Enzymes are protein molecules that function in your body as catalysts; that is, they speed up the rate of a biological reaction while they themselves remain unchanged. They are active at a pH, or level of acidity, specific to each individual enzyme. Your digestive system, from your mouth to your large intestine, operates at different pH levels at different points of the process. Therefore, your digestive enzymes are active at different pH levels, with pepsin able to work in an acidic environment and the neutral enzymes requiring neutral conditions.
Digestion
The digestive process begins in your mouth, at a neutral pH, as you chew and swallow food. The pH drops abruptly as the food reaches your stomach, where highly acidic gastric juice continues the digestive process. As food particles move from your stomach to your small intestine, they become neutralized as they mix with a bicarbonate buffer secreted from your pancreas. The digestive enzyme active in your stomach is pepsin, while the neutral enzymes include salivary amylase, pancreatic amylase, pancreatic lipase and pancreatic proteases.
Pepsin
Pepsin is a protein-cleaving molecule. When the cells lining your stomach secrete gastric juice in the presence of food, the inactive form of pepsin, called pepsinogen, changes to the active form. Then, in the acidic environment of your stomach, your food protein denatures, or unfolds, allowing activated pepsin to attack the bonds holding the amino acids of the protein together. Pepsin snips the protein strand into smaller pieces called peptides. Pepsin is most active at a pH between 1.8 and 3.5, according to Colorado State University. It loses activity at pH 5 and becomes completely inactive once it reaches the neutral pH of your small intestine.
Neutral Enzymes
Unlike pepsin, most digestive enzymes are active at a neutral pH of about 7. These neutral enzymes include amylase, which is secreted by both your salivary glands and your pancreas and digests starch; lipase, secreted by your pancreas to digest fat; and a variety of proteases, secreted by your pancreas and small intestine for the purpose of breaking down peptides into amino acids. Similar to pepsin, the pancreatic proteases exist in an inactive, precursor form to protect the cells of your digestive system from random protein degradation. The presence of food in your small intestine signals specific enzymes to activate your pancreatic proteases to continue the digestion process.
Considerations
In addition to pepsin, another digestive enzyme known as chymosin works in the low pH of gastric juice. However, this enzyme is only present in young mammals, not adults. Other neutral digestive enzymes secreted by the pancreas include elastase, gelatinase, deoxyribonuclease and ribonuclease, although these play a lesser role in the digestion of your food than other neutral enzymes.
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