Enzymes feedback control

Hundreds of metabohc reac tions take place simultaneously in cells. There are branched and parallel pathways, and a single biochemical may participate in sever distinct reactions. Through mass action, concentration changes caused by one reac tion may effect the kinetics and equilibrium concentrations of another. In order to prevent accumulation of too much of a biochemical, the product or an intermediate in the pathway may slow the production of an enzyme or may inhibit the ac tivation of enzymes regulating the pathway. This is termed feedback control and is shown in Fig. 24-1. More complicated examples are known where two biochemicals ac t in concert to inhibit an enzyme. As accumulation of excessive amounts of a certain biochemical may be the key to economic success, creating mutant cultures with defective metabolic controls has great value to the produc tion of a given produc t. 

FIG. 24-1 Feedback control. Product inhibits the first enzyme. 

Now we will return briefly to Sections 3.8-3.11 and 4.6-4.8 where we considered the general problem of multiple flows, here of H, C, N, O, S and P. We observe immediately that all the products are from the same small molecule environmental sources and are required to be formed in relatively fixed amounts using the same source of energy and a series of intermediates. Controlling all the processes to bring about optimum cellular production are feedbacks between them and linked with the code which generates proteins, and here we note particularly enzymes, i.e. catalysts. The catalysts are made from the amino acids, the synthesis of which they themselves manage, while the amino acids control the catalysts so as to maintain a restricted balanced set of reaction pathways in an autocatalytic assembly. It is also the feedback controls on both the DNA (RNA) from the same units used in the... 

Now, most metal ion/organic molecule chemical reactions inside cells also come to equilibrium rapidly. The organic products, made irreversibly available by synthesis under feedback control, contain a broad set of possible binding sites for selected metal ions mainly in soluble proteins (enzymes) and in the pumps for uptake or rejection managed at the cell membrane, as well as in the factors, transcription factors, necessary for controlled production of those organic products under the direction of the coded system. These ion-selective binding sites are common to all cells so that while all cells are based on similar major organic reactions and similar but specific biopolymer products, they also have in common a set of... 

Unlike catecholamines and indoleamines, histamine itself is not a direct inhibitor of its biosynthetic enzyme, but it exerts feedback control through the H3 autoreceptor. Perhaps the most powerful tool in the study of the histamine system is S-a-fluoromethylhistidine, a highly selective and potent suicide inhibitor of HDC [22]. This compound has been used successfully to study many of the functions of histamine in brain. 

Where the pathway generates an allosteric regulator, the opportunity for feedback or indeed feed-forward control arises. Feedback control is exerted by a product of the pathway and acts to switch off a key control enzyme when the concentration of the product reaches a threshold. This indicates the cell contains enough of that particular... 

Any inability to form haem in appropriate amounts will release the feedback control and the pathway will accelerate until the enzyme block point is reached, resulting in the accumulation of the precursors (ALA and PBG) and/or intermediates (porphyrinogens) up to the block point. 

Production of the API begins with the selection of a synthetic route, as determined in the development program. Raw materials are added into a reaction vessel. These raw materials as reactants are heated or cooled in the reaction vessel (normal range is from -15 to 140 °C purpose-built vessels are needed for extreme reactions that require lower or higher temperature controls or pressurization of reaction processes). The chemical synthesis reactions are monitored and controlled via sensor probes (pH, temperature, and pressure) with in-process feedback controls for adjustments and alarms when necessary. Samples are withdrawn at dehned intervals for analysis to determine the reaction progress. Catalysts, including enzymes, may be added to speed up and direct the reaction along a certain pathway. 

It was snbseqnently discovered that the first enzyme in the pathway for isoleucine synthesis, which is threonine deaminase, was inhibited by isoleucine in an extract of E. coli. No other amino acid caused inhibition of the enzyme. Threonine deaminase is, in fact, the rate-limiting enzyme in the pathway for isoleucine synthesis, so that this was interpreted as a feedback control mechanism (Fignre 3.13(a)). Similarly it was shown that the hrst enzyme in the pathway for cytidine triphosphate synthesis, which is aspartate transcarbamoylase, was inhibited by cytidine triphosphate (Fignre 3.13(b)). Since the chemical structures of isoleucine and threonine, or cytidine triphosphate and aspartate, are completely different, the qnestion arose, how does isolencine or cytidine triphosphate inhibit its respective enzyme The answer was provided in 1963, by Monod, Changenx Jacob. 

The first step of this sequence, which is not unique to de novo purine nucleotide biosynthesis, is the synthesis of 5-phosphoribosylpyrophosphate (PRPP) from ribose-5-phosphate and adenosine triphosphate. Phosphoribosyl-pyrophosphate synthetase, the enzyme that catalyses this reaction [278], is under feedback control by adenosine triphosphate [279]. Cordycepin interferes with thede novo pathway [229, 280, 281), and cordycepin triphosphate inhibits the synthesis of PRPP in extracts from Ehrlich ascites tumour cells [282]. Formycin [283], probably as the triphosphate, 9-0-D-xylofuranosyladenine [157] triphosphate, and decoyinine (LXXlll) [284-286] (p. 89) also inhibit the synthesis of PRPP in tumour cells, and this is held to be the blockade most important to their cytotoxic action. It has been suggested but not established that tubercidin (triphosphate) may also be an inhibitor of this reaction [193]. 

An even more striking instance of feedback control is found in the synthesis of UNA (see also Nucleoproteins and Nucleic Acids). As pointed nut in Ihat cnlry. normal UNA is composed of the nucleotides dcoxyguanosiiic. dcoxycytidine. deuxyadenosine. and thymidine, and Ihe amounts of the first and second of these are the same, as are those of the third and fourth. Obviously, close control is required of the amounts of these nucleotides that are synthesized by the cell, if they are to be made in the quantities required for DNA synthesis. Evidence has been found that ihe enzyme carbamoylphosphaie L-aspartate carbamoyl transferase, which... 

Increased clearance of steroid hormones due to induction of hepatic biotransformation enzymes following chemical exposure often has been cited as a possible mechanism by which toxicants could lower circulating testosterone or 17/3-estradiol levels. While enhanced clearance of sex steroids has been demonstrated following chemical exposure and induction of hepatic biotransformation enzymes, elegant feedback control mechanisms tend to ensure that more hormone is produced and homeostasis is maintained (Figure 17.2). Enhanced clearance of sex steroids can contribute to endocrine disruption if the toxicity also results in impaired hormone synthesis (i.e., gonadal toxicity or interference with the feedback control of hormone synthesis). 2,3,7,8-Tetrachlorodibenzodioxin appears to lower circulating sex steroid levels via this dual effect. 

Eukaryotic organisms contain a multifunctional enzyme with carbamoylphosphate synthetase, aspartate transcarbamoylase, and dihydroorotase activities. Two mechanisms control this enzyme. First, control at the level of enzyme synthesis exists the transcription of the gene for the enzyme is reduced if an excess of pyrimidines is present. Secondly, control exists at the level of feedback inhibition by pyrimidine nucleotides. This enzyme is also an example of the phenomenon of metabolic channeling aspartate, ammonia, and carbon dioxide enter the enzyme and come out as orotic acid. 

The simplest form of regulation of a metabolic pathway is the inhibition of an enzyme by the product of the pathway. In Fig. 9-5, the E, s denote enzymes, A and B are metabolites, and the circled minus sign indicates inhibition. If there were no inhibitor of the enzyme (E,) acting on A, the concentration of B would depend entirely on its rate of synthesis or utilization. If the rate of utilization of B decreased or B was supplied from an outside source, its concentration would rise, perhaps even to toxic levels. However, if B is an inhibitor of the first enzyme, then as its concentration rises, the extent of inhibition will increase and its rate of synthesis will decrease. This effect is called feedback inhibition or negative feedback control it is a concept also used in describing electronic circuits. 

Cholesterol cannot suppress HMG-CoA reductase activity in liver cells that have become malignant. However, preparations of the soluble enzyme from both rat liver and hepatoma cells behaved identically with respect to gel filtration, kinetics, and cold-inactivation.25 It was concluded that the failure of feedback control in malignant cells was not due to an alteration in the enzyme. A study of the effects of various antibodies on the decrease of HMG-CoA reductase synthesis brought about by serum lipoprotein in hepatoma cells in culture26 has led to the suggestion that regulation by lipoprotein is at a post-transcriptional level. 

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