What is the result of substrate collisions regarding catalysis?

• A. Only a small proportion of collisions lead to a reaction.
• B. Most collisions lead to a reaction.
• C. All collisions require energy input.
• D. Only a small proportion of collisions lead to a reaction, and enzymes remain unchanged after catalysis.

Answer: (D)

When substrates collide with enzymes, only a small fraction have enough energy and the right orientation to reach the transition state and form products. This is because reactions need to overcome the activation energy barrier, and not every collision provides both the needed energy and the correct alignment. Enzymes help by shaping an active site that binds substrates and lowers this activation energy, making more collisions successful. After the reaction occurs, the enzyme is not consumed; it is free to catalyze many more reactions by forming a new enzyme–substrate complex with fresh substrate molecules.