Apparent allosteric constant

According to the MWC model, in the presence of inhibitor and activator at normalized concentrations P and y an enzyme will still follow Eq. 9-65, but the allosteric constant L will be replaced by an apparent allosteric constant L (Eq. 9-70).86 Figure 9-14 shows plots of Y vs. log a for two different values of L for a tetramer with a specific value assumed for c. In both... 

Figure 9-14 Fractional saturation Y and "function of state" R for hypothetical tetrameric enzymes following the MWC model. Curves are calculated for two different values of the apparent allosteric constant L (Eq. 9-70) and for c = 0.1 (Eq. 9-66). After Rubin and Changeux.86...

The expression is identical to that for the homotropic interactions, except that L is replaced by an apparent allosteric constant L, defined as ... 

This time, an apparent allosteric constant L is a complex function ... 

In the absence of chloroquine, the apparent torsion constants for supercoiled pBR322 DNAs with normal (allosteric transitions in secondary structure, so the secondary structures of supercoiled and linear DNAs might not be identical, despite their similar torsion constants. 

Formally, the extended ternary complex model is a two-state model different agonists apparently produce cellular response by causing (quantitatively different) enrichment of the (qualitatively identical) R active receptor conformation. However, upon more careful examination it becomes clear that the allosteric constants a and/or (3 can theoretically be specific for each ligand [42], Under these circumstances the ternary complex... 

In the above equations, a, /3 and y denote the (dimensionless) normalized concentrations of S, Pj and P2, respectively and L2 are the allosteric constants of enzymes Ej and Ej, while o-j and 02 are the normalized maximum rates (in s ) of these two enzymes v and (both in s ) denote the substrate injection rate and the apparent first-order rate constant for the removal of the final product in a reaction catalysed by a Michaelian enzyme far from saturation by its substrate. 

In these equations, v and a denote, respectively, the constant rate of synthesis of ATP and the maximum velocity of adenylate cyclase, both divided by the Michaelis constant of the substrate for the active form of the enzyme a represents the concentration of ATP divided by )8 and y are the concentrations of intracellular and extracellular cAMP, divided by the dissociation constant Kf of extracellular cAMP for the receptor (a, jS and y are thus dimensionless) q = KJK -, L is the allosteric constant of the receptor-enzyme complex /c, and k denote, respectively, the apparent first-order kinetic constants for the transport of cAMP into the extracellular medium (Dinauer, MacKay ... 

The E. coli enzyme is subject to allosteric regulation it is activated by by dCDP and dCTP, and inhibited by dTTP, but not by dTDP or thy-midylate. In the presence of dCDP, the apparent Michaelis constant for thymidine decreases and the Fm increases the sigmoidal nature of the rate-... 

An allosteric situation where is constant but the apparent changes in response to effectors is termed a V system. In a V system, all v versus S plots are hyperbolic rather than sigmoid (Figure 15.12). The positive heterotropic effector A activates by raising whereas 1, the negative heterotropic effec-... 

Rice endosperm ADPGlc PPase has been purified to apparent homogeneity (43 U/ mg).153 However, electrophoretic analyses detected multiple isoforms, and no kinetic characterization was reported. However another report shows that the purified endosperm ADPGlc PPase is activated by 3PGA (40-fold) and inhibited by Pi.81 The allosteric kinetic constants are given in Table 4.1. 

Table 4 shows the effects of thioredoxin reduction on the kinetic constants of the allosteric effectors and the substrates. The activator constant has a twofold higher apparent affinity (twofold lower 5) for the reduced enzyme form. The 3-PGA activation curve of the oxidized enzyme was slightly sigmoidal (Hill n= 1.5) while the reduced enzyme activation curve for 3-PGA was essentially hyperbolic (Hill =0.8). Thus the activity at... 

Fig. 2.11. Allosteric model for glycolytic oscillations. The enzyme is formed by n subunits existing in the states R and T. The substrate (S), injected at a constant rate, binds to the two forms of the enzyme with different affinities. The complexes thus formed in the two states decompose with different rates to yield the product (P). The latter binds in an exclusive manner to the the most active, R, form of the enzyme, and disappears from the reaction medium in an apparent first-order reaction (Goldbeter Lefever, 1972 Venieratos Goldbeter, 1979 Goldbeter, 1980).

This two-variable system (Goldbeter et al, 1978) presents the additional advantage of being formally identical with the system of eqns (2.7) studied in chapter 2 for glycolytic oscillations. This similarity stems from the basic structure common to the two models a substrate, injected at a constant rate, is transformed in a reaction catalysed by an allosteric enzyme activated by the reaction product. In the cAMP-synthesizing system in D. discoideum, activation is indirect as extracellular cAMP enhances the synthesis of intracellular cAMP, which is then transported into the extracellular medium. However, the hypothesis of a quasi-steady state for intracellular cAMP is tantamount to considering that the variation of )8 is so fast that the enzyme is, de facto, activated directly by its apparent product, extracellular cAMP. 

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