Michaelis-Menten kinetics, allosteric effect

Figure 2.23 graphically depicts a plot of I/vq versus l/[Aoj. The values V and Km are obtained from the intercepts of the ordinate (1/V) and of the abscissa (—1 /Km), respectively. If the data do not fit a straight line, then the system deviates from the required steady-state kinetics e. g., there is inhibition by excess substrate or the system is influenced by allosteric effects (cf. 2.5.1.3 allosteric enzymes do not obey Michaelis-Menten kinetics). 

Fig. 2.28. The effect of substrate concentration on the catalytic reaction rate, a Enzyme obeying Michaelis-Menten kinetics b allosterically regulated enzyme with positive cooperativity c allosterically regulated enzyme with negative cooperativity...

Allosteric inhibitors bind to a separate binding site outside the active center (6). This results in a conformational change in the enzyme protein that indirectly reduces its activity (see p. 116). Allosteric effects practically only occur in oligomeric enzymes. The kinetics of this type of system can no longer be described using the simple Micha-elis-Menten model. 

The inhibition of certain enzymes by specific metabolites is an important element in the regulation of intermediary metabolism and most often occurs with cooperative enzymes that are regulated allosterically. Inhibition of enzymes that obey the Michaelis-Menten equation, noncooperative enzymes, is more commonly used by pharmacists to alter a patient s metabolism. Reversible inhibition of noncooperative enzymes is classified into three groups which can be distinguished kinetically and which have different mechanisms and effects when administered. The classes are called competitive, uncompetitive, and noncompetitive inhibition. Mixed inhibition also occurs. In all these types of inhibition, the inhibitor (usually a small molecule) binds reversibly and rapidly with the enzyme. 

---