Allosteric, defined

FIGURE 15.11 Heterotropic allosteric effects A and I binding to R and T, respectively. The linked equilibria lead to changes in the relative amounts of R and T and, therefore, shifts in the substrate saturation curve. This behavior, depicted by the graph, defines an allosteric K system. The parameters of such a system are (1) S and A (or I) have different affinities for R and T and (2) A (or I) modifies the apparent for S by shifting the relative R versus T population. 

The allosteric model just presented is called a K system because the concentration of substrate giving half-maximal velocity, defined as -K0 5, changes in response to effectors (Figure 15.11). Note that Vjnax is constant in this system. 

Thermodynamically it would be expected that a ligand may not have identical affinity for both receptor conformations. This was an assumption in early formulations of conformational selection. For example, differential affinity for protein conformations was proposed for oxygen binding to hemoglobin [17] and for choline derivatives and nicotinic receptors [18]. Furthermore, assume that these conformations exist in an equilibrium defined by an allosteric constant L (defined as [Ra]/[R-i]) and that a ligand [A] has affinity for both conformations defined by equilibrium association constants Ka and aKa, respectively, for the inactive and active states ... 

The concentration of allosteric antagonist [B] that reduces a signal from a bound amount [A ] of radioligand by 50% is defined as the IC50 ... 

AR] complex) interacts with an equilibrium association constant Ke (to yield an efficacy term x) and the allosterically altered agonist-bound receptor complex ([ABR] complex) interacts with the cell with equilibrium association constant K e (to yield an altered efficacy x7). It is useful to define a ratio of efficacies for the native and allosterically modulated receptor of x /x (denoted , where = x//x). 

Equation 7.6 defines the allosteric noncompetitive antagonism of receptor function and predicts insurmountable effects on agonist maximal response (i.e., as [A] oo) the expression for maximal response is... 

In cases where the plot of [A ]/Kd vs ICS0 is not linear, other mechanisms of antagonism may be operative. If there is a nearly vertical relationship, this be due to noncompetitive antagonism in a system with no receptor reserve (see Figure 12.2d). Alternatively, if the plot is linear at low values of [A ]/Kd and then approaches an asymptotic value the antagonism may be allosteric (the value of a defines the value of the asymptote) or noncompetitive in a system with receptor reserve (competitive shift until the maximal response is depressed, Figure 12.2d). 

Uncompetitive antagonism, form of inhibition (originally defined for enzyme kinetics) in which both the maximal asymptotic value of the response and the equilibrium dissociation constant of the activator (i.e., agonist) are reduced by the antagonist. This differs from noncompetitive antagonism where the affinity of the receptor for the activating drug is not altered. Uncompetitive effects can occur due to allosteric modulation of receptor activity by an allosteric modulator (see Chapter 6.4). 

Has a defined maximum effect that is determined by the cooperativity associated with its allosterism. 

The simplest model that can describe allosteric interactions at GPCRs is the ternary complex allosteric model [9], As shown in Figure 1, according to this model two parameters define the actions of allosteric agent (X) its affinity for the unoccupied receptor (Kx) and its cooperativity (a) with the ligand (A) that interacts at the primary binding site a < 1 represents negative cooperativity a = 1, no cooperativity a > 1, positive cooperativity. 

The first and best-studied allosteric site on GPCRs is that on the muscarinic receptor [9,10,12,19,20]. For the five subtypes of these receptors that have been cloned and pharmacologically defined as Mi to M5, various agents have been identified that allosterically regulate selectively these... 

A simplified scheme, in which only one desensitized and one open-channel state of the receptor exist, is represented in scheme 2, where R is the resting (activat-able) state, R the active (open channel) state and R the desensitized state of the receptor M is an allosteric constant defined by R7R, and K and fC are equilibrium dissociation constants for the ligand. 

Figure 18. A simple bistable pathway [96], Left panel The metabolite A is synthesized with a constant rate vi and consumed with a rate vcon V2(A) + V3(A), with the substrate A inhibiting the rate V3 at high concentrations (allosteric regulation). Right panel The rates of vsyn vi const. and vcon V2 (A) + V3(A) as a function of the concentration A. See text for explicit equations. The steady state is defined by the intersection of synthesizing and consuming reactions. For low and high influx v, corresponding to the dashed lines, a unique steady state A0 exists. For intermediate influx (solid line), the pathway gives rise to three possible solutions of A0. The rate equations are specified in Eq. 67, with parameters 0.2, 3 2.0, Kj 1.0, and n 4 (in arbitrary units).

To investigate these two questions, a parametric model of the Jacobian of human erythrocytes was constructed, based on the earlier explicit kinetic model of Schuster and Holzhiitter [119]. The model consists of 30 metabolites and 31 reactions, thus representing a metabolic network of reasonable complexity. Parameters and intervals were defined as described in Section VIII, with approximately 90 saturation parameters encoding the (unknown) dependencies on substrates and products and 10 additional saturation parameters encoding the (unknown) allosteric regulation. The metabolic state is described by the concentration and fluxes given in Ref. [119] for standard conditions and is consistent with thermodynamic constraints. 

Citrate synthase There are three properties of citrate synthase that are relevant to regnlation. The prodnct of the reaction, citrate, is an allosteric inhibitor of the enzyme. The concentration of acetyl-CoA in mnscle is weU above the ATm of citrate synthase for acetyl-CoA. Conseqnently, the activity of this enzyme is flnx-generating for the cycle pins the transfer of electrons along the electron transfer chain, i.e. the process from acetyl-CoA to molecnlar oxygen can be considered as a transmission seqnence , as defined in Chapter 3. In contrast, the concentration of oxaloacetate is weU below the ATm, so that variations in its concentration can regnlate the enzyme activity and therefore, the flnx throngh the cycle. 

The topologically defined region(s) on an enzyme responsible for the binding of substrate(s), coenzymes, metal ions, and protons that directly participate in the chemical transformation catalyzed by an enzyme, ribo-zyme, or catalytic antibody. Active sites need not be part of the same protein subunit, and covalently bound intermediates may interact with several regions on different subunits of a multisubunit enzyme complex. See Lambda (A) Isomers of Metal Ion-Nucleotide Complexes Lock and Key Model of Enzyme Action Low-Barrier Hydrogen Bonds Role in Catalysis Yaga-Ozav /a Plot Yonetani-Theorell Plot Induced-Fit Model Allosteric Interaction... 

Noncompetitive inhibition cannot be completely reversed by very high substrate concentrations. Monod et al. defined for an allosteric enzyme a function of state R (Eq. 9-71) which is the fraction of total enzyme in the R (B) conformation ... 

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