Enzymes for the MCAT: What You Need to Know

November 13, 2023
8 min read


Reviewed by:

Akhil Katakam

Third-Year Medical Student, Lewis Katz School of Medicine at Temple University


Are you preparing to study enzymes on the MCAT questions and don’t know where to start? Keep reading to find out why enzymes are important to pass the MCAT and what you need to know about them.

The Medical College Admission Test (MCAT) is a gateway to medical school admissions, challenging aspiring physicians with its comprehensive examination of scientific knowledge and critical thinking skills. 

Within the vast realm of MCAT subjects, the significance of enzymes cannot be overstated. Enzymes, the molecular catalysts that drive countless biochemical reactions in living organisms, are pivotal in understanding and acing the enzymes on the MCAT. 

To make sure you know exactly what to expect when you start studying for the MCAT, we'll delve into the world of enzymes and the role they play on the exam.

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What Are Enzymes?

To understand enzymes, the best place to start is to picture your body as a bustling chemical factory with countless reactions. These reactions are crucial for your body to function correctly, whether breaking down food, repairing tissues, or producing energy. However, these reactions can sometimes be very slow. 

Enzymes are specialized proteins that act as catalysts, speeding up chemical reactions without being consumed. Enzymes work by lowering the energy barrier required for a reaction to occur. They make it easier for molecules to collide and interact. 

Enzymes speed up reactions that would otherwise take forever, enabling your body to carry out its vital processes promptly.

Each enzyme has a specific job and has a precise shape that fits perfectly with certain molecules, known as substrates. This ensures that the reactions occur only when and where needed.

Student looking into microscope

Types of Enzymes 

There are six common types of enzymes on the MCAT. The following sections will briefly describe these classes of enzymes to prepare you for the MCAT.


These enzymes are the masters of electron transfer. They facilitate oxidation-reduction reactions by either adding or removing electrons from molecules. Think of them as electron traffic directors, ensuring the smooth flow of electrons during important biochemical processes.


These enzymes transfer functional groups between molecules, like methyl, phosphate, or amino groups. They're like molecular couriers, shuttling these groups to their proper destinations and participating in essential reactions such as protein synthesis and DNA replication.


Hydrolases are the cleanup crew of biochemical reactions, from breaking down food in your digestive system to assisting in cellular recycling processes. As the name suggests, hydrolases break bonds with the help of water. They're like molecular scissors, snipping molecules into smaller pieces by adding water molecules.


Lyases add or remove groups from molecules without water as a co-reactant. They're like molecular sculptors, reshaping molecules and creating new ones in the process. Their actions are crucial in processes such as carbohydrate metabolism and the synthesis of important molecules like DNA.


Isomerases are molecular shape-shifters. They catalyze reactions that convert molecules from one isomeric form to another, rearranging their atoms without adding or removing anything. Their flexibility is vital in processes like carbohydrate metabolism and amino acid biosynthesis.


Ligases are the enzyme equivalent of molecular glues. They catalyze the joining of two molecules by forming new chemical bonds. These enzymes are essential for processes like DNA replication and repair, where they stitch together the building blocks of life.

Enzyme Kinetics

Now that we've explored the basics of enzymes on the MCAT, let's take a closer look at their incredible speed and efficiency. 

Enzyme kinetics is the study of how enzymes work and the factors that influence their activity. By studying enzyme kinetics, we gain valuable insights into the rates of enzyme-catalyzed reactions and how they're affected by various factors.

Enzyme-Substrate Complex

One of the key concepts in enzyme kinetics is the enzyme-substrate complex. Remember how we mentioned that enzymes have a specific shape that perfectly fits certain molecules called substrates? When an enzyme and substrate come together, they form a temporary partnership known as the enzyme-substrate complex.

Enzyme kinetics helps us understand how quickly this complex forms and how efficiently it converts substrates into products. This leads us to two fundamental parameters: the reaction rate and the enzyme's affinity for its substrate.

The rate of reaction measures how quickly a reaction occurs, specifically, the speed at which the enzyme converts substrates into products. Various factors can influence the rate, including substrate concentration, enzyme concentration, temperature, and pH.

Michaelis-Menton Equation

The Michaelis-Menten equation, a cornerstone of enzyme kinetics, describes the relationship between the rate of reaction and substrate concentration. It reveals that as substrate concentration increases, the reaction rate initially increases but eventually levels off, reaching a maximum. 

On the other hand, the enzyme's affinity for its substrate refers to how tightly the enzyme binds to the substrate. The Michaelis constant, or Km, measures the enzyme's affinity. A low Km indicates high affinity, meaning the enzyme binds tightly to the substrate, while a high Km suggests lower affinity.

Understanding these concepts allows us to analyze enzyme kinetics and predict how changes in substrate concentration, enzyme concentration, or other factors will affect the rate of reaction.

Outside Factors on Enzyme Kinetics

Enzyme kinetics also delves into the effect of temperature and pH on enzyme activity. Enzymes are delicate beings, and their activity depends on the right conditions. 

Extreme temperatures or pH levels can throw off a delicate balance, but they can also impact enzyme function. Temperature affects the speed of enzyme-catalyzed reactions, while pH influences the enzyme's shape and stability.

The study of enzyme inhibitors is another important aspect of enzyme kinetics. Inhibitors are molecules that can block or reduce enzyme activity. 

Understanding how inhibitors interact with enzymes helps us develop drugs that target specific enzymes and regulate their activity. This knowledge is crucial for medical research and the development of therapeutic interventions.

Enzyme Inhibition

Enzyme inhibition is a critical concept in biochemistry, which is a subfield of general chemistry that you'll likely encounter on the MCAT. It involves the regulation of enzyme activity by various molecules within biological systems. Here are some key points to remember:

Enzyme inhibition occurs when a molecule, known as an inhibitor, binds to an enzyme and reduces or blocks its activity. This binding can take place at the enzyme's active site or at an allosteric site, depending on the type of inhibition. Let’s take a look at the different types of enzyme inhibition.

Competitive Inhibition

Competitive inhibition occurs when an inhibitor molecule closely resembles the substrate of an enzyme. It competes with the substrate for binding to the enzyme's active site.

The inhibitor binds reversibly to the active site of the enzyme, preventing the substrate from binding. This results in reduced enzyme-substrate complex formation and slower product formation.

Competitive inhibition increases the apparent Km (Michaelis constant) of the enzyme but does not change the Vmax (maximum velocity).

It is typically reversible. Increasing the substrate concentration can overcome competitive inhibition because it out-competes the inhibitor for binding to the active site. To ace the MCAT, michaelis menten plots can be used to graphically represent competitive inhibition. These plots show the relationship between the reaction rate (velocity) and substrate concentration. Take a look:

Source: ScienceDirect

Uncompetitive Inhibition

Uncompetitive inhibition involves an inhibitor binding only to the enzyme-substrate complex, not the free enzyme. The inhibitor binds to the enzyme-substrate complex and locks it into an inactive state. This prevents the complex from dissociating and releasing the product.

Uncompetitive inhibition decreases both the apparent Km and Vmax.It is usually reversible. However, increasing substrate concentration cannot fully overcome uncompetitive inhibition because it binds specifically to the enzyme-substrate complex. 

When it comes to the MCAT, Lineweaver-Burk plots are a valuable tool for visualizing enzyme kinetics and understanding competitive inhibition on the MCAT. This graphical representation displays the reciprocity of the reaction rate (1/V) against the reciprocity of the substrate concentration (1/[S]).

Source: ScienceDirect

Mixed Inhibition 

Mixed inhibition is a combination of competitive and uncompetitive inhibition characteristics.

The inhibitor can bind to both the free enzyme and the enzyme-substrate complex, but it has different affinities for these forms. It may stabilize the enzyme-substrate complex or prevent substrate binding to the free enzyme.

Mixed inhibition can increase or decrease Km, depending on the relative affinities of the inhibitor for the free enzyme and the enzyme-substrate complex. It usually decreases Vmax. When you run into mixed inhibition on the MCAT, remember it can be reversible or irreversible, depending on the specific inhibitor. 

Source: Socratic

Noncompetitive Inhibition

Noncompetitive inhibition occurs when an inhibitor binds to an allosteric site on the enzyme rather than the active site. This binding induces a conformational change in the enzyme, reducing its activity.

The inhibitor can bind to both the free enzyme and the enzyme-substrate complex, affecting their activities. Noncompetitive inhibition does not change the apparent Km but decreases Vmax.It is typically reversible, as the inhibitor can dissociate from the allosteric site, allowing the enzyme to return to its active form.

Source: ScienceDirect

In summary, these four types of enzyme inhibition—competitive, uncompetitive, mixed, and non-competitive—play crucial roles in regulating enzymatic reactions. Understanding their mechanisms, effects on enzyme kinetics, and reversibility is essential for grasping how enzymes function in various biological processes and their relevance in areas like drug design and medical treatments.

Understanding enzyme inhibition is crucial for the MCAT, as it relates to essential biochemical processes and has implications in medical practice. Mastery of the types of inhibition, their mechanisms, and their biological consequences will serve you well on the exam and in your future medical studies.

Enzyme Activity

We can't overlook the concept of enzyme activity in our exploration of enzymes for the MCAT. Enzyme activity is like a window into the powerhouse within these remarkable molecules. 

Enzyme activity refers to the speed and efficiency with which an enzyme catalyzes a specific reaction. It measures an enzyme's ability to rapidly transform substrates into products. Understanding enzyme activity is crucial for assessing the performance and potential of these molecular workhorses. 


One of the primary factors influencing enzyme activity is temperature. Enzymes have an optimal temperature range in which they function most efficiently. When the temperature strays too far from the optimum, enzyme activity can decline, hampering effectiveness.


pH, the measure of acidity or alkalinity, is another key factor impacting enzyme activity. Enzymes have an optimal pH range in which they exhibit peak performance. 

For instance, citric acid, a key player in cellular respiration's citric acid cycle, performs optimally at around pH 3.0, which is quite acidic. If the pH strays too far from this sweet spot, enzymes may lose their efficiency or stop working altogether. So, maintaining the right pH is crucial for enzymes to do their job effectively.

Deviating from this range can alter the enzyme's shape, impairing its ability to bind with substrates and slowing reaction rates. So, maintaining the right pH environment is essential for maximizing enzyme activity.

Enzyme and Substrate Concentration

Enzyme concentration is a critical factor in determining activity as well. Generally, as the concentration of enzymes increases, so does the rate of the catalyzed reaction up to a point. Beyond that point, the reaction rate decreases as all available enzyme molecules are saturated with substrates. 

Substrate concentration is another vital factor influencing enzyme activity. Initially, as substrate concentration increases, the reaction rate increases until it reaches a plateau.

This plateau occurs when substrate molecules fully occupy all the enzyme's active sites. Further increasing the substrate concentration beyond this point won't increase the reaction rate, as the enzyme works at its maximum capacity.

Enzyme Activators and Inhibitors

Enzyme activators and inhibitors also play a significant role in modulating enzyme activity. Activators are molecules that enhance the catalytic efficiency of enzymes, while inhibitors reduce or block enzyme activity. Activators or inhibitors can profoundly impact enzyme activity and are a crucial consideration in pharmacology and drug development.

Measuring enzyme activity is typically done through various ways, which provide quantitative data on a specific reaction rate. These assays allow researchers to compare enzyme activities under different conditions, assess the impact of factors like temperature and pH, and evaluate the effectiveness of potential inhibitors or activators.

Enzyme Regulation

Enzyme regulation refers to the various mechanisms that modulate the activity of enzymes. It's like having a sophisticated control system that fine-tunes the speed and efficiency of enzymatic reactions in response to the body's ever-changing needs.

Feedback Inhibition

One primary mode of enzyme regulation is through feedback inhibition. In feedback inhibition, the end product of a metabolic pathway acts as an inhibitor of one of the enzymes involved in its synthesis. 

When the concentration of the end product reaches a certain level, it binds to the enzyme and inhibits its activity. This feedback loop prevents unnecessary overproduction of the end product, ensuring resources are allocated efficiently.

Allosteric Regulation

Another mechanism of enzyme regulation is allosteric regulation. Allosteric regulation is like a molecular switch, capable of turning enzyme activity on or off. It occurs when a regulatory molecule binds to a specific site on the enzyme, causing a change in its shape and altering its activity.

Allosteric regulators can be either positive or negative. Positive regulators enhance enzyme activity, while negative regulators inhibit it.

Covalent Modification

Enzyme regulation can also occur through covalent modification. This involves adding or removing a chemical group, such as a phosphate group, to or from the enzyme protein. This modification can alter the enzyme's activity, structure, or location within the cell, effectively regulating its function. 

Outside Factors

Additionally, enzyme regulation can be influenced by factors like temperature, pH, and the availability of cofactors or coenzymes. These factors can impact enzyme activity by altering the enzyme's conformation or affecting the efficiency of its interactions with substrates.

Sample Enzymes MCAT Question

For those studying for the MCAT, sample questions are invaluable for truly grasping the content. We have included a sample question that gives you an idea of what kind of questions you can find about enzymes on the MCAT. 

Sample Question

Potassium cyanide is a poison which combines with cytochrome a3 to prevent binding of oxygen to the enzyme without altering the Km of the reaction with respect to reduced cytochrome c. Which type of inhibition does this represent?

  1. Noncompetitive inhibition
  2. Reversible inhibition
  3. Competitive inhibition

Answer: a) Noncompetitive inhibition

FAQs: Enzymes for the MCAT

Here are a few common questions about the enzymes on the MCAT.

1. How Much Should I Know About Enzymes for the MCAT?

Enzymes are one of the most heavily tested areas on the MCAT. While the test does not expect you to be an expert in every single area, you should have a strong understanding of the core components of enzymes. 

2. Do I Need to Study Enzymes for the MCAT?

Yes, enzymes are one of the most tested subjects on the MCAT. 

Final Thoughts

Mastering the MCAT requires more than memorization of the periodic table. You’ll need to rely on a deep understanding of the crucial enzymes that govern life's intricate biochemical processes. These catalysts are the unsung heroes of the MCAT, connecting the dots between various topics such as metabolism, genetics, and molecular biology. 

By recognizing the key enzymes and their functions, aspiring medical professionals can enhance their problem-solving abilities, make informed connections, and approach questions about enzymes on the MCAT confidently and precisely.

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