Positive effector

The reaction involves biotin as a carrier of activated HCO3 (Fig. 14-18). The reaction mechanism is shown in Figure 16-16. Pyruvate carboxylase is the first regulatory enzyme in the gluconeogenic pathway, requiring acetyl-CoA as a positive effector. (Acetyl-CoA is produced by fatty acid oxidation (Chapter 17), and its accumulation signals the availability of fatty acids as fuel.) As we shall see in Chapter 16 (see Fig. 16-15), the pyruvate carboxylase reaction can replenish intermediates in another central metabolic pathway, the citric acid cycle. 

Allosteric enzymes are regulated by molecules called effectors (also modifiers) that bind noncovalently at a site other than the active site. These enzymes are composed of multiple subunits, and the regula tory site that binds the effector may be located on a subunit that is not itself catalytic. The presence of an allosteric effector can alter the affinity of the enzyme for its substrate, or modify the maximal cat alytic activity of the enzyme, or both. Effectors that inhibit enzyme activity are termed negative effectors, whereas those that increase enzyme activity are called positive effectors. Allosteric enzymes usually contain multiple subunits, and frequently catalyze the commit ted step early in a pathway. 

Homotropic effectors When the substrate itself serves as an effector, the effect is said to be homotropic. Most often, an allosteric substrate functions as a positive effector. In such a case, the presence of a substrate molecule at one site on the enzyme enhances the catalytic properties of the other substrate... 

Acting to counteract any drop in ATP level, accumulating ADP acts as a positive effector for isocitrate dehydrogenases. 

The relative contribution, in intact microsomal preparations, of the two monooxygenases to the formation of N-oxygenated C=N functionalities has been frequently assessed by measurements in the presence and absence of selective enzyme inhibitors or positive effectors. These observations were supplemented by studies using highly purified native or recombinant proteins in reconstituted systems. 

Johnson EF, Schwab GE, Vickery LE. Positive effectors of the binding of an active site-directed amino steroid to rabbit cytochrome P-450 3c. J Biol Chem 1988 263 (33) 17672-17677. 

The control of ribonucleotide reductase activity is affected in the classic feedback fashion by cellular nucleotide concentrations. dATP inhibits the reduction of all four ribonucleoside diphosphates. dTTP inhibits the reduction of only CDP and UDP. ATP is the positive effector for the reduction of these two nucleotides, and both dTTP and dGTP stimulate the reduction of GDP and ADP. Hydroxyurea, an antitumor agent, inhibits ribonucleotide reductase, and this depletes the deoxyribonucleotide supply required for tumor DNA biosynthesis. 

PC requires biotin for activity. Biotin is bound to the enzyme via a peptide-like linkage involving e-NH2 groups of certain lysine residues. This type of biotin complex is biocytin (see Chapter 6). Another compound necessary for PC activity is acetyl-CoA, a positive effector. PC is activated as cellular levels of acetyl-CoA increase, as when extensive lipolysis takes place. Acetyl-CoA is produced in large amounts from fatty acids via the /8-oxidation reaction (see Chapter 19). PC can also be considered an anaplerotic reaction, those reactions that replenish crucial intermediates for metabolic pathways. In this case, oxaloacetate, an important intermediate in the Krebs cycle, is replenished by a reaction catalyzed by PC. 

Cellular levels of fructose-2,6-biphosphate, the positive effector of PFK I, are increased when blood insulin levels are high because PFK II is active as a result of a suppression of fructose-2,6-biphosphate phosphatase (dephosphorylation) and activation (dephosphorylation) of PFK II. 

Acetyl-CoA is a positive effector of pyruvate carboxylase, a biotin enzyme. The correct answer is 1,3-diphosphoglyceric acid. It is produced by the oxidation of glyceraldehyde-3-phosphate in the presence of P,. 

The second example is the biosynthesis of antibiotics. The ppGpp is a positive effector in the synthesis of antibiotics in Streptomyces. The disruption of the ppGpp synthetase relA gene of Streptomyces coelicolor (Chakraburty and Bibb, 1997) and Streptomyces antibioti-cus (Hoyt and Jones, 1999) gives phenotypes unable to produce antibiotics. The disruptants were unable to accumulate ppGpp to the level sufficient for initiation of morphological differentiation and antibiotics production. 

Aspartate carbamoyltransferase catalyzes the formation of carbamoyl aspartate from carbamoyl phosphate and aspartate in the first committed step of pyrimidine biosynthesis (Chap. 15). The enzyme from the bacterium E. coli (Mr = 310,000) consists of 12 subunits, six regulatory and six catalytic. CTP is a negative effector i.e., it inhibits the enzyme, and does so through binding to the regulatory subunits. ATP is a positive effector that acts through the regulatory subunits, while succinate inhibits the reaction by direct competition with aspartate at the active site (see Chap. 9 for more on effectors). 

Positive (+) effectors or activators. These shift the binding constant so the binding takes place at lower concentrations, or increases the rate of an enzyme at a given concentration. Positive (+) effectors decrease cooperativity. 

It can also be added binding of effectors to this model positive effectors bind to R (circles) and shift equilibrium to right, negative effectors bind to T (squares) and shift equilibrium to left... 

The reaction is nonequilibrium in type and has a AG of —5.0 kcal/mol (—20.9 kJ/mol). Although in mitochondria both NAD+- and NADP+-linked enzymes are involved in the cycle, the NAD+-linked enzyme, which is also under allosteric regulation, is the more predominant. Positive effectors are ADP and Ca +, and negative effectors are ATP and NADH. Thus, under conditions of abundance of energy the enzyme is inhibited, and under conditions of low energy the enzyme is stimulated. 

Regulation of liver 6-phosphofructokina.se and fructose-1,6-bisphosphatase. These multimodulated enzymes catalyze nonequilibrium reactions, the former in glycolysis and the latter in gluconeogenesis. Note the dual action of fructose-2,6-bisphosphate (F-2.6-BP), which activates phosphofnictokinase (PFK-1) and inactivates fructose-1,6-bisphosphatase. The activity of F-2.6-BP is under hormonal and substrate regulation (Figure 15-6). = positive effectors 0 = negative effectors. 

Carbamoyl phosphate synthesis requires amino acid acetyltransferase (N-acetylglutamate synthase, mitochondrial) and carbamoyl-phosphate synthase I (CPSI). N-Acetylglutamate (NAG) is an obligatory positive effector of CPSI. NAG synthase is under positive allosteric modulation by arginine and product inhibition by NAG. Depletion of CoA-SH decreases NAG synthesis and ureage-nesis. This situation can occur in organic acidemias (e.g., propionic acidemia Chapter 18), in which organic acids produced in excess compete for CoA-SH for formation... 

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