Digestion of proteins
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of enzyme specificity and digestion. You can test your knowledge on these below.
What enzyme catalyses the hydrolysis of starch into maltose?
Amylase
What enzyme catalyses the hydrolysis of maltose into glucose?
Maltase
What type of reaction breaks down proteins into amino acids?
Hydrolysis reaction
Topic Explainer Video
Intro to Protein Digestion
Proteins are large, complex molecules made up of long chains of amino acids. Since proteins are too large to be absorbed into the bloodstream directly, they must be broken down into their monomers (amino acids) through the action of enzymes.
Enzymes Involved in Protein Digestion
Protein digestion occurs in both the stomach and small intestine and involves different types of enzymes:
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Endopeptidases – Hydrolyse peptide bonds in the middle of the polypeptide chain.
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Example: Pepsin (works in acidic conditions in the stomach).
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Exopeptidases – Hydrolyse peptide bonds at the ends of polypeptide chains, releasing dipeptides or single amino acids.
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Example: Trypsin (produced in the pancreas, works in the small intestine).
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Dipeptidases – Hydrolyse dipeptides into individual amino acids.
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These are membrane-bound enzymes found in the epithelial cells of the small intestine.
Protein Digestion Process
1. Stomach:
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Endopeptidases are released by the stomach lining and work in acidic conditions (optimum pH ~2).
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They hydrolyse peptide bonds in the middle of the polypeptide chain breaking proteins into shorter polypeptides. This means there are more ‘ends’ available for the exopeptidases to act on.
2. Small Intestine:
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Endopeptidases from the pancreas continue breaking down polypeptides.
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Exopeptidases hydrolyse peptide bonds at the ends of polypeptide chains, releasing dipeptides or single amino acids from the ends of peptides.
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Dipeptidases in the epithelial membrane hydrolyse dipeptides into single amino acids, which are then absorbed into the bloodstream.
Absorption of Amino Acids
- Amino acids are absorbed via co-transport with sodium ions into the epithelial cells of the small intestine.
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They are then transported into the bloodstream by facilitated diffusion.
Key Terms
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Hydrolysis: The breaking of bonds using water.
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Polypeptide: A long chain of amino acids linked by peptide bonds.
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Endopeptidase: An enzyme that hydrolyses peptide bonds within the middle of a polypeptide.
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Exopeptidase: An enzyme that hydrolyses peptide bonds at the ends of a polypeptide.
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Dipeptidase: An enzyme that breaks down dipeptides into individual amino acids.
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Co-transport: The movement of molecules across a membrane using a transport protein that also moves another molecule, such as sodium ions.
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Exam Tip
Always refer to the specific enzyme when discussing digestion. Simply saying "proteins are broken down into amino acids" is not enough—mention endopeptidases, exopeptidases, and dipeptidases for full marks. Also, include the hydrolysis of peptide bonds.
No answer provided.
Describe the process of protein digestion and absorption in the human digestive system. (4 marks)
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Endopeptidases hydrolyse peptide bonds in the middle of polypeptide chains to form shorter polypeptides. (1 mark)
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Exopeptidases hydrolyse peptide bonds at the ends of the polypeptide chains, releasing dipeptides and single amino acids. (1 mark)
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Dipeptidases hydrolyse dipeptides into single amino acids. (1 mark)
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Note, the fourth mark is given for ‘hydrolysis of peptide bonds’.
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Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!