How to learn enzyme inhibition without memorizing facts

MCAT MD/PhD admissions

Title_ How to Study Efficiently for Hours On End (With the Help of a Tomato)-Jan-26-2021-08-52-45-81-PMThe 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 KM (the Michaelis-Menten constant) and Vmax (the maximum rate of catalysis). Here is the information you need to know regarding enzyme inhibition:

 

Type of inhibition

Effect on KM

Effect on Vmax

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:

  1. Enzyme and substrate are separate
  2. Enzyme and substrate are bound together
  3. 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.

Vmax is the maximum rate of catalysis. When determining Vmax, 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, Vmax 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 Vmax stays the same.

KM 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 KM, 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 KM 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, KM 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, KM can increase or decrease. Noncompetitive is a special case of mixed inhibition such that KM 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.

Jacob studied Computer Science at Harvard, where he was also on the Nordic Ski Team and served as Men's captain his senior year. After scoring in the 100th percentile on the MCAT, he went on to earn his MD at Harvard Medical School.

Comments

topicTopics
academics study skills MCAT medical school admissions SAT expository writing college admissions English MD/PhD admissions strategy writing LSAT GMAT GRE physics chemistry math biology graduate admissions academic advice ACT interview prep law school admissions test anxiety language learning premed MBA admissions career advice personal statements homework help AP exams creative writing MD study schedules computer science test prep Common Application summer activities history mathematics philosophy organic chemistry secondary applications economics supplements research 1L PSAT admissions coaching grammar law psychology statistics & probability legal studies ESL CARS SSAT covid-19 dental admissions logic games reading comprehension engineering USMLE calculus PhD admissions Spanish mentorship parents Latin biochemistry case coaching verbal reasoning DAT English literature STEM excel medical school political science skills AMCAS French Linguistics MBA coursework Tutoring Approaches academic integrity chinese letters of recommendation Anki DO Social Advocacy admissions advice algebra artificial intelligence astrophysics business cell biology classics diversity statement gap year genetics geometry kinematics linear algebra mechanical engineering mental health presentations quantitative reasoning study abroad technical interviews time management work and activities 2L DMD IB exams ISEE MD/PhD programs Sentence Correction adjusting to college algorithms amino acids analysis essay art history athletics business skills careers cold emails data science dental school finance first generation student functions information sessions international students internships logic networking poetry resume revising science social sciences software engineering tech industry trigonometry writer's block 3L AAMC Academic Interest EMT FlexMed Fourier Series Greek Health Professional Shortage Area Italian Lagrange multipliers London MD vs PhD MMI Montessori National Health Service Corps Pythagorean Theorem Python Shakespeare Step 2 TMDSAS Taylor Series Truss Analysis Zoom acids and bases active learning architecture argumentative writing art art and design schools art portfolios bacteriology bibliographies biomedicine brain teaser campus visits cantonese capacitors capital markets central limit theorem centrifugal force chemical engineering chess chromatography class participation climate change clinical experience community service constitutional law consulting cover letters curriculum dementia demonstrated interest dimensional analysis distance learning econometrics electric engineering electricity and magnetism escape velocity evolution executive function freewriting genomics graphing harmonics health policy history of medicine history of science hybrid vehicles hydrophobic effect ideal gas law immunology induction infinite institutional actions integrated reasoning intermolecular forces intern investing investment banking lab reports linear maps mandarin chinese matrices mba medical physics meiosis microeconomics mitosis mnemonics music music theory nervous system neurology neuroscience object-oriented programming office hours operating systems