Most enzymes obey the standard Michaelis Menten kinetics scheme that we have already discussed in module 4. The implications of this in the context of a metabolic pathway must now be discussed. Most of these enzymes have evolved for the many millenia to have a Km near the substrate concentration that is "normally observed" in an individual at rest. Enzymes are not generally running at a rate near their maximum but considerably less. As substrate concentrations changes with changes in physiological conditions then the rate of flux through these enzymes can change appropriately.
Let's consider two scenarios.
Shown is this picture is a graph representing the standard Michaelis Menten kinetics. The gray area shows a range of
substrate concentrations that might be observed depending on the physiological state.
The vertical arrow indicates what the concentration of substrate might be
while at rest. As substrate concetration changes the rate of conversion from S to P aslo changes accordingly.
If the concetration of [S] averaged at "0.0015" Molar or so, this would not be the case. The enzyme is already operating near its Vmax additional substrate would not appreciably increase the rate of conversion.
Since the average substrate concentration is ususally near Km we can obtain a rate of conversion nearly proportional to and changes in substrate concentration without any input or alteration of any kind.