Homotropic enzymes

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. 

The V versus [S] plot for a homotropic enzyme is shown in Figure 7-1. The following features should be noted ... 

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 symmetry model (fig. 2.4) of allostery can describe the cooperative binding of substrate to enzyme (homotropic effect), as well as the influence of effector molecules on the activity of enzymes (heterotropic effect). 

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... 

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... 

The enzyme from B. stearothermophilus is an a4 tetramer of subunit Mr 33 900. Early kinetic studies indicated that the enzyme acts in a manner that is qualitatively consistent with an MWC two-state model. The enzyme acts as a A system i.e., both states have the same value of kcal but different affinities for the principle substrate. In the absence of ligands, the enzyme exists in the T state that binds fructose 6-phosphate more poorly than does the R state. In the absence of ADP, the binding of fructose 6-phosphate is highly cooperative, and h = 3.8. The positive homotropic interactions are lowered on the addition of the allosteric effector ADP, with h dropping to 1.4 at 0.8-mM ADP.52 ADP thus binds preferentially to the R state. The allosteric inhibitor phosphoenolpyruvate binds preferentially to the T... 

Fushinobu, S., Ohta, T., and Matsuzawa, H. (1998). Homotropic activation via the subunit interaction and allosteric symmetry revealed on analysis of hybrid enzymes of L-lactate dehydrogenase./. Biol. Chem., 273, 2971-2976. 

Figures 5.11A and B show the characteristic sigmoid curves obtained as a result of interaction between the substrate site, the stimulating modulator site, and the inhibitory modulator site for K and M classes of enzymes. The curves in the absence of modulators are homotropic effects caused by cooperative binding of the substrate. Heterotropic modulators, which can be either positive or negative in their actions, are molecules other than the substrate.

If an effector of an enzyme is also the substrate, it is called a homotropic effector if it is a nonsubstrate, it is called heterotropic. 

The MWC model of the previous section accounts only for homotropic effects, as occur, for example, with 02 binding to hemoglobin, but heterotropic effects are frequently observed with regulatory enzymes. To incorporate these effects into the MWC model, we must introduce the binding... 

Relationship between the initial velocity (v) and the substrate concentration [S] for an allosteric enzyme that shows a homotropic effect. The substrate functions as a positive modulator. The profile is sigmoidal, and during the steep part of the profile, small changes in [S] can cause large changes in v. Ko.i represents the substrate concentration corresponding to half-maximal velocity. 

The allosteric kinetic effects of ATCase are shown in Figure 7-6. The interaction of substrates with the enzyme is cooperative (an example of homotropic cooperativity), as indicated by the sigmoidal shapes of the v versus [S] plots, CTP being an inhibitor and ATP an activator. These modulators compete for the same regulatory site and modulate the affinity of the enzyme for its... 

A number of multimeric enzymes do not obey the classic Michaelis-Menten kinetics, since the value of their kinetic properties, and kcat, depend on the specific binding of small molecules called effectors. Such regulatory enzymes have, in addition to the catalytic sites, regulatory sites which bind effectors and alter so the properties of the catalytic site. If these effectors are the same as S, they are called homotropic or allosteric (Monod et al., 1963). 

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