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
Simple Proteins
Made entirely of amino acids.
Example: Albumin, Globulin
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.
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 |
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 |
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.
Tests for Proteins
Biuret Test
Add Biuret reagent (NaOH + CuSO₄)
Purple color indicates presence of protein
Xanthoproteic Test
Add concentrated nitric acid
Yellow color indicates aromatic amino acids (like tyrosine, tryptophan)
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
Keratin = fibrous, structural protein
Hemoglobin = globular, functional, conjugated protein
✅ 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
Define proteins.
Define amino acids.
Distinguish between essential and non-essential amino acids.
Name four functions of proteins in living organisms.
Define peptide bond and explain how it is formed.
Structured Questions
Draw and label the general structure of an amino acid.
Explain the four levels of protein structure (primary, secondary, tertiary, quaternary).
List three examples of simple proteins and three examples of conjugated proteins.
State the Biuret test procedure for detecting proteins.
Explain the difference between fibrous and globular proteins.
Higher-Level / Application Questions
Explain why amino acids are called the building blocks of proteins.
Hemoglobin contains iron as part of its structure. What type of protein is hemoglobin and why?
A student carries out a Biuret test and gets a violet color. What does this indicate?
Explain the importance of essential amino acids in the human diet.
Compare the structural features of keratin and hemoglobin in terms of protein classification.