Amino Acids: Structure, Types, and Functions

Amino acids are the building blocks of proteins, essential for various biological functions. They are organic compounds composed of carbon (C), hydrogen (H), oxygen (O), nitrogen (N), and sometimes sulfur (S).

1. Structure of Amino Acids

Each amino acid consists of:

1. A central carbon atom (α-carbon)
2. Amino group (-NH₂) (basic)
3. Carboxyl group (-COOH) (acidic)
4. Hydrogen atom (-H)
5. R-group (Side chain) – Determines the identity and properties of the amino acid

General Structure:

   H     O 

   |     || 

H₂N—C—C—OH 

   | 

   R (Side Chain) 

2. Types of Amino Acids

Amino acids are classified based on essentiality and chemical properties:

A. Based on Essentiality

1. Essential Amino Acids (Must be obtained from diet)
• Histidine (H)
• Isoleucine (I)
• Leucine (L)
• Lysine (K)
• Methionine (M)
• Phenylalanine (F)
• Threonine (T)
• Tryptophan (W)
• Valine (V)
2. Non-Essential Amino Acids (Synthesized in the body)
• Alanine (A)
• Asparagine (N)
• Aspartic acid (D)
• Glutamic acid (E)
3. Conditionally Essential Amino Acids (Essential under certain conditions, e.g., stress, illness)
• Arginine (R)
• Cysteine (C)
• Glutamine (Q)
• Glycine (G)
• Proline (P)
• Serine (S)
• Tyrosine (Y)

B. Based on Chemical Properties

1. Non-Polar (Hydrophobic) Amino Acids
• Glycine (G)
• Alanine (A)
• Valine (V)
• Leucine (L)
• Isoleucine (I)
• Methionine (M)
• Proline (P)
• Phenylalanine (F)
• Tryptophan (W)
2. Polar (Hydrophilic) Amino Acids
• Serine (S)
• Threonine (T)
• Tyrosine (Y)
• Cysteine (C)
• Asparagine (N)
• Glutamine (Q)
3. Acidic (Negatively Charged) Amino Acids
• Aspartic acid (D)
• Glutamic acid (E)
4. Basic (Positively Charged) Amino Acids
• Lysine (K)
• Arginine (R)
• Histidine (H)

3. Functions of Amino Acids

✅ Protein Synthesis – Form peptides and proteins
✅ Enzyme Production – Act as catalysts in biochemical reactions
✅ Hormone Synthesis – Build signaling molecules like insulin
✅ Neurotransmitter Synthesis – Produce serotonin, dopamine
✅ Immune Function – Help create antibodies
✅ Energy Production – Serve as a source of ATP under extreme conditions

4. Peptide Bond Formation

• Amino acids link together via peptide bonds to form proteins.
• A peptide bond is a covalent bond formed between the carboxyl group of one amino acid and the amino group of another with the loss of water (condensation reaction).

Peptide Bond Formation Reaction:

    Amino Acid 1      Amino Acid 2 

     NH₂—CHR—COOH  +  NH₂—CHR—COOH 

       |                     | 

       H₂O is removed → Peptide Bond (-CO-NH-) formed  

Amino acids are vital for life, playing roles in protein synthesis, metabolism, and cellular function. Understanding their classification and functions helps in fields like biochemistry, nutrition, and medicine.