Enzyme are biological catalysts

A catalyst is a substance that speeds up a chemical reaction but remains unchanged at the end.

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How Enzymes Work?

Enzymes that act inside cells are called Intracellular enzyme. e.g Hydrolase and ATPase

Enzymes that act outside of cells are called Extracellular Enzymes

Enzymes are globular proteins - tertiary structure with hydrophilic R groups.

Enzyme molecules have a active site - region where another molecule can bond to.

The molecule that bonds is called the substrate.

The active site and enzyme have a perfect fit as the active site is enzyme specific.

When the substrate bonds with the active site it is called a enzyme substrate complex.

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Lock and Key

Each type of enzyme only reacts with one type of substrate as it has a certain shape. The active site will only mould to one shape.

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Induced Fit

In most enzymes when the substrate fits the active site the enzyme changes slightly so that it can acomodate and hold the substrate in exactly the right position.


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How Enzymes Work?

Which ever way the substrate bonds to the active. It has to be a perfect fit and the enzymes has to be specific for the substrate.

Enzymes may catalyse one substrate into two. But it also may catalyse two substrates into one.

Once the reaction is done the enzyme can leave enzyme and the enzyme is then ready to recieve another substrate.

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Activation Energy

In many reactions, the substrate will not be converted into its products unless it is given a bit of energy. This energy is called activation energy.

Mammals use this method, our body is maintained around 37 degrees. Our body temperaure cant be raised above 40 degress as proteins will denature.

Enzyme lower the activation energy, they do the by holding the subtrate in a way they react more easily. Reactions catalysed by enzymes take place rapidly at a much lower temperature then they would without them.

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Course Of A Reaction

Reaction begins very fast. As soon as the enzyme and substrate are mixed the enzyme are turning the substrated into products. The amount of products made in the first mins is very large.

This is because at the start the substrate molecules are in excess. They bind with the enzyme at a fast speed. At the start virtually every enzyme is bonded to a substrate. The rate of the reaction depends on how many enzymes there are and the speed at which they can catalyse the substrate.

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Course Of A Reaction

The graph then starts to have a less steep gradient as the rate of reaction slows down.

As more and more substrate is converted into products, the substrates stop becoming in excess and are starting to be used up. There is fewer and fewer substrates to bind to the active site of a enzyme. Enzyme molecules will be waiting for a substrate to bind to itself.

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Course Of A Reaction

At the end, the graph levels off as the reaction stops, there is no substrates to react with the enzyme.

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Effect of enzyme concentration

The higher the conentration of the enzyme the more enzymes there are for the substrate to bind to.

However there will become a point however much you increase the enzyme concentration the rate wont change because all of the substrates have been turned into prouducts.

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Effect of substrate concentration

The higher the substrate concentration the higher the chance of the substrate molecules colliding with a enzyme. However there will become a point where every enzyme active site is active. If any more substrate is added there will be a back log. If the enzyme is working at its maximum possible rate this is called "V max"

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Effect of temperature on enzyme activity

At low temperatures the substrate molecules dont have much kinetic energy and dont move around very fast. As the temperature increases the substrates and enzymes get more kinetic energy and move around a lot faster. This therefore increase the amount of collisions of the substrate and active site. They also have more energy so have sufficient activation energy.

After the optimum temperature the bonds are broken in the enzyme and the protein becomes denatured.

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Effect of pH on enzyme activity

In a acidic solution (low pH)  there is a high concentration of H+ ions and a lower concentration of OH- ions. This will be the opposite for a alkaline solution (high pH).

As two ions are attracted to each other, the difference when they do meet weakens the intermolecular bond and change enzyme shape making it difficult for the substrate to fit.

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Enzyme Inhibitors

A substance that slows down orr stops a enzyme is called a inhibitor.

Competitive Inhibitor

If the inhibitor binds only briefly to the site, then there is competition between itself and the substrate.

If there is more substrate than the inhibitor the substrate can bind in its usual way and enzyme function is hardly affected. However if substrate concentration falls or inhibitor concentration rises the inhibitor is more likely to bind to the empty site. 

It is reversible as it can leave and join the enzyme.


If a molecule can bind to a different part of the enzyme, this can cause problems with the active site and disform it. This means it may struggle to fit the substrate.

It can be reversible and irreversible

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Competitive and Non-Competitive Inhibitors


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Inhibitor graphs

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Cofactors and Coenzymes

Most enzymes require the presence of another substance in order to function. These substance are called cofactors and coenzymes.

Cofactors - simple molecules or inorganic molecules

Coenzyme - large molecules

Both work by binding to the enzyme, the slighlty alter the shape so it bind more effectively with its substrate and sometimes they help the enzyme to transfer a particular group of enzyme from one molecule to the other.


Chloride enzyme act as cofactors, bind with amaylase slightly changing its shape making it easier for starch to bind.

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