Achievable logoAchievable logo
MCAT
Sign in
Sign up
Purchase
Textbook
Practice exams
Feedback
Community
How it works
Exam catalog
Mountain with a flag at the peak
Textbook
Introduction
1. CARS
2. Psych/soc
3. Bio/biochem
4. Chem/phys
4.1 Translational motion, forces, work, energy, and equilibrium
4.2 Fluids in circulation of blood, gas movement, and gas exchange
4.3 Electrochemistry and electrical circuits and their elements
4.4 How light and sound interact with matter
4.5 Atoms, nuclear decay, electronic structure, and atomic chemical behavior
4.6 Unique nature of water and its solutions
4.7 Nature of molecules and intermolecular interaction
4.8 Separation and purification methods
4.9 Structure, function, and reactivity of bio-relevant molecules
4.10 Principles of chemical thermodynamics and kinetics, enzymes
4.10.1 Bioenergetics, thermochemistry and thermodynamics
4.10.2 Rate processes in chemical reactions - Kinetics and equilibrium
4.10.3 Enzymes
Wrapping up
Achievable logoAchievable logo
4.10.3 Enzymes
Achievable MCAT
4. Chem/phys
4.10. Principles of chemical thermodynamics and kinetics, enzymes
Our MCAT course is in "early access"; the content on this page is a work-in-progress.

Enzymes

4 min read
Font
Discuss
Share
Feedback

Classification by reaction type

Enzymes are typically categorized according to the type of reaction they facilitate.

Examples:

  • Oxidoreductases (handle redox processes)
  • Transferases (move functional groups),
  • Hydrolases (cleave bonds using water),
  • Lyases (break bonds without hydrolysis or oxidation),
  • Isomerases (rearrange molecular structures), and
  • Ligases (form new bonds, often using ATP).

Mechanism

They lower the activation energy by stabilizing transition states, aligning substrates properly, or providing catalytic residues that facilitate electron flow. Through such general catalysis, enzymes increase the reaction rate constant without altering equilibrium constants or overall free energy changes.

  • Substrates and enzyme specificity
    Enzymes exhibit high specificity toward their substrates based on intricate three-dimensional complementarity. This selectivity arises from well-defined binding pockets that recognize particular shapes, charges, and functional groups. The enzyme’s specificity ensures that only the correct substrates undergo reaction, reducing unwanted side reactions.

  • Active site model
    The active site is a specialized region on the enzyme where substrate binding and catalysis occur. In the lock-and-key representation, the substrate fits into a rigid cavity that is pre-formed to complement its structure.

  • Induced-fit model
    Refining the above concept, the induced-fit model proposes that the enzyme adjusts its shape upon substrate contact. This conformational shift more precisely orients catalytic residues and the substrate, thereby enhancing transition state stabilization and facilitating the chemical transformation.

  • Cofactors, coenzymes, and vitamins
    Some enzymes depend on additional non-protein components known as cofactors to function properly. These can be metal ions (e.g., Fe²⁺, Zn²⁺) or coenzymes, which are organic cofactors often derived from vitamins (e.g., B-complex vitamins). They enable reaction types the enzyme’s amino acids cannot perform on their own, such as redox transfers (e.g., NAD⁺ ⇌ NADH).

Kinetics

Enzyme kinetics explores how factors like substrate concentration or enzyme concentration influence reaction rates. The Michaelis–Menten equation models this behavior, describing how velocity depends on substrate binding and turnover until reaching a maximum rate (Vmax).

General (catalysis)

Enzymes accelerate reactions by lowering the activation energy. They do not alter the equilibrium position or the reaction’s overall thermodynamics. Both forward and reverse reactions become faster, but the net free energy change (ΔG) remains unchanged.

Michaelis–Menten

Under the Michaelis–Menten framework, the reaction velocity (v) increases with substrate concentration [S] and approaches Vmax at saturating levels of substrate. Two key parameters emerge: Km (the substrate concentration at half Vmax) and Vmax (the maximum rate).

Cooperativity

Certain multimeric enzymes display cooperativity, in which substrate binding to one subunit changes the affinity of other subunits. Positive cooperativity means binding enhances additional substrate uptake, generating a sigmoidal dependence of velocity on [S].

Effects of local conditions on enzyme activity

Environmental factors—pH, temperature, and ionic strength—profoundly influence enzyme structure and reactivity. Each enzyme has an optimal pH and temperature, and deviations can denature or inactivate it. Cells often maintain microenvironments or localization strategies to keep enzymes at their ideal conditions.

Inhibition

Enzymes can be inhibited in several ways:

  • Competitive inhibitors vie for the active site, raising the substrate concentration needed to reach a given velocity.
  • Non-competitive inhibitors bind allosterically, reducing maximal activity regardless of substrate level.
  • Mixed or uncompetitive inhibitors offer more nuanced impacts by binding to either free enzyme or the enzyme–substrate complex, altering kinetic parameters in distinct ways.

Regulatory enzymes

Many pathways rely on regulatory enzymes that coordinate metabolic flow. Modulation can be:

  • Allosteric: Effectors bind separate sites, inducing conformational shifts that up- or down-regulate activity.
  • Covalently modified: Enzymes are switched on or off via phosphorylation, methylation, or acetylation.
    Zymogens are inactive precursors that require a chemical change (e.g., peptide cleavage) to become catalytically active.

Sign up for free to take 5 quiz questions on this topic

All rights reserved ©2016 - 2025 Achievable, Inc.