IB Biology How to Solve: IB Biology – Enzyme Kinetics & Competitive/Non-Competitive Inhibition

How to Solve: IB Biology – Enzyme Kinetics & Competitive/Non-Competitive Inhibition

Complete Guide

Introduction

"Mastering enzyme kinetics and inhibition isn’t just about memorising graphs—it’s how you score 7s on IB Biology Paper 2 data questions, where 20% of your marks come from interpreting enzyme experiments. Miss this, and you’re leaving easy points on the table."

WHAT YOU NEED TO KNOW FIRST

1. Enzyme basics: Enzymes are biological catalysts that speed up reactions by lowering activation energy. They have an active site where substrates bind.
2. Michaelis-Menten kinetics: How reaction rate (V) changes with substrate concentration ([S]). You should recognise the hyperbolic curve and know that Vmax is the maximum rate.
3. Lineweaver-Burk plots: Double-reciprocal plots (1/V vs. 1/[S]) that linearise Michaelis-Menten data. Used to distinguish inhibition types.

KEY TERMS & FORMULAS

Key Terms

Formulas

1. Michaelis-Menten equation (given on IB exam sheet): [ V = \frac{V_{max} [S]}{K_m + [S]} ]
2. V = reaction rate
3. Vmax = maximum reaction rate
4. [S] = substrate concentration

5. Km = Michaelis constant

6. Lineweaver-Burk equation (derived from Michaelis-Menten): [ \frac{1}{V} = \frac{K_m}{V_{max}} \cdot \frac{1}{[S]} + \frac{1}{V_{max}} ]

7. MEMORISE THIS: It’s a straight line (y = mx + c), where:
   • y-intercept = 1/Vmax
   • x-intercept = –1/Km
   • Slope = Km/Vmax

STEP-BY-STEP METHOD

How to Solve Enzyme Kinetics Problems

Step 1: Identify the type of data given - Is it a Michaelis-Menten curve (hyperbolic) or a Lineweaver-Burk plot (straight line)? - Are inhibitors present? If yes, determine if they’re competitive or non-competitive.

Step 2: Extract Vmax and Km from the graph - For Michaelis-Menten curves: - Vmax = plateau (maximum rate at high [S]). - Km = [S] at V = Vmax/2. - For Lineweaver-Burk plots: - Vmax = 1 / (y-intercept). - Km = –1 / (x-intercept).

Step 3: Compare Vmax and Km with and without inhibitor - Competitive inhibition: - Vmax stays the same. - Km increases (enzyme affinity decreases). - Non-competitive inhibition: - Vmax decreases. - Km stays the same.

Step 4: Answer the question - If asked to identify the inhibitor type, compare Vmax and Km changes. - If asked to calculate Vmax or Km, use the Lineweaver-Burk plot. - If asked to explain the effect of an inhibitor, describe how it binds and affects the enzyme.

WORKED EXAMPLES

Example 1 – Basic: Michaelis-Menten Curve

Question: The following data was obtained for an enzyme-catalysed reaction. Determine Vmax and Km.

Solution:
1. Plot the data: V vs. [S] (hyperbolic curve).
2. Vmax = plateau at high [S] = 8.0 μmol/min.
3. Km = [S] at V = Vmax/2 = 8.0/2 = 4.0 μmol/min → [S] = 4 mM.

What we did and why: - We identified Vmax as the maximum rate (plateau). - We found Km by locating the substrate concentration at half Vmax.

Example 2 – Medium: Lineweaver-Burk Plot

Question: The following Lineweaver-Burk plot was obtained for an enzyme. Determine Vmax and Km.

Solution:
1. Plot 1/V vs. 1/[S] (straight line).
2. y-intercept = 0.1 → Vmax = 1 / 0.1 = 10 μmol/min.
3. x-intercept = –0.2 → Km = –1 / (–0.2) = 5 mM.

What we did and why: - We used the y-intercept to find Vmax (1/Vmax). - We used the x-intercept to find Km (–1/Km).

Example 3 – Exam-Style: Inhibition Type

Question: An enzyme has a Vmax of 12 μmol/min and a Km of 3 mM. With an inhibitor, Vmax remains 12 μmol/min, but Km increases to 6 mM. What type of inhibition is this?

Solution:
1. Compare Vmax and Km with and without inhibitor: - Vmax unchanged. - Km increased.
2. Competitive inhibition matches this pattern (Vmax same, Km increases).

What we did and why: - We compared the changes in Vmax and Km to the expected patterns for each inhibition type. - Since Vmax stayed the same but Km increased, we concluded it was competitive inhibition.

COMMON MISTAKES

EXAM TRAPS

1-MINUTE RECAP

"Okay, listen up—this is your 60-second crash course for enzyme kinetics. First, memorise the Michaelis-Menten equation: V = (Vmax [S]) / (Km + [S]). Vmax is the maximum rate, Km is the substrate concentration at half Vmax. If you see a Lineweaver-Burk plot, remember: y-intercept = 1/Vmax, x-intercept = –1/Km. For inhibition, competitive inhibitors keep Vmax the same but increase Km—they compete for the active site. Non-competitive inhibitors decrease Vmax but keep Km the same—they bind somewhere else and change the enzyme’s shape. If the question gives you a graph, always label Vmax and Km first. If it’s about inhibition, compare the changes. And don’t forget units! Now go ace that exam."