Amino acids

By /

Introduction

  • Amino acids are the basic structural units of proteins and are essential for the formation of all proteins present in living organisms.
  • They are often called the building blocks of life because enzymes, hormones, antibodies, transport proteins, and structural proteins are all composed of amino acids.
  • In biochemistry, amino acid chemistry is important because the physical, chemical, and biological properties of proteins depend on the nature of their amino acids.
  • Chemically, each amino acid contains an amino group (-NH₂), a carboxyl group (-COOH), a hydrogen atom, and a variable side chain (R group) attached to the same alpha carbon (α-carbon).
  • The R group or side chain determines the identity, chemical behavior, polarity, and biological function of each amino acid.
  • Amino acids possess both acidic and basic groups, so they behave as amphoteric molecules, meaning they can act as either acids or bases depending on the pH.
  • Because of this amphoteric nature, amino acids exist as zwitterions in aqueous solution, carrying both positive and negative charges simultaneously.

Basic Structure of Amino Acids

The general formula of an amino acid is:

Where:

  • NH₂ = amino group
  • COOH = carboxyl group
  • R = side chain
  • CH = alpha carbon

The alpha carbon is attached to four different groups:

  • Amino group
  • Carboxyl group
  • Hydrogen atom
  • Side chain (R group)

Classification of Amino Acids

A. Based on Nutritional Requirement

A. Essential Amino Acids

These cannot be synthesized by the human body and must be obtained from diet:

  • Leucine
  • Isoleucine
  • Valine
  • Lysine
  • Methionine
  • Phenylalanine
  • Threonine
  • Tryptophan
  • Histidine

B. Non-Essential Amino Acids

Synthesized within the body:

  • Alanine
  • Aspartate
  • Glutamate
  • Serine
  • Glycine

C. Semi-essential Amino Acids

Required during growth or special conditions.

  • Arginine
  • Histidine

B. Classification Based on Side Chain Polarity

A. Nonpolar (Hydrophobic) Amino Acids

These amino acids are insoluble in water and usually found inside proteins.

  • Glycine
  • Alanine
  • Valine
  • Leucine
  • Isoleucine
  • Methionine
  • Phenylalanine
  • Tryptophan
  • Proline

B. Polar Uncharged Amino Acids

These contain polar groups and can form hydrogen bonds.

  • Serine
  • Threonine
  • Asparagine
  • Glutamine
  • Tyrosine
  • Cysteine

C. Acidic Amino Acids

These contain extra carboxyl groups.

  • Aspartic acid
  • Glutamic acid

D. Basic Amino Acids

These contain extra amino groups.

  • Lysine
  • Arginine
  • Histidine

C. Amino Acid Classification Based on Structure

Amino acids can be classified according to the structure of their side chain (R group). The nature of the side chain determines their chemical properties, polarity, and biological functions.

1. Aliphatic Amino Acids

These amino acids contain open-chain hydrocarbon side chains.

  • Glycine
  • Alanine
  • Valine
  • Leucine
  • Isoleucine

Characteristics:

  • Mostly hydrophobic
  • Nonpolar in nature
  • Important in protein core formation

2. Aromatic Amino Acids

These contain an aromatic benzene or indole ring in the side chain.

  • Phenylalanine
  • Tyrosine
  • Tryptophan

Characteristics:

  • Absorb ultraviolet light
  • Important in protein spectroscopy
  • Participate in hormone and neurotransmitter synthesis

3. Hydroxy Amino Acids

These contain a hydroxyl (−OH) group in the side chain.

  • Serine
  • Threonine
  • Tyrosine

Characteristics:

  • Polar in nature
  • Participate in hydrogen bonding
  • Important in phosphorylation reactions

4. Sulfur-Containing Amino Acids

These contain sulfur atoms in their structure.

  • Cysteine
  • Methionine

Characteristics:

  • Cysteine forms disulfide bonds
  • Important for protein stability
  • Methionine acts as a methyl group donor

5. Acidic Amino Acids and Their Amides

Acidic Amino Acids

Contain an extra carboxyl group.

  • Aspartic acid
  • Glutamic acid

Amides of Acidic Amino Acids

Formed when the carboxyl group is converted into an amide group.

  • Asparagine
  • Glutamine

Characteristics:

  • Polar compounds
  • Important in nitrogen transport

6. Basic Amino Acids

These contain additional amino or nitrogen-containing groups.

  • Lysine
  • Arginine
  • Histidine

Characteristics:

  • Positively charged at physiological pH
  • Important in nucleic acid binding
  • Participate in enzyme active sites

7. Heterocyclic Amino Acids

These contain a ring structure with atoms other than carbon.

  • Proline
  • Histidine
  • Tryptophan

Characteristics:

  • Influence protein folding
  • Proline produces bends in protein chains

8. Imino Acid

Proline is often classified separately as an imino acid because it contains an imino (−NH−) group instead of a primary amino group.

Characteristics:

  • Cyclic structure
  • Restricts protein flexibility
  • Important in collagen structure

D. Amino Acid Classification Based on Their Metabolic Fate

Based on their metabolic fate, amino acids are classified into:

  1. Glucogenic amino acids
  2. Ketogenic amino acids
  3. Both glucogenic and ketogenic amino acids

1. Glucogenic Amino Acids

These amino acids are metabolized into compounds that can be converted into glucose through gluconeogenesis.

Their carbon skeletons usually form:

  • Pyruvate
  • Oxaloacetate
  • α-ketoglutarate
  • Succinyl-CoA
  • Fumarate

Examples

  • Alanine
  • Glycine
  • Serine
  • Cysteine
  • Aspartate
  • Asparagine
  • Glutamate
  • Glutamine
  • Histidine
  • Methionine
  • Valine
  • Arginine
  • Proline

Characteristics

  • Produce glucose during fasting
  • Help maintain blood glucose level
  • Important during starvation and diabetes mellitus

2. Ketogenic Amino Acids

These amino acids are degraded into compounds that form ketone bodies or fatty acids.

They produce:

  • Acetyl-CoA
  • Acetoacetate

Purely Ketogenic Amino Acids

  • Leucine
  • Lysine

Characteristics

  • Cannot produce net glucose
  • Form ketone bodies
  • Important during prolonged fasting

3. Both Glucogenic and Ketogenic Amino Acids

These amino acids produce intermediates that can participate in both glucose and ketone body formation.

Examples

  • Isoleucine
  • Phenylalanine
  • Tyrosine
  • Tryptophan
  • Threonine

Characteristics

  • Produce both glucose and ketone bodies
  • Participate in multiple metabolic pathways

Properties of Amino Acids

Amino acids possess unique physical and chemical properties because they contain both an amino group (−NH₂) and a carboxyl group (−COOH). These properties are important in protein structure, metabolism, and biochemical reactions.

The properties of amino acids can be divided into:

  1. Physical properties
  2. Chemical properties

I. Physical Properties of Amino Acids

1. Color and Appearance

  • Most amino acids are:
    • Colorless
    • Crystalline solids
  • Usually have a sweet or tasteless nature

2. Solubility

  • Amino acids are generally:
    • Soluble in water
    • Insoluble in organic solvents like ether and chloroform

Reason:

  • Presence of polar amino and carboxyl groups

3. High Melting Point

Amino acids possess high melting points because they exist as zwitterions.

+H3N−CH(R)−COO−

Characteristics:

  • Strong ionic interactions
  • High thermal stability

4. Optical Activity

  • Most amino acids are optically active due to the presence of an asymmetric carbon atom.
  • They exist in:
    • D-form
    • L-form
  • Proteins mainly contain L-amino acids.
  • Glycine is optically inactive because it lacks a chiral carbon.

5. Amphoteric Nature

Amino acids act as both:

  • Acids
  • Bases

Therefore, they are called amphoteric compounds.

They can:

  • Donate H⁺ ions
  • Accept H⁺ ions

6. Buffer Action

Amino acids resist sudden changes in pH because they contain both acidic and basic groups.

Importance:

  • Maintenance of body pH
  • Protein stability

II. Chemical Properties of Amino Acids

1. Formation of Zwitterions

In aqueous solution, amino acids exist as dipolar ions called zwitterions.

H2N  −  CH(R)  −  COOH  ⇌  +H3N  −  CH(R)  −  COO−

Characteristics:

  • Electrically neutral
  • Responsible for ionic behavior

2. Reaction with Acids

The amino group reacts with acids forming salts.

RNH2+HCl→RNH3Cl

3. Reaction with Bases

The carboxyl group reacts with bases.

RCOOH+NaOH→RCOONa+H2O

4. Peptide Bond Formation

Two amino acids combine through a peptide bond.

−COOH+−NH2→−CO−NH−+H2O

Importance:

  • Formation of proteins
  • Formation of peptides

5. Ninhydrin Reaction

Amino acids react with ninhydrin producing a blue-violet color.

Importance:

  • Detection of amino acids
  • Protein analysis
  • Forensic applications

6. Decarboxylation

Removal of the carboxyl group forms amines.

Example:

  • Histidine → Histamine

Importance:

  • Formation of biologically active compounds

7. Deamination

Removal of the amino group produces keto acids and ammonia.

Importance:

  • Amino acid metabolism
  • Energy production

Biological Importance of Amino Acids

Amino acids are essential biomolecules that play a vital role in the structure, function, metabolism, and regulation of the human body. They are not only the building blocks of proteins but also participate in energy production, synthesis of hormones, neurotransmitters, and many other biologically important compounds.

1. Building Blocks of Proteins

The most important function of amino acids is the formation of proteins.

Amino acids join together through peptide bonds to form:
  • Peptides
  • Polypeptides
  • Proteins

Proteins formed include:

  • Enzymes
  • Structural proteins
  • Transport proteins
  • Antibodies

2. Growth and Tissue Repair

Amino acids are necessary for:

  • Growth of body tissues
  • Repair of damaged cells
  • Muscle development
  • Wound healing

Essential amino acids are especially important during:

  • Childhood
  • Pregnancy
  • Recovery from illness

3. Formation of Enzymes

Most enzymes are proteins made from amino acids.

Functions:

  • Catalyze biochemical reactions
  • Increase metabolic rate
  • Maintain normal cellular activities

Examples:

  • Digestive enzymes
  • Metabolic enzymes

4. Energy Production

During fasting or starvation, amino acids are used as energy sources.

They:

  • Enter the TCA cycle
  • Produce ATP
  • Form glucose or ketone bodies

5. Synthesis of Hormones

Many hormones are synthesized from amino acids.

Examples:

Amino Acid Hormone Formed
Tyrosine Thyroxine, Epinephrine
Tryptophan Melatonin
Histidine Histamine

6. Formation of Neurotransmitters

Amino acids act as precursors for neurotransmitters.

Examples:

Amino Acid Neurotransmitter
Tyrosine Dopamine, Norepinephrine
Tryptophan Serotonin
Glutamate GABA

Functions:

  • Brain function
  • Mood regulation
  • Nerve impulse transmission

7. Maintenance of Acid–Base Balance

Amino acids act as buffers due to their amphoteric nature.

Functions:

  • Maintain blood pH
  • Stabilize intracellular pH
  • Prevent acidosis and alkalosis

8. Transport and Storage of Nutrients

Certain amino acids help in transport and storage.

Examples:

  • Hemoglobin transports oxygen
  • Albumin transports fatty acids and drugs

9. Formation of Biologically Important Compounds

Amino acids form several important compounds.

Examples

Amino Acid Compound Formed
Glycine Heme, Creatine
Arginine Nitric oxide
Methionine S-adenosyl methionine
Cysteine Glutathione

10. Immune Function

Amino acids help in:

  • Antibody formation
  • Immune cell growth
  • Tissue defense mechanisms

Glutamine is especially important for immune cells.

11. Detoxification

Certain amino acids participate in detoxification processes.

Examples:

  • Glycine conjugates toxic substances
  • Methionine helps methylation reactions

12. Clinical Importance

Abnormal amino acid metabolism causes diseases such as:
  • Phenylketonuria (PKU)
  • Alkaptonuria
  • Maple syrup urine disease
  • Homocystinuria

Peptides and Peptide Bonds

Peptides are compounds formed when two or more amino acids join together through peptide bonds.

According to the number of amino acids:

  • Dipeptide → 2 amino acids
  • Tripeptide → 3 amino acids
  • Polypeptide → Many amino acids

Proteins are long polypeptide chains.

Peptide Bond

A peptide bond is a covalent bond formed between:

  • Carboxyl group (−COOH) of one amino acid
  • Amino group (−NH₂) of another amino acid

During bond formation, one molecule of water is removed (condensation reaction).

−COOH+−NH2→−CO−NH−+H2O

The bond formed is called a peptide bond or amide bond.

Characteristics of Peptide Bond

  • Strong covalent bond
  • Rigid and planar structure
  • Important for protein stability
  • Links amino acids into chains

Biological Importance

  • Formation of proteins
  • Essential for growth and tissue repair
  • Important in enzymes and hormones
  • Maintain structure and function of cells

Related Posts

Leave a Comment

Your email address will not be published. Required fields are marked *

Scroll to Top
Enable Notifications OK No thanks