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Enzymes: 2. How do enzymes work as catalysts? Describing and explaining the 'lock and key' reaction model

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There are various sections to work through, after 1 they can be read and studied in any order.

Sub-index of biology notes on enzymes and digestion


(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

(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.

 

Key and lock mechanism for producing smaller molecules from larger ones

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

 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

C6H12O6(aq) ===> 2C2H5OH(aq) + 2CO2(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)


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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(Enzymes are also dealt with in my GCSE chemistry notes chemistry - biotechnology)

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