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.

Michaelis-Menten CurveIn 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.Effects of CTP and ATP on the Michells-Menten curve of the positively cooperative enzyme asparate transcarbamylase

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