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Enzymes speed up chemical reactions by acting as biological catalysts. They catalyse metabolic reactions in your body, eg digestion and respiration.

Enzymes are proteins with an active site which has a specific shape. The active site is the part of the enzyme where the substrate molecules bind to.
Enzymes are highly specific due to their tertiary structure.

In a chemical reaction, a certain amount of energy needs to be supplied to the chemicals before the reaction will start. This is called the activation energy. It's often provided as heat.
Enzymes lower the activation energy needed, often making reactions happen at a lower temperature than they could without an enzyme.
This speeds up the rate of reaction.
When a substrate fits into an enzyme's active site it forms an enzyme-substrate complex. It's this that lowers the activation energy because if two substrate molecules need to be joined together, being attached to the enzyme holds them close together, reducing any repulsion between molecules so they can bond more easily. Also, if the enzyme is catalysing a breakdown reaction, fitting into the active site puts a strain on bonds in the substrate, so the substrate molecule breaks up more easily.

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The lock and key, and induced fit models

The lock and key model:
Enzymes only work with the substrates that fit their active site. Scientists came up with the 'lock and key' model to explain how they work.
This is where the substrate fits into the active site as their shapes are complementary.

The induced fit model:
However, new evidence suggested that the enzyme-substrate complex changed shape slightly to complete the fit. This locks the substrate even more tightly to the enzyme.
The substrate doesn't only have to be the right shape to fit the active site but it has to make the active site change shape in the right way as well, which is why enzymes are so specific.

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Enzyme properties are related to their tertiary structure. 
They are very specific and usually only catalyse one reaction. This is because only one substrate will fit into the active site. The active site's shape is determined by the enzyme's tertiary structure, which is determined by it's primary structure.
If the substrate doesn't match the active site, the reaction won't be catalysed.

If the tertiary structure of a protein is altered in any way, the shape of the active site will change. This means that the substrate won't fit into the active site so the enzyme cannot carry out it's function, so the enzyme is denatured.

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Rate of reaction

You can measure the rate of reaction by:
Measuring the amount of product produced at different times during the experiment.
Measuring the amount of substrate left at different times during the experiment.

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Enzyme activity

Factors affecting enzyme activity:

Temperature - the rate of an enzyme-controlled reaction increases when the temperature increases. More heat means more kinetic energy so molecules move faster. However if the temperature gets too high, the vibration of the enzyme breaks som of the bonds that hold the enzyme in shape. This changes the shape of the active site so the substrate doesn't fit with the enzyme any more, so it is denatured and the rate of reaction decreases before stopping.

pH - all enzymes have an optimum pH value. Most human enzymes work best at pH7 but there are exceptions, such as pepsin. Above and below the optimum pH, the H+ and OH- ions found in acids and alkalis disrupt the ionic bonds and hydrogen bonds that hold the tertiary structure in place. This denatures the enzyme.

Substrate concentration - the higher the substrate concentration, the faster the reaction as a collision is more likely. This is only true up to a saturation point as after that the enzymes' active sites are full and adding more makes no difference.

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Enzyme inhibitors

Competitive inhibitors - they have a similar shape to that of substrate molecules so compete with the substrate to bind to the active site, though no reaction takes place as they just block the active site so that no substrate molecules can fit in it.

Non-competitive inhibitors - they bind to the enzyme away from its active site. This causes the active site to change shape so the substrate molecules can no longer bind to it. As the enzymes have a different shape, increasing the substrate concentration will have no effect.

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