Allosteric enzymes positive allosterism

Regulatory or allosteric enzymes like enzyme 1 are, in some instances, regulated by activation. That is, whereas some effector molecules such as F exert negative effects on enzyme activity, other effectors show stimulatory, or positive, influences on activity. 

To describe the all-or-none transition between distinct conformational states of enzymes or receptors — an allosteric transition. In keeping with this usage, the constant that describes the position of the equilibrium between the states (e.g., E0 in the schemes of Figures 1.11 and 1.28) is sometimes described as the allosteric constant. 

To account for positive cooperativity and sigmoidal rate equations, a number of theoretical models for allosteric regulation have been developed. Common to most models is the assumption (and requirement) that enzymes act as multimers and exhibit interactions between the units. We briefly mention the most... 

When binding of a substrate molecule at an enzyme active site promotes substrate binding at other sites, this is called positive homotropic behavior (one of the allosteric interactions). When this co-operative phenomenon is caused by a compound other than the substrate, the behavior is designated as a positive heterotropic response. Equation (6) explains some of the profile of rate constant vs. detergent concentration. Thus, Piszkiewicz claims that micelle-catalyzed reactions can be conceived as models of allosteric enzymes. A major factor which causes the different kinetic behavior [i.e. (4) vs. (5)] will be the hydrophobic nature of substrate. If a substrate molecule does not perturb the micellar structure extensively, the classical formulation of (4) is derived. On the other hand, the allosteric kinetics of (5) will be found if a hydrophobic substrate molecule can induce micellization. 

The fact that ATP and CTP bind to the same site follows from the observation that adding ATP to the inhibited enzyme by CTP reduces or reverses the inhibition, presumably because ATP competes with CTP for the same site. The fact that CTP binds to an allosteric site (i.e., it is not a competitive inhibitor) follows from the so-called desensitization effect. Addition of mercurials [e.g., p-mercuribenzoate (PMB)] reduces or eliminates the inhibition by CTP. However, it has no effect on the enzymatic activity of ATCase, presumably because the mercurials affect the regulatory subunits but not the catalytic site. As for the mechanism of cooperativity (both positive and negative), it is known that CTP does induce changes in the quaternary structure of the enzyme. 

Enzymes that are subject to allosteric regulation by either positive or negative effectors exhibit cooperativity. 

Goldbeter, A. and Nicolis, G. (1976). An allosteric enzyme model with positive feedback applied to glycolytic oscillations. J. Theor. Biol., 4, 65-160. 

Homotropic allosteric enzymes generally are multisubunit proteins and, as noted earlier, the same binding site on each subunit functions as both the active site and the regulatory site. Most commonly, the substrate acts as a positive modulator (an activator), because the subunits act cooperatively the binding of one molecule... 

FIGURE 6-29 Substrate-activity curves for representative allosteric enzymes. Three examples of complex responses of allosteric enzymes to their modulators, (a) The sigmoid curve of a homotropic enzyme, in which the substrate also serves as a positive (stimulatory) modulator, or activator. Note the resemblance to the oxygen-saturation curve of hemoglobin (see Fig. 5-12). (b) The effects of a positive modulator (+) and a negative modulator (—) on an allosteric enzyme in which K0 5 is altered without a change in Zmax. The central curve shows the substrate-activity relationship without a modulator, (c) A less common type of modulation, in which Vmax is altered and /C0.sis nearly constant. 

When fructose 2,6-bisphosphate binds to its allosteric site on PFK-1, it increases that enzyme s affinity for its substrate, fructose 6-phosphate, and reduces its affinity for the allosteric inhibitors ATP and citrate. At the physiological concentrations of its substrates ATP and fructose 6-phosphate and of its other positive and negative effectors (ATP, AMP, citrate), PFK-1 is virtually inactive in the absence of fructose 2,6-bisphosphate. Fructose 2,6-bisphosphate activates PFK-1 and stimulates glycolysis in liver and, at the same time, inhibits FBPase-1, thereby slowing gluconeogenesis. 

Pyruvate carboxylase is a regulatory enzyme and is virtually inactive in the absence of acetyl-CoA, its positive allosteric modulator. Whenever acetyl-CoA, the fuel for the citric acid cycle, is present in excess, it stimulates the pyruvate carboxylase reaction to produce more oxaloacetate, enabling the cycle to use more acetyl-CoA in the citrate synthase reaction. 

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... 

Carbamoyl phosphate synthetase I produces carbamoyl phosphate in the mitochondria from CO2, NH3, and two ATP molecules. The enzyme, which has an absolute requirement for its positive allosteric effector, N-acetylglutamate, is the rate-limiting step in the cycle. 

Feedback can also be positive. Since AMP is a product of the hydrolysis of ATP, its accumulation is a signal to activate key enzymes in metabolic pathways that generate ATP. For example, AMP causes allosteric activation of glycogen phosphorylase, which catalyzes the first step in the catabolism of glycogen. As is shown in Fig. 11-5, the allosteric site for AMP or IMP binding is more than 3 nm away from the active site. Only a... 

Figure 10.4 The abolition of positive cooperativity on the binding of allosteric effectors to some enzymes. Note the dramatic increases in activity at low substrate concentrations on the addition of adenosine monophosphate to isocitrate dehydrogenase, of deoxycytosine diphosphate to deoxythymidine kinase, and of fructose 1,6-diphosphate to pyruvate kinase this shows how the activity may be switched on by an allosteric effector (PEP = phosphoenolpyruvate). [From J. A. Hathaway and D. E. Atkinson, J. Biol. Chem. 238,2875 (1963) R. Okazaki and A. Kornbcrg, J. Biol. Chem. 239,275 (1964) R. Haeckel, B. Hess, W. Lauterhom, and K.-H. Wurster, Hoppe-Seyler s Z. Physiol. Chem. 349, 699 (1968).]...

Control in allosteric enzymes may take two extreme forms.4 In K (= binding) systems, the ones discussed so far, the substrate and the effector molecules have different affinities for the R and T states. The binding of an effector alters the affinity of the enzyme for the substrate, and vice versa. The R and the T states can have the same intrinsic value of cat, and activity is modulated by changes in affinity for the substrate. In V (= rate) systems, the substrate has the same affinity for both states, but one state has a much higher value of kCM. The effector molecule binds preferentially to one of the two states, and so modulates activity by changing the equilibrium position between the two. As the substrate binds... 

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