Cooperative Versus Allosteric Effects

It is obvious that aU living organisms have a need for a high degree of control over metabohc processes so as to permit orderly change without precipitating unwanted progress towards thermodynamic equihbrium. 

The activity of majority of enzymes is regulated by the concentration of their own substrates in a MichaeUs-Menten fashion. However, such a control may be insufficient for some metabolic purposes. For example, in order to increase the velocity of a simple Michaelian enzyme from o.iV to 0.9V, it is necessary to increase the substrate concentration from Xm/9 to that is, an 81-fold increase. Similarly, an 81-fold increase in inhibitor concentration is required to reduce the velocity from 90% to 10% of the uninhibited value. On one hand, the concentration of metabolites in vivo vary within relatively narrow limits wltile, on the other hand, the activity of specific enzymes must be increased or decreased within very large limits. Consequently, in addition to a simple Michaelian kinetics, nature has a need for additional control mechanisms for the regulation of enzyme activity in vivo. 

many ofthe enzymes at control points in metabolism display the property of responding with exceptional sensitivity to changes in metabolite concentrations, a response that does not obey the usual Michaelis-Menten kinetics. This property is reserved for regulatory enzymes, which are usually found at the beginning, at the end or at a branchpoint of a given metabolic pathway. 

Two types of phenomena are responsible for such regulatory properties of enzymes. First are the cooperative phenomena in proteins and enzymes, and second are the allosteric properties of proteins and enzymes (Kurganov, 1982). Cooperativity is the apparent change in affinity or activity-that deviates from Michaelis-Menten kinetics-of an enzyme or protein with its substrate or other ligand as the concentration of the ligand changes. Cooperativity often requires that the enzyme or protein is built up of interacting subunits, but may also arise 

Lactate dehydrogenase, an enzyme isolated from the beef heart, is an example of atypical Michaelian enzyme. This enzyme is atetramertic oUgomer, composed of four identical subunits, but each subunit binds its substrates independently of other subunits, in a typical noncooperative fashion (Holbrook et al, 1975). 

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FIGURE 17.6 Allosteric effects in phosphofructokinase. At high [ATP], phosphofructokinase behaves cooperatively, and the plot of enzyme activity versus [fructose-6-phosphate] is sigmoidal. High [ATP] thus inhibits PFK, decreasing the enzyme s affinity for fructose-6-phosphate. 

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... 

Fig. 20.13. Allosteric regulation of isocitrate dehydrogenase (ICDH). Isocitrate dehydrogenase has eight subunits, and two active sites. Isocitrate, NAD, and NADH bind in the active site ADP and Ca are activators and bind to separate allosteric sites. A. A graph of velocity versus isocitrate concentration shows positive cooperativity (sigmoid curve) in the absence of ADP. The allosteric activator ADP changes the curve into one closer to a rectangular h5 perbola, and decreases the (S0.5) for isocitrate. B. The allosteric activation by ADP is not an all-or-nothing response. The extent of activation by ADP depends on its concentration. C. Increases in the concentration of product, NADH, decrease the velocity of the enzyme through effects on the allosteric activation.

Figure 8.7. (a) Simulation of the effects of varying the effective number of active sites in an enzyme (w) on the shape of the initial velocity versus substrate concentration curve for a cooperative enzyme, (b) Simulation of the effects of varying the allosteric constant (L) on the shape of the initial velocity versus substrate concentration curve for a cooperative enzyme. 

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