Heterotropic allosteric interaction

The tryptophan synthase bienzyme complex from enteric bacteria provides an important example wherein RSSF has been used to good advantage for the study of both enzyme mechanism and protein structure-function relationships. This enzyme complex is composed of heterologous a- and P2-subunits arranged in a nearly linear a-(3-(l-a array (81). The a-subunit catalyzes the aldolytic cleavage of IGP to indole and G3P, while the P-subunit catalyzes the PLP-dependent condensation of i-Ser and indole to yield i-Trp. The aP-reaction is essentially the sum of the individual a- and P-reactions (scheme I). Indole, the common intermediate produced at the a-site, is direcdy channeled to the P-active site via a tunnel located in the interior of the protein complex which directly interconnects the a- and P-catalytic centers (81-84). Although the individual subunits may be isolated and are functional, formation of the bienzyme complex not only increases the catalytic activities of the separate subunits by nearly 100-fold, but also alters the thermodynamic stability of P-site reaction intermediates and introduces heterotropic allosteric interactions between sites. 

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. 

Heme—chemical models 307 Hemoglobin 289, 304-307 allosteric interactions 289-292, 302 Henderson-Hasselbalch equation 170 Heterotropic 290 Hexokinase 23, 51, 364 Hill constant 299, 300-302, 304 Hill equation 297 - 300 Hinge motions 48 HIV protease 486 Holoe nzyme 458 Homology 8, 9 Homology modeling 537 Homotropic 290 Hpr (histidine-containing... 

According to the concerted model, an allosteric activator shifts the conformational equilibrium of all subunits toward the R state, whereas an allosteric inhibitor shifts it toward the T state. Thus, ATP (an allosteric activator) shifted the equilibrium to the R form, resulting in an absorption change similar to that obtained when substrate is bound. CTP had a different effect. Hence, this allosteric inhibitor shifted the equilibrium to theT form. Thus, the concerted model accounts for the ATP-induced and CTP-induced (heterotropic), as well as for the substrate-induced (homotropic), allosteric interactions of ATGase. 

Let us first define two terms. Homotropic effects are allosteric interactions that occur when several identical molecules are bound to a protein. The binding of substrate molecules to different sites on an enzyme, such as the binding of aspartate to ATGase, is an example of a homotropic effect. Heterotropic effects are allosteric interactions that occur when different substances (such as inhibitor and substrate) are bound to the protein. In the ATGase reaction, inhibition by GTP and activation by ATP are both heterotropic effects. 

Outline the effects of heterotropic and homotropic allosteric interactions on the equilibrium between the T and R forms of ATCase. 

Homotropic allosteric effect appears in the system with identical ligands, and heterotropic allosteric effect is manifested for interaction between different ligands. 

Allosteric behavior of the heterotropic variety is seen in the interaction between polymer and detergent or polymer and polymer (Shirahama, 1974 Arai et al., 1973 Tsuchida and Osada, 1973). Shinkai et al. (1977b) observed a sigmoid profile of rate constant vs. concentration of cationic detergents in the acyl transfer reaction from p-nitrophenyl acetate (PNPA) to copolymers (7). 

Allosteric binding occurs when two molecules bind to different sites on the target. When the two molecules are identical, it is termed homotropic interaction. If the molecules differ from each other, it is termed heterotropic interaction. Binding is competitive when two different ligand molecules compete for the same site. We discuss ligand binding further in Chapter 3. The specificity of ligand-receptor interaction is illustrated in Exhibit 2.9. 

Metabolic activators and inhibitors are structurally dissimilar to substrates. These effectors exert regulatory control over catalysis by binding at an allosteric site quite distinct from the catalytic site. Such heterotropic interactions are mediated through conformational changes, often involving subunit interactions. Allosteric effectors can alter the catalytic rate by changing the apparent substrate affinity (K system) or by altering the... 

Heterotropic transition Heterotropic interactions may be transmitted to the regulatory sites by the tower helices and by changes at the subunit contacts. The simultaneous binding of allosteric activator and allosteric inhibitor gives rise to a structure intermediate between the T and the R structures. The heterotropic effect is exerted throughout the subunit interface contacts. 

Heterotropic interactions with multiple allosteric ligands... 

This equation expresses the second fundamental property of the MWC model, namely, that the heterotropic effect of an allosteric ligand upon the saturation function for another allosteric ligand should be to modify the homotropic interaction of the latter. 

The capacities for cooperativity and allosteric regulation elevate enzymes from the level of simple catalysts to that of the regulators of metabolism. In fact, cooperativity and allosteric regulation share a common mechanism—the alteration of the properties of the catalytic site by binding of a ligand to a second site on the enzyme. Cooperativity may be thought of as the homotropic interaction of identical catalytic sites, and allostery as the heterotropic interaction of a catalytic site and a dissimilar site which binds an allosteric modifier. 

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