An enzyme is a protein that speeds up a chemical reaction in a living thing without being used up. Enzymes are catalysts that work at around body temperature.
Catalase in the liver breaks hydrogen peroxide down into water and oxygen, 2\mathrm{H_2O_2} \longrightarrow 2\mathrm{H_2O} + \mathrm{O_2}, just as the inorganic catalyst manganese(IV) oxide does.
Boiled liver gives no bubbles, because heating denatures the catalase, which is a protein.
The substance that an enzyme acts on is called its substrate. The substrate binds to a part of the enzyme called the active site, and reacts there.
Only a substrate whose shape fits the active site can bind, like a key fitting a lock, so each enzyme acts only on particular substances. This is called substrate specificity.
After the reaction the products leave, and the enzyme, unchanged, can act on the next substrate molecule.
As the temperature rises, a reaction catalysed by an enzyme speeds up at first, like other reactions.
Above about 40 °C, however, the enzyme is denatured and the reaction slows down. The temperature at which an enzyme works fastest is called its optimum temperature, and for many enzymes in the human body it is about 35 to 40 °C.
With an inorganic catalyst such as manganese(IV) oxide, the reaction keeps getting faster as the temperature rises.
Each enzyme also works fastest at a particular pH, called its optimum pH. Away from this pH the shape of the enzyme changes and it works more slowly.
Pepsin in the stomach works best at about pH 2, amylase in saliva at about pH 7, and trypsin in the small intestine at about pH 8.
The optimum pH of each enzyme suits the place where it works.
The food we eat is broken down by enzymes into small molecules that can be absorbed. Amylase breaks starch down into maltose, and maltase breaks maltose down into glucose.
Sucrase breaks sucrose down into glucose and fructose, and lactase breaks lactose down into glucose and galactose. Pepsin and trypsin break proteins down into peptides, and peptidases break these down into amino acids.
Lipase breaks fats and oils down into fatty acids and monoglycerides. All of these reactions are hydrolysis reactions.