Enzymes

Definition

Activation energy is the energy barrier separating reactants from the transition state. An enzyme stabilizes a productive transition-state pathway, so a larger fraction of molecular collisions can lead to products at the same temperature.

Active Site and Specificity

The active site has chemical and geometric properties that favor particular substrates. Binding can position reactants, strain bonds, or create a local chemical environment that supports the reaction. Specificity is based on interactions, not on a rigid lock that never changes shape.

Interpreting Rate Changes

Increasing substrate concentration raises rate only while enough active sites remain available. Temperature and pH can change collision frequency or disrupt the interactions that maintain enzyme shape. A rate decrease must be connected to a mechanism rather than described only as “the enzyme stopped.”

Worked Example

Question: A competitive inhibitor resembles the substrate and binds reversibly to the active site. What happens when substrate concentration is greatly increased?

Reasoning: More substrate molecules compete for the same active sites. The inhibitor does not permanently remove enzyme molecules in this model.

Answer: The reaction rate can approach the uninhibited maximum because substrate increasingly occupies the active sites, although more substrate is required to reach a given rate.

Common Mistakes

• Saying enzymes supply energy to the reaction.
• Claiming enzymes change the equilibrium position or overall free-energy change.
• Assuming every inhibitor binds the active site; noncompetitive mechanisms act differently.

Key Takeaways

• Enzymes change reaction rate by lowering the activation barrier.
• Rate depends on molecular interactions, concentrations, and conditions.
• Experimental conclusions should distinguish correlation from a proposed molecular mechanism.