BIOLOGY QUALITY WORKSPACE · B3–B19

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LESSON · bio-metabolism-2026.1-draft

BIO-CMB-MET-L01

Draft
LESSON PACKET · BIO-CMB-MET-L01

Enzymes and activation energy

Predict how an enzyme or environmental change alters reaction rate while keeping kinetics separate from thermodynamics.

Lower the barrier, not the endpoints

An enzyme stabilizes a route to the transition state, lowering activation energy for both forward and reverse reactions. It does not change reactant or product free energy.

  • Activation energy changes
  • ΔG and the equilibrium constant do not

Fit is chemical and dynamic

Active-site shape, charge, and local chemistry position substrates for reaction. Temperature and pH can improve collision conditions within a range, then disrupt the protein or key interactions outside that range.

  • Think induced fit
  • Separate a faster collision rate from denaturation

Read the rate pattern

More substrate increases rate only while free active sites remain. At saturation, enzyme concentration limits the maximum observed rate. Inhibitor patterns depend on where and how the inhibitor binds.

  • Plateau means active-site saturation
  • More enzyme can raise capacity
WORKED EXAMPLE

One enzyme has relative rates of 15, 62, 100, 58, and 12 at pH 3, 5, 7, 9, and 11. What conclusion is supported?

  1. Locate the maximum: the measured optimum is near pH 7 under these conditions.
  2. Compare both sides: activity falls in strongly acidic and strongly basic solutions, consistent with disrupted active-site ionization or protein structure.
  3. Respect the evidence limit: rate data alone do not show that ΔG, equilibrium, or enzyme quantity changed.
The data support a pH-dependent catalytic optimum near 7; they do not support a change in reaction equilibrium.
RETRIEVAL ANSWERS

If an enzyme speeds both directions of a reversible reaction, what happens to equilibrium?

It is reached sooner, but its position does not change.

Lower activation energy accelerates both directions without changing the free-energy difference.

Why can a rate-versus-substrate curve plateau?

Available active sites become occupied most of the time.

At saturation, adding substrate cannot raise turnover unless enzyme capacity also changes.

Why can warming first increase and then decrease enzyme activity?

Collisions initially increase, but excessive heat disrupts structure and active-site chemistry.

The rising and falling regions have different mechanistic causes.

SOURCES TO OPEN AND VERIFY
ADA DAT User’s Manual 2026 · Biology content specifications ↗OpenStax Biology 2e · 6.5 Enzymes ↗
INDEPENDENT REVIEW RECORD

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CONTROLLED LIFECYCLE

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  1. 01Draft
  2. 02Science reviewed
  3. 03Assessment reviewed
  4. 04Accessibility reviewed
  5. 05Pilot eligible
  6. 06Approved
  7. 07Revise
  8. 08Retired