PROTEINS – COMPLETE NOTE

Introduction

Proteins are essential biological molecules made of amino acids linked together by peptide bonds.
They are one of the macronutrients needed for growth, repair, and maintenance of the body.

Proteins play important roles in living organisms, such as 

• Building and repairing tissues

• Acting as enzymes and hormones

• Supporting the immune system

• Providing energy when necessary

Structure of Proteins

Proteins have four levels of structure:

1. Primary Structure

• The sequence of amino acids in a polypeptide chain.

• Determines the unique characteristics of the protein.

2. Secondary Structure

• Folding of the polypeptide chain into patterns like:

  • Alpha-helix (α-helix)

  • Beta-pleated sheet (β-sheet)

• Stabilized by hydrogen bonds.

3. Tertiary Structure

• Three-dimensional shape of the protein.

• Determined by interactions among R-groups (side chains) of amino acids.

4. Quaternary Structure

• Present in proteins with more than one polypeptide chain.

• Example: Hemoglobin (made of 4 polypeptide chains).

Composition of Proteins

Proteins are made of carbon (C), hydrogen (H), oxygen (O), and nitrogen (N).
Some proteins also contain sulfur (S), phosphorus (P), or iron (Fe).

Types of Proteins

1. Simple Proteins

   • Made entirely of amino acids.

   • Example: Albumin, Globulin

2. Conjugated Proteins

   • Contain a non-protein group (prosthetic group) in addition to amino acids.

   • Example: Glycoprotein (protein + carbohydrate), Hemoglobin (protein + heme)

Functions of Proteins

 

Function	Example/Explanation
Structural	Collagen in connective tissues, keratin in hair
Enzymatic	Amylase, lipase (catalyze reactions)
Hormonal	Insulin, glucagon (regulate metabolism)
Transport	Hemoglobin (carries oxygen)
Immune	Antibodies (fight infection)
Storage	Ferritin (stores iron)
Energy	Proteins can be broken down for energy if carbs/fats are low

Classification Based on Shape

• Fibrous Proteins: Long, strand-like, structural proteins.

  • Example: Keratin, Collagen

• Globular Proteins: Spherical, functional proteins.

  • Example: Enzymes, Hemoglobin, Antibodies

AMINO ACIDS

Amino acids are the building blocks of proteins. Each amino acid has the general formula:

H₂N-CH(R)-COOH

Where R = side chain unique to each amino acid

Examples of Amino Acids

• Essential amino acids (cannot be made by the body):

  • Lysine, Methionine, Leucine, Valine, etc.

• Non-essential amino acids (can be made by the body):

  • Glycine, Alanine, Serine, etc.

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1. Essential Amino Acids

These cannot be made by the body and must be obtained from the diet.

Amino Acid	3-Letter Code	Source / Notes
Lysine	Lys	Meat, eggs, beans
Methionine	Met	Fish, eggs, nuts
Leucine	Leu	Meat, soy, dairy
Isoleucine	Ile	Meat, eggs, cheese
Valine	Val	Meat, dairy, legumes
Threonine	Thr	Eggs, meat, milk
Phenylalanine	Phe	Dairy, soy, meat
Tryptophan	Trp	Milk, chocolate, turkey
Histidine	His	Meat, poultry, fish

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2. Non-Essential Amino Acids

These can be synthesized by the body.

Amino Acid	3-Letter Code	Source / Notes
Glycine	Gly	Gelatin, collagen
Alanine	Ala	Meat, poultry
Serine	Ser	Soy, dairy, eggs
Cysteine	Cys	Poultry, eggs, garlic
Tyrosine	Tyr	Meat, eggs, dairy
Aspartic acid	Asp	Meat, eggs
Glutamic acid	Glu	Meat, cheese, soy
Proline	Pro	Collagen-rich foods
Asparagine	Asn	Dairy, asparagus
Glutamine	Gln	Meat, eggs

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3. Special Amino Acids

• Selenocysteine (Sec, U) – sometimes called the 21st amino acid; contains selenium.

• Pyrrolysine (Pyl, O) – found in some microorganisms.

4. Key Points

• Amino acids are linked together by peptide bonds to form proteins.

• Each amino acid has:

  • Amino group (–NH₂)

  • Carboxyl group (–COOH)

  • Hydrogen atom (H)

  • Side chain (R group)

• Essential amino acids are important in diet; non-essential can be made in the body.

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Tests for Proteins

1. Biuret Test

   • Add Biuret reagent (NaOH + CuSO₄)

   • Purple color indicates presence of protein

2. Xanthoproteic Test

   • Add concentrated nitric acid

   • Yellow color indicates aromatic amino acids (like tyrosine, tryptophan)

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Summary

• Proteins are polymers of amino acids linked by peptide bonds.

• They have primary, secondary, tertiary, and quaternary structures.

• Proteins serve structural, functional, enzymatic, and regulatory roles.

• Proper protein intake is essential for growth, repair, immunity, and metabolism.

• Keratin = fibrous, structural protein

• Hemoglobin = globular, functional, conjugated protein

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✅ Tips for Students:

• Always link structure → function when answering protein questions.

• Remember fibrous = structural, globular = functional.

• Include examples and chemical/structural details for high marks.

Objective Questions

1. Proteins are polymers of:
A. Sugars
B. Amino acids
C. Fatty acids
D. Nucleotides

2. The bond linking amino acids in a protein is called:
A. Glycosidic bond
B. Peptide bond
C. Hydrogen bond
D. Ionic bond

3. Which of the following is an essential amino acid?
A. Glycine
B. Alanine
C. Lysine
D. Serine

4. Which protein is primarily structural in hair and nails?
A. Hemoglobin
B. Albumin
C. Keratin
D. Globulin

5. A protein that contains a non-protein prosthetic group is called:
A. Simple protein
B. Conjugated protein
C. Fibrous protein
D. Globular protein

6. Which of the following is non-essential?
A. Methionine
B. Leucine
C. Glycine
D. Isoleucine

7. The secondary structure of proteins can be:
A. Alpha-helix or beta-pleated sheet
B. Single chain of nucleotides
C. Fatty acid tail
D. Glycosidic linkage

8. Which test is used to detect proteins?
A. Benedict’s test
B. Biuret test
C. Iodine test
D. Molisch test

9. Hemoglobin is an example of:
A. Fibrous protein
B. Conjugated protein
C. Simple protein
D. Enzyme

10. Which element is found in all amino acids but not in carbohydrates or fats?
A. Carbon
B. Nitrogen
C. Hydrogen
D. Oxygen

Theory Questions

Short Answer

1. Define proteins.

2. Define amino acids.

3. Distinguish between essential and non-essential amino acids.

4. Name four functions of proteins in living organisms.

5. Define peptide bond and explain how it is formed.

Structured Questions

6. Draw and label the general structure of an amino acid.

7. Explain the four levels of protein structure (primary, secondary, tertiary, quaternary).

8. List three examples of simple proteins and three examples of conjugated proteins.

9. State the Biuret test procedure for detecting proteins.

10. Explain the difference between fibrous and globular proteins.

Higher-Level / Application Questions

11. Explain why amino acids are called the building blocks of proteins.

12. Hemoglobin contains iron as part of its structure. What type of protein is hemoglobin and why?

13. A student carries out a Biuret test and gets a violet color. What does this indicate?

14. Explain the importance of essential amino acids in the human diet.

15. Compare the structural features of keratin and hemoglobin in terms of protein classification.