The MCAT contains LOTS of material that can often feel quite overwhelming. With this mountain before you, it can feel like the best thing to do is to memorize as many facts as possible to simply regurgitate on test day. I’m here to tell you: this isn’t your only option!
When I was studying for the MCAT, I found that rote memorization of material wasn’t helpful for me when trying to efficiently move through problems. Instead, an intuitive understanding of the material was instrumental in my capacity to quickly solve problems. After practicing how to derive facts on my own, I was able to answer problems without depending on memorization.
Let’s use enzyme inhibitor kinetics as an example of how to gain an intuitive understanding of concepts that appear on the MCAT. The MCAT tests you on the effect of inhibitors on enzyme kinetics. For each type of inhibition, you need to know the effect on K_{M} (the Michaelis-Menten constant) and V_{max} (the maximum rate of catalysis). Here is the information you need to know regarding enzyme inhibition:
Type of inhibition |
Effect on K_{M} |
Effect on V_{max} |
Competitive |
Increases |
Stays the same |
Uncompetitive |
Decreases |
Decreases |
Noncompetitive |
Stays the same |
Decreases |
Mixed |
Increases or decreases |
Decreases |
You could simply memorize this information, but we’ll go through how to think about it at a more basic level to avoid that memorization.
First, a review of the different types of inhibition. When an enzyme binds with a substrate, there are three different states:
- Enzyme and substrate are separate
- Enzyme and substrate are bound together
- Enzyme and product are separate
This can be summed up as:
1 2 3
E + S <—> ES <—> E + P
where E is the enzyme, S is the substrate, and P is the product.
In competitive inhibition, the inhibitor competes with the substrate to bind with the enzyme in state 1, making it harder to get to state 2.
In uncompetitive inhibition, the inhibitor competes with the enzyme-substrate complex in state 2, making it harder to get to state 3.
Mixed inhibition is like a combination of competitive and uncompetitive: the inhibitor acts on both states 1 and 2. Noncompetitive inhibition is a special case of mixed inhibition.
V_{max} is the maximum rate of catalysis. When determining V_{max}, we can assume that the concentration of substrate is high, and thus state 2 is the important state to consider. In uncompetitive, noncompetitive, and mixed inhibition, the inhibitor acts on state 2, decreasing [ES] even when [S] is high. Thus, V_{max} decreases for these types of inhibition. In competitive inhibition, the inhibitor only acts on state 1, not state 2. With enough substrate, the same [ES] can be achieved (this is an aspect of competitive inhibition), and thus V_{max} stays the same.
K_{M} is thought of as a measure of the amount of substrate needed to get an enzyme to act at its maximum capacity. Thus, to reason about K_{M}, we need to consider how the inhibitor is influencing the equilibrium between states 1 and 2. On the one hand, if the inhibitor causes the equilibrium to shift to the left (favoring state 1), then more substrate is needed to shift the equilibrium back to the right such that there is the same [ES] as without an inhibitor. Since more substrate is required for the same [ES] (a marker of maximum capacity), then K_{M} increases. This is the case with competitive inhibition, since the inhibitor is competing with the substrate in state 1.
On the other hand, if the inhibitor causes the equilibrium to shift to the right (favoring state 2), when we remove substrate, the equilibrium will shift to the left such that there is the same [ES] as without an inhibitor. Thus, in this case, K_{M} decreases. This is the case with uncompetitive inhibition, since the enzyme competes with the enzyme-substrate complex in state 2. In mixed inhibition, the inhibitor is competing with states 1 and 2, so in some cases the equilibrium will shift to the left, while in other cases, it will shift to the right. Thus, K_{M} can increase or decrease. Noncompetitive is a special case of mixed inhibition such that K_{M} stays the same (for this one, you may need to simply memorize the fact).
I hope this helped you gain a more intuitive understanding of enzyme inhibition kinetics so that test day is just a little bit less stressful!
Note: thanks to Berg et al.’s Biochemistry, Eighth Edition textbook, as well as Dr. Meyer’s slides in Dixie State University’s Chemistry 3510 course for giving me the tools to gain an intuitive understanding of this material.
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