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Allosteric enzymes and enzymes that exhibit cooperative effects do not display conventional Michaelis-Menten kinetics.
Figure 8-25 compares the Michaelis-Menten curves for enzymes exhibiting noncooperativity, positive cooperativity, and negative cooperativity.
The curve in Figure 8-25a shows the normal hyperbolic binding pattern exhibited by most enzymes.
In this example, an 81-fold increase in substrate concentration is required to elevate enzyme activity from 10 to 90% of its maximum level. The curve in Figure 8-25b depicts the sigmoid pattern characteristic of positive cooperativity. Here, only a nine fold increase in substrate concentration elevates enzyme activity from 10 to 90% of maximum. Note the resemblance of this curve to that for hemoglobin oxygenation.
Negative cooperativity produces the curve shown in Figure 8-25c. Although the curve appears hyperbolic, it actually is not. An increase in substrate concentration greater than 6000-fold would be required to elevate the enzyme activity from 10 to 90% of maximum. The curves have been drawn so that a substrate concentration of 1 unit corresponds to 50% of maximum enzyme activity.
Enzymes may exhibit cooperativity and also be affected by the binding of effectors to regulatory sites. ATCase, the enzyme discussed above, is a good example; it is negatively affected by CTP, positively affected by ATP, and shows positive cooperativity among its substrate binding sites. Figure 8-26 shows the effects of positive and negative effectors on ATCase.
