Allosteric enzymes behavior

The two principal models for allosteric enzyme behavior are called the concerted model and the sequential model. 

The group of Phil Evans, MRC Laboratory of Molecular Biology, Cambridge, UK, has determined x-ray structures of bacterial PFK both in the R and the T states. These studies have confirmed the above conclusions and given insight into how an allosteric enzyme accomplishes its complex behavior. 

Because this enzyme catalyzes the committed step in fatty acid biosynthesis, it is carefully regulated. Palmitoyl-CoA, the final product of fatty acid biosynthesis, shifts the equilibrium toward the inactive protomers, whereas citrate, an important allosteric activator of this enzyme, shifts the equilibrium toward the active polymeric form of the enzyme. Acetyl-CoA carboxylase shows the kinetic behavior of a Monod-Wyman-Changeux V-system allosteric enzyme (Chapter 15). 

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. 

When an enzyme reacts to effectors (substrates, activators, or inhibitors) with conformational changes that increase or reduce its activity, it is said to show allosteric behavior (see p. 116). Allosteric enzymes are usually oligomers with several subunits that mutually influence each other. 

B. i. Kurganov (1982) Allosteric Enzymes Kinetic Behavior, Wiley,... 

The Kinetic Properties of Allosteric Enzymes Diverge from Michaelis-Menten Behavior... 

The activity of allosteric enzymes is adjusted by reversible binding of a specific modulator to a regulatory site. Modulators may be the substrate itself or some other metabolite, and the effect of the modulator may be inhibitory or stimulatory. The kinetic behavior of allosteric enzymes reflects cooperative interactions among enzyme subunits. 

Fig. 9-10 Behavior of an MWC allosteric enzyme in the presence of positive and negative heterotropic effectors. The activator term, y, in Eq. (9.62) causes the curve to become more hyperbolic, whereas the inhibitor term (j3) renders it more sigmoidal. The curves were constructed using Eq. (9.62) with L = 1,000 and n - 4.

Allosteric Effectors. Many enzymes are subject to metabolic regulation through interaction with metabolites that often act at allosteric sites, which are distinct from the active site. The kinetic behavior of such enzymes is often more complex than the behavior we have discussed above, and such complex kinetics may serve as an indication that you are dealing with an allosteric enzyme. Further discussion of this subject is found in Experiments 9 and 15. 

We ve been working on the behavior of zinc in enzymes and also what are called allosteric enzymes, that is, enzymes that undergo conformational change when they have different subunits, different proteins, to make a... 

Segel, 1. H., Emyme Kinetics Behavior and Analysis of Rapid Equilibrium and Steady-State Enzyme Systems. Wiley-Interscience (1975). This book starts at the same elementary level as Biochemical Calculations and progresses to the modern subjects of steady-state kinetics of mullireac-tant enzymes, allosteric enzymes, isotope exchange, and membrane transport. 

Two theoretical models that attempt to explain the behavior of allosteric enzymes are the concerted model and the sequential model. In the concerted (or symmetry) model, it is assumed that the enzyme exists in only two states T(aut) and R(elaxed). Substrates and activators bind more easily to the R conformation, whereas inhibitors favor the T conformation. The term concerted is applied to this model because the conformations of all the protein s protomers are believed to change simultaneously when the first effector binds. (This rapid concerted change in conformation maintains the protein s overall symmmetry.) The binding of an activator shifts the equilibrium in favor of the R form. An inhibitor shifts the equilibrium toward the T conformation. 

We have recently established (B. Belleau, A. Paturet, and M. Saucier, in preparation) that erythrocyte-bound AChE behaves as a genuine allosteric enzyme displaying explosive cooperative behavior which suggests that it probably plays a role in the control of membrane configuration. A phase transition must be first nucleated by appropriate effectors before the esteratic center can accept the substrate ACh. We conclude that the regulatory units of membrane-bound AChE possess the biophysical qualifications for a receptor-like role on excitable membranes. 

Yeast invertase,30 32 33 acid phosphatase,29 32 35 37 39 urease,29 /3-glucosi-dase,29 dCMP-amino hydrolase31 and malic enzyme34 36 have been immobilized in gel form on both flat and capillary membranes. Cellulosic and polyamide polymers have been used as supporting membrane matrices. In all instances, immobilized enzymes behave in a manner almost identical to their behavior in homogenous solution, independent of the nature of the polymer. Neither allosteric nor pseudo-allosteric enzymes, proteins whose kinetic behavior is affected by the presence of particular compounds in the reaction environment (ligands), show different kinetic behavior, as they do when subjected to less gentle immobilization procedures.31 34 36... 

When ATGase catalyzes the condensation of aspartate and carbamoyl phosphate to form carbamoyl aspartate, the graphical representation of the rate as a function of increasing substrate concentration (aspartate) is a sigmoidal curve rather than the hyperbola obtained with nonallosteric enzymes (Figure 7.2a). The sigmoidal curve indicates the cooperative behavior of allosteric enzymes. In this two-substrate reaction, aspartate is the substrate for which the concentration is varied, while the concentration of carbamoyl phosphate is kept constant at high levels. 

The key to allosteric behavior, including cooperativity and modifications of cooperativity, is the existence of multiple forms for the quaternary structures of allosteric proteins. The word allosteric is, derived from alio, other, and stetic, shape, referring to the fact that the possible conformations affect the behavior of the protein. The binding of substrates, inhibitors, and activators changes the quaternary structure of allosteric proteins, and the changes in structure are reflected in the behavior of those proteins. A substance that modifies the quaternary structure, and thus the behavior, of an allosteric protein by binding to it is called an allosteric effector. The term effector can apply to substrates, inhibitors, or activators. Several models for the behavior of allosteric enzymes have been proposed, and it is worthwhile to compare them. 

Allosteric enzymes exhibit different behaviors compared to nonallosteric enzymes, and the Michaelis-Menten equations are not applicable. 

The two principal models for the behavior of allosteric enzymes are the concerted model and the sequential model. They were proposed in 1965 and 1966, respectively, and both are currently used as a basis for interpreting experimental results. The concerted model has the advantage of comparative simplicity, and it describes the behavior of some enzyme systems very well. 

Recall How is the cooperative behavior of allosteric enzymes reflected in a plot of reaction rate against substrate concentration ... 

Recall Does the behavior of allosteric enzymes become more or less cooperative in the presence of inhibitors ... 

Recall Distinguish between the concerted and sequential models for the behavior of allosteric enzymes. 

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