Proteins are polypeptides, meaning they are long linear chains of amino acids connected by peptide bonds. Think of them as beads (amino acids) arranged in a specific sequence to form a functional molecule.
Unlike carbohydrates or lipids, proteins are heteropolymers, which means they are made up of different types of monomers (20 amino acids). This diversity is what gives proteins their vast functional variety.
Chemically, proteins contain → Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), Sometimes Sulphur (S)
Sources of Proteins
Proteins can be obtained from:
Animal sources: meat, fish, eggs, milk
Plant sources: pulses, beans, nuts, seeds, soy
Amino Acids – The Building Blocks
Amino acids are classified into two categories:
Essential Amino Acids These cannot be synthesized by the body and must be obtained through diet. Example: lysine, leucine, tryptophan, etc.
Non-Essential Amino Acids These can be synthesized by the body. Example: alanine, glutamine
Key Properties of Proteins
Solubility
Globular proteins → water-soluble (e.g., enzymes)
Fibrous proteins → insoluble (e.g., keratin)
Denaturation When proteins lose their structure due to heat, pH changes, or chemicals, they lose their function. Example: boiling egg → protein solidifies.
Classification of Proteins
1. Based on Shape
Type
Characteristics
Solubility
Examples
Fibrous Proteins
Long, thin, thread-like; arranged in parallel chains or sheets; provide strength and durability (high tensile strength)
Insoluble in water (non-polar nature)
Collagen, Keratin, Elastin
Globular Proteins
Compact, spherical; folded into a three-dimensional structure; functionally active
Soluble in water (hydrophilic amino acids on surface)
Haemoglobin, Insulin, Antibodies
2. Based on Constitution
Type
Composition
Description
Examples
Simple Proteins
Only amino acids
Yield only amino acids upon hydrolysis
Albumin, Globulin, Prolamins
Conjugated Proteins
Amino acids + Prosthetic group
Contain non-protein components
See subtypes below
Derived Proteins
From simple/conjugated proteins
Formed by action of heat, enzymes, or chemicals
Proteases, Peptones, Peptides
A prosthetic group is a non-protein component that is tightly bound to a protein and is essential for that protein’s biological function.
Subtypes of Conjugated Proteins
Subtype
Prosthetic Group
Description
Examples
Glycoproteins
Carbohydrate
Protein + carbohydrate group
Mucins (in mucus)
Lipoproteins
Lipid
Protein + lipid
High-Density Lipoprotein (HDL)
Metalloproteins
Metal ions
Protein + metal ion
Haemoglobin (iron)
Phosphoproteins
Phosphate group
Protein + phosphate
Casein (milk)
Chromoproteins
Pigment
Protein + coloured pigment
Haemoglobin (heme)
Nucleoproteins
Nucleic acids
Protein + DNA/RNA
Ribosomes, Chromatin
3. Based on Nature of Molecules
Type
Characteristics
Examples
Acidic Proteins
Rich in acidic amino acids
Glutamic acid, Aspartic acid
Basic Proteins
Rich in basic amino acids
Lysine, Arginine
4. Based on Nutritional Value
Type
Definition
Sources/Examples
Complete Proteins
Contain all essential amino acids in adequate proportions
Protein combining involves combining two or more incomplete protein sources together to provide adequate amounts of all the essential amino acids. E.g., nuts or seeds with whole grains and whole grains with beans.
Functions of Proteins
S. No.
Function
Explanation
Examples
1
Structural Support
Provide structural framework and strength to cells and tissues
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