Enzymes (Working Mechanism)

Enzymes are specialized protein complexes that serves as catalysts and accelerate the rate of reactions.

Enzymes vary in their specificity. While there are some enzymes that are only compatible with a single substrate, there are other enzymes that are compatible with several substrates, such as those with similar side chains, positions on a chain, or functional groups.

The substrate can interact with the active site through opposite charges, hydrogen bonding, non-polar interaction, and coordinate covalent bonding to metal ion activator.

The working mechanism of enzymatic action  are described by two hypothesis which are:

  1. Lock and Key Hypothesis
  2. Induced Fit Hypothesis

Lock and key Hypothesis

This model was proposed by Emil Fischer in 1890 to explain binding between active site of an enzyme and a substrate molecule. The active site is thought to have a fixed structure (Lock), which exactly match the structure of a specific substrate (Key). However it doesn’t explain the stabilization of enzyme (i.e. when an enzyme has a substrate entered into its active site, the enzyme will change its shape slightly to match the substrate).

Active Site (Lock) + Specific Substrate (Key) = Enzyme-Substrate Complex (Lock-Key Complex)


Lock and Key Hypothesis


In lock and key hypothesis, the shape of active site matches the shape of its substrate molecules. This makes enzymes highly specific. Each type of enzyme can usually catalyze only one type of reaction. When a substrate molecule collides with an enzyme whose active site shape is complementary, the substrate will fit into the active site and an Enzyme-Substrate complex will form. Enzyme will catalyze the reaction, and products will form. According to this model, it is possible for an enzyme to catalyze a reverse reaction.


Induced Fit Hypothesis

This model was proposed by Daniel Koshland in 1958. It states that since enzymes are so flexible, the active site is constantly being reshaped by its interaction with substrate.

The induced-fit theory assumes that substrate plays a role in determining final shape of an enzyme and that the enzyme is partially flexible. This explains why certain compounds can bind to an enzyme but do not react because enzyme has been distorted too much. Other molecules may be too small to induce proper alignment and therefore cannot react. Only the proper substrate is capable of inducing proper alignment of active site.


Induced Fit Model of Enzymatic Action


It states that shape of active sites in enzymes are not exactly complementary, but can change shape in presence of a specific substrate to become complementary as amino acid side-chains that are a part of active site are molded into a specific position. When a substrate molecule collides with an enzyme and if its composition is specifically correct, shape of active site of an enzyme will change so that substrate fits into it and an Enzyme-Substrate Complex can form. The reaction is then catalyzed and an Enzyme-product Complex forms leading to formation of products.


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