An enzyme is a protein that functions as a biological catalyst
– a substance that speeds up a chemical reaction without being changed by the reaction. 
Examples of enzymes
In this example, the enzyme splits one molecule into two smaller ones ...
... but other enzymes join small molecules together to make a larger one. 
Effect of temperature
As with ordinary chemical reactions, the rate of an enzyme-catalysed reaction increases as the temperature increases.
However, at high temperatures the rate decreases again because ...
... the enzyme becomes denatured and can no longer function as a biological catalyst. 
Lock and key model
Enzymes are folded into complex shapes that allow smaller molecules to fit into them.
The place where these molecules fit is called the active site.
In the lock and key model, the shape of the active site matches the shape of its substrate molecules.
This makes enzymes highly specific – each type of enzyme can catalyse only one type of reaction (or just a few types of reactions). 
How can enzymes be denatured
If the shape of the enzyme changes, its active site may no longer work.
We say that the enzyme has been denatured.
Enzymes can be denatured by ...
- high temperatures
- extremes of pH. 
Effect of pH
Changes in pH alter the shape of an enzyme’s active site.
Different enzymes work best at different pH values.
The optimum pH for an enzyme depends on where it normally works.
For example, intestinal enzymes have an optimum pH of about 7.5, but ...
... stomach enzymes have an optimum pH of about 2. 
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