🔐 Lock-and-Key Model (Proposed in 1894)

🧠 Proposed by Emil Fischer, a German chemist.

Explanation:

• The enzyme’s active site is like a rigid lock.
• The substrate must be an exact fit, like a specific key.
• The enzyme doesn’t change shape.

✅ Easy to understand
❌ But not true for all reactions

🧰 Induced Fit Model (Proposed in 1958)

🧠 Proposed by Daniel Koshland, an American biochemist.

Explanation:

• The active site is not rigid.
• When the substrate binds, the enzyme changes its shape slightly.
• This better fits the substrate and helps the reaction.

🧤 Analogy:
Like a rubber glove adjusting to the shape of your hand — flexible and precise.

✅ More accurate
✅ Explains more enzyme behaviors

✅ Summary – Key Points to Remember

Concept	Easy Explanation
Activation Energy	Minimum energy needed to start a reaction
Enzyme	Biological helper that speeds up a reaction
Enzyme Efficiency	Reactions happen faster — even millions of times faster
Enzyme Specificity	One enzyme works on one specific substrate
Enzyme-Substrate Complex	Temporary fit between enzyme and substrate
Metabolic Pathway	Series of enzyme-controlled steps
Feedback Inhibition	End product turns off the first step to avoid waste
Lock-and-Key Model	Enzyme and substrate fit like puzzle pieces
Induced Fit Model	Enzyme changes shape to fit the substrate better