Acid Hydrolysis of Proteins Chemistry Tutorial

Key Concepts

• Proteins are polypeptides, polymers made up of a large number of amino acid units joined together by peptide bonds (amide links).
• Hydrolysis refers to a reaction with water.
• Acid hydrolysis refers to a hydrolysis reaction occurring under acidic conditions.
• Acid hydrolysis of a protein breaks the polymer down into its constituent amino acids.

  protein

  H2O, H+ →

  amino acids

• During hydrolysis water (H₂O) adds across the peptide bond linking the amino acid residues together in the protein:

  R1 |   O ||   H |   R2 |   O ||   H2N− CH − C − N − CH − C −OH

  +

  HO-H

  H+(aq) →

  R1 |   O ||   H2N− CH − C −OH

  +

  H |   R2 |   O ||   H− N − CH − C −OH

• Digestion of protein in the human body involves breaking the polymer down into its constituent amino acids using enzyme catalysed hydrolysis reactions.

  protein

  H2O, enzyme →

  amino acids

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Acid Hydrolysis of Proteins

A protein is a polypeptide, a polymer made up of many amino acid monomer units, referred to as residues or units, which are joined together by peptide bonds (amide links).
The sequence of amino acid units (amino acid residues) is referred to as the primary structure of the protein.
Each amino acid has a 3 letter symbol, so we might represent a section of protein with coloured boxes for the amino acid units joined by a line representing the peptide bond (amide link):

We can break the peptide bonds (amide links) joining the amino acid units together by using acidified water in an acid hydrolysis reaction.
This disrupts the primary structure of the protein and the protein breaks down into smaller pieces, eventually resulting in many amino acids.
We can represent this process of acid hydrolysis of a section of protein as:

The structural formula for this section of protein is shown below:

During acid hydrolysis, water (H₂O) adds across the peptide bond (-CO-NH-) resulting in the formation of carboxyl functional groups (-COOH) and amine functional groups (-NH₂) as shown below:

When humans eat food containing proteins, like meat, fish and eggs, the proteins are broken down into amino acids using various enzymes collectively called peptidases or proteases.

The enzyme catalysed hydrolysis of a section of protein can be represented as shown below:

Different enzymes catalyse the hydrolysis of the peptide bond (amide link) between different amino acid residues.

For example, the enzyme trypsin helps break the peptide bond when the amino acid residues arginine or lysine are contributing the -CO- functional group to the peptide bond.

NH2 |         CH2 |         CH2 |         CH2         |     H |   CH2 | O || − N − C − C       | H	−	|       H |   CH2 |   O ||   N − C − C −     | H	H2O, trypsin →	NH2 |           CH2 |           CH2 |           CH2           |       H |   CH2 | O ||   H− N − C − C −OH       | H	+	|         H |   CH2 |   O ||   H− N − C − C −OH       | H
lysine contributes CO to peptide bond			H2O, trypsin →	lysine	+	phenylalanine

If the -CO- functional group of the peptide bond is contributed by phenylalanine, tyrosine or tryptophan, then the enzyme chymotrypsin is used to help break the peptide bond.

|       H |   CH2 |   O || − N − C − C       | H	−	NH2 |       CH2 |       CH2 |       CH2       |   H |   CH2 | O || N − C − C−     | H	H2O, chymotrypsin →	|         H |   CH2 |   O ||   H− N − C − C −OH       | H	+	NH2 |           CH2 |           CH2 |           CH2           |       H |   CH2 | O ||   H− N − C − C −OH       | H
phenylalanine contributes CO to peptide bond			H2O, chymotrypsin →	phenylalanine	+	lysine

Digestion of Proteins in the Human Body

Many foods that we eat contain proteins: meat, fish, eggs and beans all contain protein.

Chewing food in our mouth breaks large pieces of food into smaller pieces.

In the stomach and small intestine, enzyme catalysed hydrolysis reactions break proteins down into amino acids.

These amino acids undergo condensation polymerisation reactions to produce the proteins our body needs.
For example, amino acids are used to make the proteins found in the tissues of our bodies (skin, hair, muscle, tendons), enzymes, antibodies, and the haemoglobin in blood used to transport gases.

If we produce an excess of amino acids from the break down of proteins, these can be broken in the liver to provide glucose for immediate energy, or, fat for storage.
Waste products produced are carbon dioxide (CO₂) which is expelled via the lungs, water (H₂O) and ammonia (NH₃) which can be converted to urea (H₂NCONH₂) and expelled in urine.