Final answer:
The initial rate plot of an allosteric enzyme as a function of substrate concentration typically shows a sigmoidal curve, with a slow initial reaction rate increase at low substrate levels, a steeper increase at moderate substrate levels, and a plateau at high substrate levels where the enzyme is saturated.
Step-by-step explanation:
The plot of initial rate as a function of substrate concentration for an allosteric enzyme typically shows a sigmoid (S-shaped) curve rather than the hyperbolic curve observed with Michaelis-Menten kinetics. At low substrate concentrations, the reaction rate increases slowly with substrate addition. As the substrate concentration continues to increase, a more rapid increase in reaction rate is observed until a point of saturation is approached. Beyond this saturation point, the rate plateaus as further increases in substrate concentration do not lead to an increased rate of product (P) formation because the enzyme active sites are fully occupied.
The plot can be understood in several phases: at low substrate concentrations, there is a less-than-proportional increase in the reaction rate as the allosteric sites begin to bind substrate. As substrate concentration continues to increase, the enzyme undergoes a conformational change that leads to a more active state, thereby steepening the slope of the curve. At high substrate concentrations, the enzyme's active sites are saturated, leading to the plateau in the curve where the reaction is proceeding at its maximum rate.
Allosteric enzymes do not follow Michaelis-Menten kinetics due to their multiple active sites and their ability to be regulated by other molecules that bind to different sites on the enzyme, leading to changes in their activity and the characteristic sigmoidal plot.