The substrate

• The substrates of an enzyme are the reactants that are activated by the enzyme.
• Enzymes are specific to their substrates.
• The specificity is determined by the active site.

The Lock and Key Hypothesis (Emil Fischer in 1890)

• Fit between the substrate (S) and the active site of the enzyme (E) is exact.
• Like a key fits into a lock very precisely.
• Temporary structure called the enzyme-substrate complex formed.
• Products (P) have a different shape from the substrate.
• Once formed, they are released from the active site.
• Leaving it free to become attached to another substrate.

This explains enzyme specificity.

This explains the loss of activity when enzymes denature.

The Induced Fit Hypothesis (Daniel Koshland in 1959)

• Some proteins can change their shape (conformation).
• When a substrate combines with an enzyme, it induces a change in the enzyme’s conformation.
• The active site is then moulded into a precise conformation.
• Making the chemical environment suitable for the reaction.
• The bonds of the substrate are stretched to make the reaction easier (lowers activation energy).

This explains the enzymes that can react with a range of substrates of similar types

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