Enzyme Kinetics

⚡ Introduction to Enzymes

Enzymes are biological catalysts — proteins that speed up chemical reactions without being consumed. They are essential for life because most biological reactions would occur too slowly without them.

Key Concepts

• Enzymes lower the activation energy ($E_a$) of reactions
• They do NOT change the equilibrium or ΔG of a reaction
• They are highly specific — each enzyme catalyzes one type of reaction
• Most enzymes are proteins (some RNA molecules are catalytic — ribozymes)

How Enzymes Work

The induced fit model explains enzyme action:

1. Substrate binds to the enzyme's active site
2. The enzyme changes shape to fit the substrate more tightly
3. The reaction occurs (bonds are stressed, broken, or formed)
4. Products are released
5. The enzyme is unchanged and can be reused

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Activation Energy

Every chemical reaction requires an initial input of energy — the activation energy ($E_a$).

Enzymes can speed up reactions by factors of $10^6$ to $10^{12}$!

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Enzyme Kinetics

Reaction Rate

The rate of an enzyme-catalyzed reaction is how fast product appears (or substrate disappears):

$\text{rate} = \frac{\text{change in amount of product}}{\text{change in time}}$

In a typical experiment you measure product at several times and plot product against time. The initial rate is the slope at the start, before substrate has been used up.

How Rate Responds to Substrate

1. At low substrate concentration, the rate rises as substrate is added — more substrate means more enzyme-substrate collisions.
2. The rise slows and the rate levels off (plateaus).
3. At the plateau, every active site is occupied; the enzyme is saturated, and adding more substrate cannot speed the reaction.

Formal kinetic constants used in college biochemistry are not tested on the AP exam. You need to explain the shape of the curve and calculate rates from data.

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Substrate and Product Concentration Effects

The relative amounts of substrate and product determine how efficiently an enzymatic reaction proceeds.

• Low [substrate]: the rate depends strongly on how much substrate is present.
• High [substrate]: the rate levels off because the enzyme is saturated. The only way to go faster is to add more enzyme (more active sites).
• Over time in a closed tube: substrate is used up and product accumulates, so the rate slows. A product-vs-time curve therefore starts steep and flattens out even though the enzyme is still active.

Worked example — reading a product curve

1. Rate from 0 to 2 min: 8 ÷ 2 = 4 μmol/min.
2. Rate from 6 to 8 min: (18 − 17) ÷ 2 = 0.5 μmol/min.
3. Explanation: the rate falls because substrate is running out, not because the enzyme was destroyed. Adding fresh substrate at 8 minutes would speed the reaction up again.

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Temperature & pH Effects

Temperature

• Increasing temperature increases reaction rate (more kinetic energy, more collisions)
• Until the optimal temperature is reached
• Above optimal: enzyme denatures (3D structure unfolds)
• Most human enzymes: optimal ~37°C
• Thermophilic bacteria: optimal 70-80°C

pH

• Each enzyme has an optimal pH
• Deviations disrupt ionic bonds and hydrogen bonds in the tertiary structure

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Enzyme & Substrate Concentration

Effect of Enzyme Concentration

• At fixed [S], increasing [enzyme] increases rate linearly
• More enzyme = more active sites available

Effect of Substrate Concentration

• At fixed [enzyme], increasing [S] increases rate until the rate levels off
• Eventually, all active sites are occupied (saturation)

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Enzyme Inhibition

Competitive Inhibition

• Inhibitor resembles the substrate and binds to the active site
• Competes directly with substrate for the active site
• Binds reversibly, so it can be overcome by increasing substrate concentration
• On a rate-vs-[S] graph: the inhibited curve rises more slowly but reaches the same plateau at very high [S]

Noncompetitive Inhibition

• Inhibitor binds to an allosteric site (not the active site)
• Changes enzyme shape so the active site no longer fits the substrate
• Cannot be overcome by increasing [S]
• On a rate-vs-[S] graph: the inhibited curve levels off at a lower rate, no matter how much substrate is added

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Allosteric Regulation

Allosteric Enzymes

Some enzymes have allosteric sites — binding sites separate from the active site.

Feedback Inhibition

The end product of a metabolic pathway inhibits an early enzyme in the pathway.

Example: In the pathway A → B → C → D, product D inhibits the enzyme that converts A → B.

This is a form of negative feedback that prevents overproduction.

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Cofactors & Coenzymes

Cofactors

Cofactors are non-protein molecules required for enzyme activity.

Coenzymes you will meet in Unit 3

A cofactor is a component of the enzymatic system. If it is missing or changed, the enzyme's function or efficiency changes, just as it would if the protein itself were altered.

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Testing for a Cofactor: Experimental Design

Suppose you suspect that an enzyme needs $Mg^{2+}$ to work. A clean experiment changes one variable and measures its effect.

Worked example — interpreting the results

1. Compare tubes 1 and 2: adding $Mg^{2+}$ raises the rate 15-fold (6.0 ÷ 0.4).
2. Check the control: tube 3 makes no product, so the ion is not doing the catalysis itself.
3. Claim: the enzyme requires $Mg^{2+}$ as a cofactor for full activity.

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Enzyme Problem-Solving Workshop

AP Exam Strategy for Enzyme Questions

1. Identify the enzyme property being tested
2. Draw or visualize the energy diagram if relevant
3. Distinguish inhibition types by where the inhibitor binds and whether extra substrate overcomes it
4. Connect to biological context (what pathway? what regulation?)

Common Mistakes to Avoid

• Enzymes do NOT provide energy for reactions
• Competitive inhibition CAN be overcome by adding more substrate; noncompetitive inhibition cannot
• Denaturation ≠ competitive inhibition
• Enzymes are NOT consumed in reactions

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Practice Scenarios

Scenario 1

A pharmaceutical company designs a drug that has a similar shape to the substrate of a disease-causing enzyme. Predict how this drug works.

Answer: The drug acts as a competitive inhibitor — it binds to the active site and blocks the natural substrate.

Scenario 2

A student adds increasing amounts of substrate to an enzyme-catalyzed reaction. The rate increases and then plateaus. Explain.

Answer: At low [S], increasing substrate increases the rate because more enzyme-substrate complexes form. At high [S], all active sites are occupied (saturation), so the rate plateaus.

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Enzyme Synthesis & AP Review

Key Enzyme Concepts for AP Biology

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AP Exam Tips

• Be able to interpret enzyme kinetics graphs (rate vs. [S] curves)
• Know how competitive and noncompetitive inhibitors change a rate-vs-[S] curve
• Understand how pH and temperature affect enzyme activity
• Connect enzyme regulation to metabolic pathways
• Practice experimental design questions about enzyme activity

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