Enzymes
Biological Catalysts
Explore how enzymes speed up reactions, the lock-and-key model, factors that affect activity, and enzyme kinetics for the AP Biology exam.
AP Biology
What Are Enzymes?
Enzymes are biological catalysts: proteins (and occasionally RNA molecules) that dramatically increase the rate of chemical reactions without being consumed. Nearly every metabolic process in a cell depends on enzymes to proceed fast enough to sustain life.
Catalyst
Speeds up a reaction by lowering the activation energy barrier
Protein
Most enzymes are globular proteins with a unique 3D shape
Specific
Each enzyme catalyzes one reaction or a small set of related reactions
Essential Vocabulary
1
Substrate: The specific reactant molecule that an enzyme acts upon.
2
Active site: The region of the enzyme where the substrate binds and the reaction occurs.
3
Enzyme-substrate complex: The temporary structure formed when a substrate binds to the active site.
4
Product: The molecule(s) produced after the enzyme catalyzes the reaction.
5
Activation energy (\(E_a\)): The minimum energy required to initiate a chemical reaction.
Lowering Activation Energy
Lock-and-Key vs Induced Fit
How Enzymes Work
1
Substrate approaches: The substrate drifts into the enzyme's active site via random molecular motion.
2
Binding (induced fit): The active site adjusts its shape, forming the enzyme-substrate complex through hydrogen bonds, ionic interactions, and hydrophobic contacts.
3
Catalysis: The enzyme lowers \(E_a\) by straining bonds, positioning reactants optimally, or providing a favorable microenvironment.
4
Product release: Products no longer fit the active site and are released. The enzyme returns to its original shape, ready to catalyze again.
Factors Affecting Enzyme Activity
Temperature
Increasing temperature raises reaction rate until the optimum. Beyond that, the enzyme denatures as heat disrupts hydrogen bonds.
pH
Each enzyme has an optimal pH (e.g., pepsin at pH 2, trypsin at pH 8). Extreme pH changes ionization of amino acids, altering shape.
Substrate Concentration
Rate increases with \[S\] until all active sites are occupied: saturation. Maximum rate is \(V_{max}\).
Temperature and pH Curves
Michaelis-Menten Kinetics
Competitive vs Non-Competitive Inhibition
Inhibition Comparison
| Competitive | Non-Competitive | |
|---|---|---|
| Binding Site | Active site | Allosteric site |
| Resembles Substrate? | Yes | No |
| Effect on \(V_{max}\) | Unchanged | Decreased |
| Effect on \(K_m\) | Increased (apparent) | Unchanged |
| Overcome by more \[S\]? | Yes | No |
| Example | Malonate inhibiting succinate dehydrogenase | Heavy metals binding enzyme |
Allosteric Regulation
Many enzymes have allosteric sites: regulatory binding sites separate from the active site. When a molecule binds an allosteric site, it changes the enzyme's conformation, either activating or inhibiting catalysis.
Activators
Stabilize the active conformation, increasing substrate affinity
Inhibitors
Stabilize the inactive conformation, reducing catalytic rate
Feedback Inhibition
Cofactors and Coenzymes
Many enzymes require non-protein helpers to function. These helpers assist in catalysis by accepting or donating electrons, functional groups, or atoms.
Cofactors
Inorganic ions (e.g., \(Zn^{2+}\), \(Fe^{2+}\), \(Mg^{2+}\)) that bind to the enzyme and help stabilize the active site or participate in catalysis.
Coenzymes
Organic molecules (often derived from vitamins) like \(NAD^+\), \(FAD\), and coenzyme A that carry chemical groups between enzymes.
Example: Reading a Kinetics Graph
Example: Temperature Experiment
A student tests an enzyme (extracted from human saliva) at five temperatures and measures reaction rate. The results are shown below:
| Temperature | Rate (units/min) |
|---|---|
| \(10^\circ C\) | 12 |
| \(20^\circ C\) | 28 |
| \(37^\circ C\) | 54 |
| \(50^\circ C\) | 18 |
| \(70^\circ C\) | 2 |
Common Mistakes
✕ Enzymes provide energy
Enzymes lower activation energy but do not add energy to the reaction. They are catalysts, not fuel.
✕ Enzymes are consumed
Enzymes are recycled. They are released unchanged after each catalytic cycle and can be used again.
✕ Denaturation = destruction
Denaturation unfolds the protein's tertiary structure, not its primary sequence. However, it is typically irreversible under biological conditions.
Enzyme Summary
| Concept | Key Detail |
|---|---|
| Role | Biological catalysts that lower \(E_a\) |
| Structure | Globular proteins with a specific active site |
| Model | Induced-fit (enzyme molds around substrate) |
| \(V_{max}\) | Maximum rate when all active sites are saturated |
| \(K_m\) | \[S\] at half-\(V_{max}\): measures enzyme-substrate affinity |
| Competitive Inhibitor | Binds active site: increases apparent \(K_m\), \(V_{max}\) unchanged |
| Non-Competitive Inhibitor | Binds allosteric site: decreases \(V_{max}\), \(K_m\) unchanged |
| Feedback Inhibition | End product inhibits first enzyme in a pathway |
How to Approach Enzyme Problems
1
Identify the enzyme and substrate. What reaction is being catalyzed? What is the product?
2
Check for inhibitors or activators. Is anything binding the active site or an allosteric site?
3
Examine environmental conditions. Temperature, pH, and \[S\] all affect rate. Is the enzyme near its optimum?
4
Analyze the graph. On kinetics graphs, check for changes in \(V_{max}\) and \(K_m\) to identify the type of inhibition.
5
Connect to big ideas. Relate enzyme behavior to homeostasis, energy flow, or metabolic regulation in the organism.
Question 1 of 5
Enzymes increase the rate of a reaction by:
Enzymes lower the activation energy (\(E_a\)) by stabilizing the transition state, allowing more molecules to react at a given temperature. They do not change \(\Delta G\).
Question 2 of 5
A competitive inhibitor affects enzyme kinetics by:
A competitive inhibitor competes with the substrate for the active site. More substrate is needed to reach half-\(V_{max}\), so apparent \(K_m\) increases. At very high \[S\], the substrate outcompetes the inhibitor, so \(V_{max}\) is unchanged.
Question 3 of 5
An enzyme has an optimal pH of 2. This enzyme is most likely found in which location?
The stomach has a pH of approximately 1.5 to 3.5 due to hydrochloric acid secretion. Pepsin, a digestive protease, functions optimally in this highly acidic environment. The other locations have near-neutral pH (7.0 to 8.0).
Question 4 of 5
In feedback inhibition of a metabolic pathway, which enzyme is typically inhibited by the end product?
The end product inhibits the first enzyme in the pathway. This is the most efficient control point because it prevents all downstream intermediates from being produced unnecessarily, conserving resources.
Question 5 of 5
An enzyme is heated to 80 \(^\circ C\) and then cooled back to 37 \(^\circ C\). The enzyme shows no catalytic activity. The most likely explanation is:
High heat disrupts the hydrogen bonds, ionic bonds, and hydrophobic interactions that maintain the enzyme's tertiary (3D) structure. The primary structure (peptide bonds) remains intact, but the protein cannot refold correctly. This irreversible denaturation destroys the active site shape.