is often the first enzyme unique to the specific biosynthetic pathway for the product that is inhibited. When a cell makes two or more isoenzymes, only one of them may be inhibited by a particular product. For example, in Fig. 11-1 product P inhibits just one of the two isoenzymes that catalyzes conversion of A to B the other is controlled by an enzyme modification reaction. In bacteria such as E. coli, three isoenzymes, which are labeled I, II, and III in Fig. 11-3, convert aspartate to (3-aspartyl phosphate, the precursor to the end products threonine, isoleucine, methionine, and lysine. Each product inhibits only one of the isoenzymes as shown in the figure. 

Regulation of purine biosynthesis. Red arrows show points of inhibition 0 or activation . In addition to the feedback inhibition, GTP stimulates ATP synthesis, and ATP stimulates GTP synthesis, thus helping to ensure a balance between the pools of the two nucleoside triphosphates. The full biosynthetic pathways are shown in figures 23.10 and 23.11. 

The 10 amino acids essential in the human diet (Arg, His, He, Leu, Lys, Met, Phe, Thr, Trp, Val) are synthesized by non-human organisms by multistep pathways starting from simple metabolic precursors. Amino acid biosynthesis is controlled by feedback inhibition and suppression of synthesis of biosynthetic enzymes. The ability of an amino acid analogue to block biosynthesis of the parent amino acid often contributes to the toxicity of the analogue. Mutants resistant to the toxic effects of the analogue can be valuable tools for studying various aspects of cellular mechanism (examples to be given below). 

To evaluate such kinetic expressions in the cell obviously requires a method for determining the reaction rates and the concentrations of all the important substrates and effectors. This is now possible for some very well studied carbon breakdown pathways using whole-cell NMR spectroscopy, but extension of these methods to synthesis in cells is so far untouched. From a qualitative viewpoint, biosynthetic pathways are known to employ feedback product inhibition in order to regulate the flow of a starting precursor to a variety of different products required by the cell (see Fig. 4). 

The L-threonine biosynthetic pathway consists of five enzymatic steps from L-aspartate. E. coli has three aspartate kinase isoenzymes, key enzymes which catalyze the first reaction of the L-threonine biosynthetic pathway. The aspartate kinase isoenzymes I, II, and III encoded by the thrA, metL, and lysC genes, respectively, are affected by feedback inhibition by L-threonine, L-methionine, and L-lysine, respectively. C. glutamicum has only one aspartate kinase encoded by the lysC gene, which is subjected to feedback inhibition by L-lysine and... 

Fig. I The biosynthetic pathway of L-threonine, and its regulation in E. coli. Dotted lines indicate feedback inhibition. Thick dotted lines indicate transcriptional attenuation regulation...

Alkaloid metabolism in lupine was proved by Wink and Hartmann to be associated with chloroplasts (34). A series of enzymes involved in the biosynthesis of lupine alkaloids were localized in chloroplasts isolated from leaves of Lupinus polyphylls and seedlings of L. albus by differential centrifugation. They proposed a pathway for the biosynthesis of lupanine via conversion of exogenous 17-oxosparteine to lupanine with intact chloroplasts. The biosynthetic pathway of lupinine was also studied by Wink and Hartmann (35). Two enzymes involved in the biosynthesis of alkaloids, namely, lysine decarboxylase and 17-oxosparteine synthetase, were found in the chloroplast stoma. The activities of the two enzymes were as low as one-thousandth that of diaminopimelate decarboxylase, an enzyme involved in the biosynthetic pathway from lysine to diaminopimelate. It was suggested that these differences are not caused by substrate availability (e,g., lysine concentration) as a critical factor in the synthesis of alkaloids. Feedback inhibition would play a major role in the regulation of amino acid biosynthesis but not in the control of alkaloid formation. 

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