Enzymes as Biological Catalysts

Enzymes are proteins that catalyze reactions in the body. A catalyst is any substance that increases the rate of a chemical reaction without being consumed in the process — enzymes bind their reaction partners, help the reaction proceed, and are released unchanged, ready to be reused.

Key Takeaways

• Enzymes are catalysts: they speed up reactions without being consumed.

• Enzymes lower activation energy — that's the sole mechanism behind their effect on rate.

• Enzymes affect kinetics only. They never change K_eq or ΔG.

• The six enzyme classes (LIL HOT): ligases, isomerases, lyases, hydrolases, oxidoreductases, transferases — each defined by reaction type, not substrate identity.

• On an activation-energy diagram, a catalyzed reaction has the same start/end points as the uncatalyzed one, just a lower peak.

What Is a Catalyst?

Every reaction has to pass through a high-energy, unstable intermediate called the transition state before reactants can become products. The energy required to reach that transition state is the reaction's activation energy. Enzymes work by lowering this activation-energy barrier, which makes it easier for molecules to reach the transition state and makes the reaction happen faster.

Lowering activation energy is the single mechanism behind every effect an enzyme has on a reaction — everything else follows from it.

Enzymes Affect Kinetics, Not Thermodynamics

The most important distinction to lock in early: enzymes speed reactions up, but they don't change whether a reaction is favorable, and they don't change how much product forms at equilibrium.

• Enzymes do not alter the equilibrium constant (K_eq). K_eq reflects the ratio of products to reactants once a reaction has stabilized — enzymes don't shift that ratio, they just help the system reach it faster.

• Enzymes do not change the overall Gibbs free energy (ΔG) of a reaction. A reaction with a positive ΔG (nonspontaneous) stays nonspontaneous even with an enzyme present. Enzymes only speed up favorable reactions; they can't make an unfavorable one happen.

In short: enzymes are a kinetics tool, not a thermodynamics tool. They change how fast a system gets to equilibrium, never where that equilibrium sits.

The Essential Features of Enzymes

These seven properties are always true of enzymes and are common territory for "which of the following is true" MCAT questions:

Enzymes DO:

• Lower the activation energy of a reaction

• Increase the rate of a reaction

• Get reused — they're not consumed or permanently changed

• Show specificity for a particular substrate or closely related group of substrates

• Remain sensitive to environmental conditions like pH and temperature

Enzymes DON'T:

• Alter the equilibrium constant (K_eq)

• Change the overall free energy (ΔG) of a reaction

Any answer choice claiming an enzyme "shifts equilibrium" or "makes a reaction more thermodynamically favorable" is testing this exact distinction — and is wrong.

The Six Classes of Enzymes

Enzymes are grouped into six major classes based on the type of reaction they catalyze. A common mnemonic is LIL HOT (Ligases, Isomerases, Lyases, Hydrolases, Oxidoreductases, Transferases).

Each class name tells you what the enzyme is doing at the molecular level — recognizing the suffix pattern (e.g., "-ase" reaction types) helps you predict function from an unfamiliar enzyme name on the exam.

The Activation Energy Diagram

Reactions are classified by their free-energy change, ΔG:

• An endergonic reaction requires energy input to proceed (positive ΔG).

• An exergonic reaction releases energy (negative ΔG).

Regardless of which type a reaction is, it still has to clear an activation-energy barrier to reach the transition state. On a reaction-progress diagram, the uncatalyzed reaction traces a curve with a high peak at the transition state. With an enzyme present, the curve reaches the same starting and ending points — but the peak is lower. That lower peak is the enzyme's entire effect: it doesn't move the start or end of the graph (that would mean changing ΔG), it only lowers the height of the barrier in between.

Common MCAT Mistakes

• Thinking an enzyme shifts equilibrium. Enzymes speed up how fast a reaction reaches equilibrium — they never change K_eq or the ratio of products to reactants once equilibrium is reached.

• Thinking an enzyme can make an unfavorable reaction favorable. Enzymes don't change ΔG. A reaction with a positive ΔG stays nonspontaneous with or without an enzyme present; enzymes only speed up reactions that are already thermodynamically possible.

• Misreading the activation-energy diagram. A catalyzed reaction's curve keeps the same starting and ending energy levels as the uncatalyzed curve — only the height of the peak (the transition state) drops. Moving the start or end point would mean changing ΔG, which enzymes don't do.

• Sorting enzyme classes by substrate instead of reaction type. The six enzyme classes (LIL HOT) are defined by the type of reaction catalyzed — oxidation-reduction, group transfer, hydrolysis, non-hydrolytic bond cleavage, isomerization, or ATP-driven joining — not by what specific molecule the enzyme acts on.

MCAT-Style Concept Check

Question: An enzyme catalyzes the transfer of a phosphate group from ATP to a substrate molecule. Based on the type of reaction being catalyzed, which of the six enzyme classes does this enzyme belong to?

• A) Hydrolase

• B) Transferase

• C) Ligase

• D) Oxidoreductase

Answer: B

Explanation: Transferases move a functional group — such as a phosphate group — from one molecule to another; a kinase transferring a phosphate from ATP to a substrate is a classic transferase reaction. Option A is incorrect because hydrolases break bonds using water, not by moving a functional group between molecules. Option C is incorrect because ligases join two large molecules together using ATP as an energy source, rather than transferring a single functional group. Option D is incorrect because oxidoreductases catalyze oxidation-reduction reactions involving electron transfer, not group transfer.

FAQ

Do enzymes change the equilibrium of a reaction?

No. Enzymes don't alter the equilibrium constant (K_eq) or shift the ratio of products to reactants at equilibrium — they only change how fast the system gets there.

What is activation energy, and how do enzymes lower it?

Activation energy is the energy required for reactants to reach the high-energy transition state before becoming products. Enzymes bind their substrates in a way that stabilizes this transition state, lowering the energy barrier and making the reaction proceed faster.

What are the six classes of enzymes?

The six classes, remembered with the mnemonic LIL HOT, are ligases, isomerases, lyases, hydrolases, oxidoreductases, and transferases — each defined by the type of reaction it catalyzes, not by the specific substrate involved.

Are enzymes used up during a reaction?

No. Enzymes are catalysts, meaning they're released unchanged after the reaction and can be reused to catalyze the same reaction again.