Enzyme regulation feedback inhibition

The yeast enzyme is a homodimer of Mr2 X 102 500 and has 49% sequence identity to the muscle enzyme. The yeast enzyme is more simply regulated feedback inhibition by the allosteric inhibitor glucose-6-phosphate and activation by a 3 -5 -cyclic AMP-dependent protein kinase or a yeast phosphorylase that phosphorylates Thr-10.55... 

Pyrimidine biosynthesis in E. coli is regulated by the feedback inhibition of aspartate transcarbamoylase, the enzyme that catalyzes the committed step. CTP inhibits and ATP stimulates this enzyme. The feedback inhibition of glutamine-PRPP amidotransferase by purine nucleotides is important in regulating their biosynthesis. 

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. 

Initially, the first and last enzymes of the lysine branch of the aspartate pathway illustrated in Fig. 1 were isolated from higher plants. The first enzyme, dihydrodipicolinate synthase (10), is sensitive to inhibition by the pathway product, lysine (Wallsgrove and Mazelis, 1981). Results obtained with extensively purified DHDP synthase from wheat suspension cultures indicate that lysine is a competitive inhibitor with respect to aspartate semialdehyde and a noncompetitive inhibitor with respect to pyruvate (Kumpaisal et ai, 1987). In vivo regulation of this enzyme by feedback inhibition is supported by the isolation of mutants which overproduce lysine and are characterized by a lysine-insensitive DHDP synthase (Negrutiu et ai, 1984). 

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. 

In this scheme, F symbolizes an essential metabolite, such as an amino acid or a nucleotide. In such systems, F, the essential end product, inhibits enzyme 1, xAie first step in the pathway. Therefore, when sufficient F is synthesized, it blocks further synthesis of itself. This phenomenon is called feedback inhibition or feedback regulation. 

In die metabolic pathway to an amino add several steps are involved. Each step is die result of an enzymatic activity. The key enzymatic activity (usually die first enzyme in the synthesis) is regulated by one of its products (usually die end product, eg die amino add). If die concentration of die amino add is too high die enzymatic activity is decreased by interaction of die inhibitor with the regulatory site of die enzyme (allosteric enzyme). This phenomenon is called feedback inhibition. 

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.

Since biosynthesis of IMP consumes glycine, glutamine, tetrahydrofolate derivatives, aspartate, and ATP, it is advantageous to regulate purine biosynthesis. The major determinant of the rate of de novo purine nucleotide biosynthesis is the concentration of PRPP, whose pool size depends on its rates of synthesis, utilization, and degradation. The rate of PRPP synthesis depends on the availabihty of ribose 5-phosphate and on the activity of PRPP synthase, an enzyme sensitive to feedback inhibition by AMP, ADP, GMP, and GDP. 

As the rate-limiting enzyme, tyrosine hydroxylase is regulated in a variety of ways. The most important mechanism involves feedback inhibition by the catecholamines, which compete with the enzyme for the pteridine cofactor. Catecholamines cannot cross the blood-brain barrier hence, in the brain they must be synthesized locally. In certain central nervous system diseases (eg, Parkinson s disease), there is a local deficiency of dopamine synthesis. L-Dopa, the precursor of dopamine, readily crosses the blood-brain barrier and so is an important agent in the treatment of Parkinson s disease. 

Aiming at a computer-based description of cellular metabolism, we briefly summarize some characteristic rate equations associated with competitive and allosteric regulation. Starting with irreversible Michaelis Menten kinetics, the most common types of feedback inhibition are depicted in Fig. 9. Allowing all possible associations between the enzyme and the inhibitor shown in Fig. 9, the total enzyme concentration Er can be expressed as... 

Metabolism is tightly regulated by a number of mechanisms feedback inhibition, compartmentalization, covalent modification of enzymes (e.g., phosphorylation), and hormone action, among others. 

To provide a mechanism for the feedback inhibition of these enzymes, the allosteric model was put forward in 1963. It was proposed that the enzyme that regulates the flux through a pathway has two distinct binding sites, the active site and a separate site to which the regulator binds. This was termed the allosteric site. The word allosteric means different shape , which in the context of this mechanism means a different shape from the substrate. The theory further proposed that when the regnlator binds to the allosteric site, it canses a conformational change in... 

Finally, the activity of key enzymes can be regulated by ligands (substrates, products, coenzymes, or other effectors), which as allosteric effectors do not bind at the active center itself, but at another site in the enzyme, thereby modulating enzyme activity (6 see p. 116). Key enzymes are often inhibited by immediate reaction products, by end products of the reaction chain concerned feedback inhibition), or by metabolites from completely different metabolic pathways. The precursors for a reaction chain can stimulate their own utilization through enzyme activation. 

A form of regulation in a metabolic pathway in which an end product (or even an intermediate in the pathway) binds to and inhibits an enzyme which catalyzes an earlier reaction in that same pathway. For example, consider the metabolic scheme A B C Din which the steps are respectively catalyzed by the enzymes Ei, E2, and E3. Feedback inhibition would be seen when elevated concentrations of D (or C) inhibited Ei. 

The control via activation or inhibition of the rate(s) of an enzyme-catalyzed reaction(s). This control includes the increase or decrease in the stability or half-life of the enzyme(s). There are many different means by which control can be achieved. These include 1. Substrate availability and reaction conditions (e.g., pH, temperature, ionic strength, lipid interface activation) 2. Magnitude of Vraax sud valucs) 3. Activation (particularly, feedforward activation) 4. Isozyme formation 5. Com-partmentalization and channeling 6. Oligomerization/ polymerization 7. Feedback inhibition and cooperativity (particularly, allosterism and/or hysteresis) 8. Covalent modification and 9. Gene regulation (induction repression)... 

The flow of metabolites through the citric acid cycle is under stringent regulation. Three factors govern the rate of flux through the cycle substrate availability, inhibition by accumulating products, and allosteric feedback inhibition of the enzymes that catalyze early steps in the cycle. 

The most responsive regulation of amino acid synthesis takes place through feedback inhibition of the first reaction in a sequence by the end product of the pathway. This first reaction is usually irreversible and catalyzed by an allosteric enzyme. As an example, Figure 22-21 shows the allosteric regulation of isoleucine synthesis from threonine (detailed in Fig. 22-15). The end product, isoleucine, is an allosteric inhibitor of the first reaction in the sequence. In bacteria, such allosteric modulation of amino acid synthesis occurs as a minute-to-minute response. 

Heterotropic effectors The effector may be different from the substrate, in which case the effect is said to be heterotropic. For example, consider the feedback inhibition shown in Figure 5.17. The enzyme that converts A to B has an allosteric site that binds the end-product, E. If the concentration of E increases (for example, because it is not used as rapidly as it is synthesized), the initial enzyme in the pathway is inhibited. Feedback inhibition provides the cell with a product it needs by regulating the flow of substrate molecules through the pathway that synthesizes that product. [Note Heterotropic effectors are commonly encountered, for example, the glycolytic enzyme phosphofructokinases allosterically inhibited by citrate, which is not a substrate for the enzyme (see p. 97).]... 

The two-step reduction of HMG-CoA to mevalonate (Fig. 22-1, step a)n 15 is highly controlled, a major factor in regulating cholesterol synthesis in the human liver.121617 The N-terminal portion of the 97-kDa 888-residue mammalian FlMG-CoA reductase is thought to be embedded in membranes of the ER, while the C-terminal portion is exposed in the cytoplasm.16 Tire enzyme is sensitive to feedback inhibition by cholesterol (see Section D, 2). The regulatory mechanisms include a phosphorylation-dephosphorylation cycle and control of both the rates of synthesis and of proteolytic degradation of this key en-... 

Many lines of evidence indicate that the first committed step in de novo purine nucleotide biosynthesis, production of 5-phosphoribosylamine by glutamine PRPP amidotransfer-ase, is rate-limiting for the entire sequence. Consequently, regulation of this enzyme is probably the most important factor in control of purine synthesis de novo (fig. 23.24). The enzyme is inhibited by purine-5 -nucleotides, but the most inhibitory nucleotides vary with the source of the enzyme. Inhibition constants (A, ) are usually in the range 10-3-10-5 M. The maximum effect of this end-product inhibition is produced by certain combinations of nucleotides (e.g., AMP and GMP) in optimum concentrations and ratios, indicating two kinds of inhibitor binding sites. This is an example of a concerted feedback inhibition. 

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