Homotropic effectors

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... 

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. 

Figure 2.12 Plot of F1/2 for Hb as a function of imidazole concentration that illustrates the influence of homotropic effector equilibrium reaction 14 on the ease of reduetion and level of cooperativity (inset). Parameters obtained by spectroelectrochemistry max is defined in Figure 2.4. Conditions Pt mesh electrode [heme] = 0.1-0.23 mM [Ru(NH3)6Cl3] = 0.30-1.1 mM [NaN03] = 200mM [MOPS] = 50 mM at pH 7.1 20 °C. Figure adapted from ref. 11 and used with permission.

In general, the presence of homotropic effectors will stabilise the R-state and shift the Eij2 to lower values i.e. the Hb is more easily oxidised). This is illustrated by the data in Figure 2.12, where the 1/2 and n parameter variations are due to the homotropic effector equilibrium shown in reaction (2.14), where L = imidazole. 

Homotropic effectors enhance the binding of subsequent S molecules, i.e. they exhibit a cooperative S binding, resulting in a sigmoid relationship between v and [S]. 

The separation between allosteric effectors and cooperativity lies in the molecule doing the affecting. If the effector molecule acts at another site and the effector is not the substrate, the effect is deemed allosteric and heterotropic. If the effector molecule is the substrate itself, the effect is called cooperative and/or homotropic. 

The architecture of various CYPs may accommodate entities of different shapes [139]. Several CYPs, particularly CYP3A4 [140,141] and CYP2C9 [142], may exhibit atypical (non-Michaelis-Menten) kinetics such as heterotropic activation, homotropic activation, substrate inhibition and partial inhibition, all in a substrate-effector-dependent manner [143]. Several hypotheses have been proposed to account for the observation of atypical kinetics, including simultaneous occupancy of the CYP active site by two substrates (or one substrate and one effector simultaneously) [144] and allosteric changes in CYP architecture due to binding of an effector [145,146]. Along... 

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). 

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... 

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). 

In this case, the homotropic cooperativity occurs with a monomeric enzyme through substrate binding to both a catalytic and an effector site, and these sites may not be equivalent (Shou et al., 1999). This results in two parameter estimates and two y ax parameter estimates, as discussed below. One of the most commonly used equations describing sigmoidal kinetic profiles is the Hill equation (Eq. 4.5) ... 

Homotropic effect refers to allosteric effects produced by enzyme s own substrate and heterotropic effects are due to metabohtes that are not stmcturaUy related to enzyme s substrates. Positive effects are related to enzyme activation and negative effects to enzyme inhibition. Thus, fmctose-diphosphate is a positive allosteric effector of pymvate kinase. 

The glutamine-dependent CP synthase has been extensively purified in E. coli and shown to be a biotin-containing enzyme with marked allosteric properties [74,75]. Homotropic interaction is evident from the sigmoidal kinetics obtained with ATP as substrate. Negative effectors are UMP, UDP and UTP, and purine nucleotides, particularly IMP, act as positive effectors. 

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