Amino Acids

Proteins are the most abundant organic molecules in the living system. They occur in every part of the cell and constitute about 50% of the cellular dry weight. Proteins form the fundamental basis of the structure and function of life.

Amino Acids

• Amino acids are the structural units (monomers) of proteins.
• An amino acid comprises two functional groups—amino (–NH₂), and carboxyl (–COOH). They also contain a hydrogen atom and a side chain (R) linked to the carbon atom.
• Amino acids differ from each other in their side chains.

General Structure

Every amino acid has a basic structure consisting of:

• Central carbon atom: This is often referred to as the alpha-carbon.
• Amino group: A nitrogen-containing group (-NH2). (Basic group)
• Carboxyl group: A carboxylic acid group (-COOH). (Acidic group)
• Hydrogen atom: Attached to the alpha-carbon.
• Sidechain (R group): Each amino acid’s unique part determines its properties and function.

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Classification of amino acids

• Based on the variable side chain
• Based on the nutritional requirements of amino acids
• Based on the metabolic products of amino acids
• Based on the nature or polarity of the side chain.

1.  Based on the variable side chain

Amino acids with aliphatic side chains (GAVLI)

1. Glycine
2. Alanine
3. Valine
4. Leucine
5. Isoleucine

Amino acids containing hydroxyl (–OH) groups (ST)

6. Serine
7. Threonine

Sulphur-containing amino acids (CM)

8. Cysteine
9. Methionine

Acidic amino acids and their amides (GAGA)

10. Aspartic acid
11. Asparagine
12. Glutamic acid
13. Glutamine

Basic amino acids (HAL)

14. Lysine
15. Arginine
16. Histidine

Aromatic amino acids (PTT)

17. Phenylalanine
18. Tyrosine
19. Tryptophan

Imino acid

20. Proline

2. Based on nutritional requirement

• Essential – Cannot be synthesized in the body, supplied from diet. Examples are phenylalanine, Valine, Tryptophan, Threonine, Isoleucine, Methionine, Histidine, Arginine, Lysine, and (PVT TIM HALL)
• Semi essential – Growing children required them in the food, but not essential in the adults. Example –
• Nonessential – This can be synthesized in the body, hence not required in the diet. Example – All the other 10 amino acids.

3. Based on metabolic fate

• Ketogenic – Amino Acids that are converted into ketone bodies.

Example – Leucine, Lysine.

• Glucogenic – Amino Acids that enter into glucose.

Example – All the other 14 amino acids

• Both glucogenic and ketogenic – Both are converted into glucose and ketone bodies. Examples are phenylalanine, Isoleucine, Tyrosine, and Tryptophan.

4. Based on the polarity

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Biologically important compounds formed by amino acids

SN.	Amino acid	Biologically important compound
1.	Tyrosine	Hormones, e.g., adrenaline and thyroxine. Skin pigment, e.g., melanin
2.	Glycine, arginine and methionine	Creatine
3.	Glycine and cysteine	Bile salts
4.	Glycine	Heme
5.	Aspartic acid and glutamic acid	Pyrimidine bases
6.	Glycine, aspartic acid and glutamine	Purine bases
7.	β-alanine	Coenzyme-A
8.	Tryptophan	Vitamin, e.g., niacin

Important of amino acids

1. Protein Synthesis
2. Enzyme Function
3. Hormone Synthesis
4. Neurotransmitters
5. Energy Source
6. Immune Function
7. Biosynthesis of Other Molecules

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Isoelectric pH

The isoelectric point (pI) of a molecule, particularly an amino acid or protein, is the pH at which the molecule carries no net electrical charge.

Each amino acid contains at least two ionizable groups:

1. Amino group (-NH₃⁺)
2. Carboxyl group (-COO⁻)

• The amino acid is protonated at low pH (acidic) and has a positive charge.
• The amino acid is deprotonated at high pH (basic) and has a negative charge.
• At the pI, the amino acid has no net charge, which makes it electrically neutral.

Importance of the Isoelectric Point

1. Protein Purification
2. Solubility
3. Biological Function

Various ionic forms of amino acid at different pH

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Zwitterion

A zwitterion is a molecule with both a positive and a negative charge but is neutral overall.

Example: Amino Acids

In water, amino acids can become zwitterions because:

• The amino group (-NH₂) can accept a proton and become positively charged (NH₃⁺).
• The carboxyl group (-COOH) can lose a proton and become negatively charged (-COO⁻).

At a certain pH (near neutral), these charges balance out, making the amino acid a zwitterion—meaning it has both charges but no overall charge.

Importance of Zwitterions

• Buffering Capacity
• Solubility
• Protein Structure

Sorenson’s titration curves of valine

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Peptide bond

• A peptide bond is a covalent bond between two amino acids during protein synthesis.
• It is a key linkage in the structure of proteins, holding together long chains of amino acids called polypeptides.
• A condensation reaction forms a peptide bond, where one amino acid’s carboxyl group (-COOH) reacts with another amino acid’s amino group (-NH₂).

Breaking of a Peptide Bond

• Hydrolysis is the reverse process of forming a peptide bond, where water is added to break the bond, releasing individual amino acids.
• This occurs during digestion when proteins are broken down into amino acids by enzymes like proteases.

Importance of Peptide Bonds

• Protein chemistry Structure: Peptide bonds link amino acids in specific sequences to form polypeptides, which fold into functional proteins.
• Enzymatic Function: As seen in digestive enzymes or protein synthesis, many enzymes work by cleaving or forming peptide bonds.
• Biological Signaling: Peptides (short chains of amino acids) can act as hormones or signalling molecules, such as insulin.