Test for Protein

What are Proteins?

Proteins are the essential biomolecules, consisting of one or more long chains of amino acid residues. They are large macromolecules, which play a vital role in growth and development, therefore it is called as the building blocks of the body. They are the class of biologically important compounds with high molecular weight.

Proteins are perhaps the most complex organic materials produced in nature. Plants build up their proteins from carbon dioxide, water, and minerals in the presence of sunlight, whereas animals derive it from eating plants.

Among various types of proteins, Albumin and globulin are the two different types of protein present in our blood cells and they play an important role in :

  • Protecting the immune system.

  • Maintaining the oncotic pressure of the blood plasma.

  • Prevents the fluid from leaking out of the blood vessels.

  • Helps in the transportation of ions, lipids, and other hormones.

Also read: Proteins 

Theory

Proteins are the main constituents of all living cells and are the building blocks of life. They play an essential role in the growth and development of cells and tissues. Proteins have a high molecular mass, long chain polymer, composed of α-amino acids and contain carbon, hydrogen, nitrogen, oxygen and sometimes phosphorus and sulfur.

The following are the simple tests carried to find the presence of proteins in the food given samples.

  • Biuret test

The compounds with peptide linkage undergo this test. Proteins are polypeptides of amino acids linked together by peptide bonds. An alkaline solution of protein is treated with a drop of aqueous copper sulfate and a bluish violet color is obtained.

Note: Formation of violet coloration confirms the presence of Proteins.

  • Xanthoproteic test

Proteins on treatment with nitric acid give a yellow or orange color. Concentrated nitric acid is used for nitration. On the treatment of nitric acid, proteins give yellow precipitate which turns to orange color on treatment with alkali.

Note: The appearance of a yellow color solution confirms the presence of proteins.

  • Millions test

Phenolic group of tyrosine of proteins reacts with mercuric sulfate in the presence of sodium nitrite and sulfuric acid to give red color. Millon’s test is given by proteins containing phenolic amino acids. Gelatin does not give this test. First, a white precipitate is formed when proteins are treated with millions reagent and then turns to brick red color on boiling, this confirms the presence of proteins.

Note: The appearance of brick red color solution confirms the presence of proteins.

  • Ninhydrin test

Proteins react with pyridine solution of ninhydrin and change to a colored solution from a deep blue to violet-pink or sometimes even to a red color. Ninhydrin solution is prepared by dissolving 0.1gm of ninhydrin in about 100ml of distilled water. But this solution of ninhydrin is unstable and can be kept for two days.

Note: The appearance of violet color solution confirms the presence of proteins.

Tests to conform to the presence of Protein

Aim

To perform a simple test to identify the presence of proteins in the given plant and animals food sample.

Materials Required

  • Stirrer
  • Dropper
  • Water bath
  • Distilled water
  • Bunsen burner
  • Test tube holder
  • Clean and dried test tubes
  • Food sample – Milk, Apple, Paneer, Butter, Cheese, etc.

If you are using solid sample like paneer, or apple, first crush or grind into a smooth paste using mortar and pestle.

Chemical required

  • Nitric acid
  • Sulfuric acid
  • Sodium nitrite
  • Mercuric sulfate
  • Pyridine solution
  • Ninhydrin reagent
  • Sodium hydroxide
  • Copper sulfate solution
  • Biuret reagent solution or solution of copper sulfate and caustic soda.

Procedure

(a) Biuret Test:

  1. Take a cleaned and dried test tube.
  2. Add the food samples of your choice into the test tubes.
  3. Add 2ml of sodium hydroxide and 5 to 6 drops of copper sulfate solution to it.
  4. Shake the test tube gently to mix the ingredients thoroughly and allow the mixture to stand for 4 – 5 minutes.
  5. If there is the appearance of bluish- violet color indicates the presence of protein.

(b) Xanthoproteic Test:

  1. Take a cleaned and dried test tube.
  2. Add the food samples of your choice into the test tubes.
  3. Add a few drops of concentrated sulfuric acid and shake the test tube.
  4. Heat the test tube gently on a Bunsen burner.
  5. If there is a formation of yellow precipitate then the presence of protein is confirmed.

(c) Millions Test:

  1. Take a cleaned and dried test tube.
  2. Add the food samples of your choice into the test tubes.
  3. Add 2-3 drops of Millon’s reagent and shake well.
  4. Observe the change.
  5. If there is the formation of white precipitate or if the sample changes to brick red on heating then the presence of protein is confirmed.

(d) Ninhydrin Test:

  1. Take a cleaned and dried test tube.
  2. Add the food samples of your choice into the test tubes.
  3. Add 1-2ml of ninhydrin solution to it and shake the test tube.
  4. Boil the mixture and observe the change.
  5. If there is the appearance of blue color then the presence of protein is confirmed.

Apparatus Setup:

Test of Proteins

Observation and Inference

Test Observation
 Biuret test  The appearance of bluish- violet color confirms the presence of Proteins.
 Xanthoproteic test  The appearance of yellow color confirms the presence of proteins.
 Millions test  The appearance of brick red color confirms the presence of proteins.
 Ninhydrin test  The appearance of violet color confirms the presence of proteins.

Results and Discussions

The result is positive.

The given food sample – Milk, Apple, Paneer, Butter, Cheese, etc contains proteins in it.

The protein test is used to measures the total amount of protein present in the given blood samples. Usually, Doctors prescribe this test if a patient is suffering from any of the food deficiency diseases or other disorders like fatigue, weight loss, kidney or liver disease, etc.

Precautions:

  1. Use droppers to take the reagents from the bottle.
  2. Wear lab aprons and hand gloves during the experiment.
  3. While performing the experiment handle chemicals with care.
  4. Refer to the lab manual before beginning with the experiment.
  5. Be careful while handling chemicals and other acidic solutions.

This was a simple experiment which is used to check for the presence of protein and peptide bonds in a protein.

For more additional information about protein, its structure, functions, visit BYJU’S Biology

Important Questions

  • How do you test the presence of Proteins?

The presence of protein is tested by the Biuret test for proteins. The Biurette reagent made of sodium hydroxide and copper (II) sulphate helps in determining the presence of protein in a sample.

  • Why is it important to determine the Proteins?

It is important to identify the proteins to understand their functions. It helps in characterizing the binding of proteins to other proteins, to nucleic acids, or too small molecules. It also plays a major role in drug discovery.

  • Which technique is used to separate proteins according to their size?

Gel electrophoresis is used to separate proteins according to their size.

  • What are Proteins?

Proteins consist of hundreds or thousands of smaller units called amino acids that are attached in long chains to each other. It is possible to combine 20 different types of amino acids to make a protein.

  • What are the two types of Proteins?

The two types of proteins are globular proteins and fibrous proteins.

  • What happens when proteins undergo hydrolysis?

Proteins undergo hydrolysis resulting in the formation of α-amino acids.

  • How do amino acids form a polypeptide together?

Two amino acids are linked together with the loss of a water molecule. A number of amino acids joined by peptide bonds form a chain of polypeptides, and each unit of amino acids in a polypeptide is called a residue.

  • What is the monomer and polymer of Proteins?

A monomer is a molecule that forms larger polymers molecules and is a building block of protein. For instance, an amino acid acts as the protein building blocks.  The polymer of protein is a monomer series.

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