(2) How do enzymes work? - the 'key and lock' mechanism theory

The reaction profile of a catalysed reaction compared to an uncatalysed reaction.

Enzymes reduce the activation energy, the minimum kinetic energy needed by reactant molecules to react by breaking bonds and forming new bonds in the product molecules.

So, by reducing the activation energy, at the same temperature, more molecules can react per unit time (rate), so increasing the reaction rate of the enzyme reaction compared to the uncatalysed reaction..

The enzyme helps break reactant molecule bonds more easily than without a catalyst, so facilitating a faster reaction without increasing concentration or temperature - the collision rate doesn't increase, but there is more chance a fruitful collision producing the product molecules because less kinetic energy is needed.

These arguments apply irrespective of whether the enzyme is functioning in its optimum conditions or not.

 

A substrate molecule is a reactant which is to be changed into the product by way of the specific enzyme.

The substrate molecule (or molecules) must fit neatly into the active site on an enzyme and weakly bond to it.

The enzyme, or more specifically, the active site, is referred to as the 'lock', and in an analogy with door locks, the substrate molecules are referred to as the 'key or keys'.

The action by which enzymes function is called the 'key and lock' mechanism. This is illustrated below.

The following diagrams illustrate two examples of the 'key and lock' mechanism - how an enzyme works.

The active site is where the chemical change from substrate to product takes place and its shape is very important.

The mechanism is discussed in more detail in the next section.

 

Many biochemistry reactions either involve synthesis of a larger molecule by joining smaller ones together or breaking down and splitting a larger molecule into smaller ones.

It is sometimes quoted as a hypothesis, but there is a vast amount of evidence to show this mechanism is correct.

 

Each enzyme is shaped precisely to accept the substrate molecules, otherwise the reaction will NOT take place. This is why a particular enzyme can only catalyse a specific reaction. The substrate must fit into the active site!

The complete molecular structure of some enzymes has been determined by X-ray crystallography.

From a computer database you generate the structure of the enzyme and with advanced computer graphics you can 'virtually' examine the 3D active site.

You can then bring in a substrate molecule (real or theoretical) to see how it fits (or not) into the unique structure of the active site.

It is now possible to design drugs to block enzyme reactions to treat a particular medical condition.

You can then synthesise the drug and thoroughly test to see if it works AND has no harmful side effects.

 

If the enzyme is not the right shape e.g. the protein structure-active site is damaged, the substrate molecule cannot 'key in' or 'dock in' so the enzyme cannot function and the reaction does not take place.

This protein structure damage is referred to as a denaturing of the enzyme.

Enzyme damage (denaturing) can be caused by too high a temperature or the medium may be too acid (too low a pH) or too alkaline (too high a pH) - see later section on factors affecting the rate of enzyme reactions.

 

Examples of the 'key and lock' mechanism for enzyme action

A good example of using a scientific model and well supported by scientific evidence

This illustrates making a larger molecule from smaller ones e.g. a stage in protein synthesis

(Stage 1) is the 'docking in' of the substrate molecules into the active site, they are held there just sufficiently strongly to allow the chemical transformation to take place.

The active site is considered the 'lock'.

(Stage 2) happens on the active site where the substrates are catalytically changed to products which are then released from the enzyme.

The substrate reactant molecule is considered the 'key'.

e.g. the key and lock mechanism for synthesising a larger molecule from smaller molecules.

Sequence key e.g. for a larger molecule being made from two smaller molecules, perhaps a stage in protein synthesis

E = free enzyme (the 'lock')

S = free substrate reactant molecules (the 'keys'), the three then combine together - 'lock' together

ES = enzyme-substrate complex ==> EP = enzyme-product complex

The chemical change at the active site, this complex then breaks down to give the free enzyme and product.

E = free enzyme, P = free product

The diagram simulates two amino acids joined together to make a dipeptide, or you can just think of one of the substrate molecules being a longer partially made protein molecule and another amino acid is added to the end of the chain.

Note that it isn't always this 'simple' for enzyme synthesis e.g. protein production, in some versions of this scheme, known as the induced fit model, the active site slightly adjusts its shape to fit the substrate more snugly - this is called induced fit.

 

Key and lock mechanism for producing smaller molecules from larger ones e.g. a stage in digestion

Sequence key e.g. for a larger molecule being broken down into two smaller molecules, perhaps in digestion where large carbohydrate molecules are broken down into small sugar molecules like glucose.

E = free enzyme (the 'lock')

S = free substrate reactant molecule (the 'key'), the two then combine together - 'lock' together

ES = enzyme-substrate complex ===> EP = enzyme-products complex

The chemical change at the active site, this complex then breaks down to give the free enzyme and products.

E = free enzyme, P = free products

Note again that it isn't always this 'simple' for enzyme reactions e.g. protein or carbohydrate digestion, in some versions, known as the induced fit model, the active site slightly adjusts its shape to fit the substrate more snugly - this is called induced fit.

 

Apart from water molecules, the diagram actually matches the hydrolysis of sucrose to glucose and fructose by the enzyme invertase.

sucrose  +  water  ===> glucose + fructose

C₁₂H₂₂O₁₁  +  H₂O  ===> C₆H₁₂O₆  +  C₆H₁₂O₆ 

(Note that the water molecule is NOT shown in the diagram, so the reaction is more complicated than the 'key and lock' diagram.

 

The anaerobic fermentation reaction

glucose (sugar) == enzyme zymase ==> ethanol + carbon dioxide

C₆H₁₂O_6(aq) ===> 2C₂H₅OH_(aq) + 2CO_2(g) 

is another example of an enzyme breaking down a larger molecule into four smaller ones, there are only two shown in the diagram!

Please bear in mind that these reactions are more complicated than the simple scheme above.

They often involve multiple stages and therefore several enzymes.

 

See also Enzymes and Biotechnology (gcse chemistry notes)

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Key points Based on the syllabus-specifications for students taking the AQA, Edexcel and OCR GCSE level biology examinations (~US grades 9-10).

Key ideas on the structure of enzymes and how enzymes work

Enzymes: Structure, Function, and Biological Importance

1. What Are Enzymes?

• Enzymes are biological catalysts—proteins that speed up chemical reactions without being used up or permanently changed.

• They are highly specific, meaning each enzyme usually catalyses one specific reaction or a set of similar reactions.

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2. How Enzymes Work

a) The Lock and Key Mechanism

• The enzyme has a specific active site that has a complementary shape to the substrate (the molecule it acts on).

• When the substrate fits into the active site, this forms an enzyme-substrate complex, allowing the reaction to proceed efficiently.

• This is like a key (substrate) fitting perfectly into a lock (enzyme).

b) The Induced Fit Model

• In some versions of the model, the active site slightly adjusts its shape to fit the substrate more snugly - this is called induced fit.

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3. Lowering Activation Energy

• Activation energy is the minimum amount of energy needed for a chemical reaction to start.

• Enzymes lower this barrier by:

  • Bringing substrates together in the right orientation.

  • Stabilising the transition state of the reaction.

• As a result, reactions occur faster and at lower temperatures than they would without enzymes—essential in living organisms where extreme conditions would be harmful.

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4. Factors Affecting Enzyme Activity

• Temperature: Too high = enzyme denatures; too low = activity slows.

• pH: Each enzyme has an optimum pH.

• Substrate concentration: More substrate usually means faster reactions—up to a point.

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5. Importance in Human Biology

• Enzymes enable:

  • Efficient digestion of food.

  • Cellular respiration to release energy (e.g. ATP production via enzyme-controlled pathways).

  • DNA replication and repair—vital for cell division and inheritance.

  • Detoxification in the liver (e.g. catalase breaks down hydrogen peroxide).

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6. Why Understanding Enzymes Matters

• Helps us grasp:

  • How metabolism is controlled and regulated.

  • Why homeostasis requires enzymes to work within optimal temperature and pH ranges.

  • The basis for medical treatments (e.g. enzyme supplements, inhibitors used in drugs).

  • Biotechnology applications like using enzymes in detergents or food production.

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Summary of learning objectives and key words or phrases

Know an enzyme catalyst lowers the activation energy of a reaction.

Be able to draw a diagram to show and explain the key and lock mechanism of enzyme reactions in biochemistry.

Know that most reactions in cell chemistry are catalysed by enzymes.

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