For a cell to function correctly, a large number of chemical reactions need to take place. However, these reactions can often be quite slow. One way to speed up the rate of chemical reactions is by using catalysts.

Enzymes act as biological catalysts. They increase the rate of chemical reactions in living organisms without being used up or altered in the process. All metabolic reactions in the body (e.g. breaking down molecules) need enzymes.

There are two models of enzyme action:

  • Lock and key model
  • Induced fit model

The Lock and Key Model

Enzymes are large proteins, so they are made up of long chains of amino acids, which are folded into complex shapes. This means they have a unique shape.

Smaller molecules fit into an active site of enzymes. Due to the unique shape of the enzyme, the shape of the active site is complementary to its substrate molecules. Therefore, the substrate fits into the enzyme’s active site like a key in a lock. This means that in most cases, each type of enzyme can only catalyse one type of reaction.

The diagram below shows how an enzyme breaks down a substrate molecule.

Diagram illustrating enzyme action on a substrate. Two hexagonal molecules, one red and the other yellow, labelled 'Substrate', are connected by a line marked 'Bond'. Below this is a blue U-shaped structure termed 'Enzyme' with an indicated 'Active Site'. Arrows show the substrate fitting into the enzyme, forming an 'Enzyme-Substrate Complex'. While in this complex, text 'Bond Breaks Under Stress' highlights the separation of the two hexagonal molecules. After separation, they are shown as individual products numbered 1 (red) and 2 (yellow). Further arrows depict the release of these products and the enzyme returning to its original state.

However, this can also happen in reverse:

Diagram showing the action of an enzyme on substrates. Two kidney-shaped molecules coloured pink and purple, labelled '1' and '2' respectively, represent 'Substrates'. Beneath them is an orange U-shaped structure named 'Enzyme', featuring a wavy line indicating its 'Active Site'. Blue arrows demonstrate the substrates fitting into the enzyme, forming the 'Enzyme-Substrate Complex' where the two substrates merge. This complex then transforms into a single large purple circle labelled 'Product'. Further arrows show the release of the product, with the enzyme returning to its initial state.

In this case, multiple substrates combine to form a single product. All the enzyme does is speed up this process. If the substrate doesn’t fit the active site of the enzyme, then the reaction will not be catalysed.

The Induced Fit Model

Initially, scientists thought that the substrate had to perfectly fit the active site. But we now know, that when a substrate binds to an active site, the enzyme changes shape slightly to fit the substrate perfectly.

A sequential diagram illustrating the enzyme-catalysed reaction process. Starting with a peach-coloured 'Substrate' above a blue 'Enzyme' with a designated 'Active Site'. Green arrows guide the substrate into the enzyme's active site, leading to a stage where the 'Substrate Binds'. Following this, the enzyme slightly alters its shape, and the bound substrate transitions into two differently shaped 'Products'. Finally, the products are released from the enzyme, which remains unchanged and ready for another round of reaction. Throughout the process, labels such as 'Substrate Enters Active Site', 'Enzyme/Substrate Complex Forms', 'Substrate is Converted to Products', and 'Products Leave Active Site' further describe each stage.

Comparing the Two Models of Enzyme Action

Lock and keyInduced fit
Early theory for enzyme actionThe better, widely accepted theory
The enzyme’s active site must have a specific shape to fit the substrateThe active site can change its shape to fit the substrate
Enzymes are fixed structuresEnzymes are flexible structures

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