If a problem gives you the Km value and Vmax value for multiple enzymes and asks which enzyme works fastest at X concentration of a reactant... How would you go about solving it? What equation am I supposed to use?
The overall rate of Enzymes are determined by the slow-step (rate determining step), which for MM Kinetics is represented by: rate = k[ES]. But, because [ES] is not something we could quantifyably measure, we have to derive this fancy equation called the Michaelis Menten equation:
Initial Velocity = Vmax(S)/ Km + (S) (given after some assumptions).
This is definitely not an equation you would need to know for the MCAT, but it's possible it was presented in a passage with fairly easy numbers. In this case, to determine the rate at some concentration for a given enzyme, you plug those concentrations into the equation above (along with the other parameters: Vmax and Km) to determine the velocity at that concentration.
If they told you the enzymes were saturated with substrate, such that you had an infinite amount compared to free enzyme, then the equation simplifies down to: Vmax - in this case, you would simply compare which enzyme has the highest Vmax. More than likely though, they gave you concentrations of substrate much sooner before any saturation kinetics are achieved.
