How Enzymes Work — Step-by-Step Process

Key Concepts

Activation Energy Effect:

• Without enzyme: HIGH activation energy barrier → slow reaction
• With enzyme: LOWERED activation energy barrier → faster reaction
• Equilibrium position: UNCHANGED ($\Delta G$ unchanged)
• Enzyme: NOT consumed, regenerated after each cycle

Lock-and-Key Model (Fischer, 1894):

• Active site = rigid, pre-formed
• Substrate fits exactly like a key into a lock
• Limitation: cannot explain conformational changes

Induced Fit Model (Koshland, 1958) — MORE ACCEPTED:

• Active site = flexible
• Active site reshapes upon substrate binding (like hand in glove)
• Explains allosteric regulation and observed conformational changes

NEET One-Liner

"Enzymes lower activation energy ONLY. They do NOT change equilibrium, $\Delta G$, or reaction direction."