Amino Acids

Amino acids are organic molecules that form proteins when linked together in chains. These proteins are large macromolecules, or polyanions, that play crucial roles in biological systems. Examples of biomolecules include macromolecules like proteins, carbohydrates, lipids, and nucleic acids and smaller molecules such as primary metabolites, secondary metabolites, and natural products.

1.0Introduction

Amino acids are colourless, nonvolatile, crystalline solids that melt and decompose at temperatures above 200°C. These high melting points resemble those of inorganic salts rather than typical amines or organic acids. This behaviour suggests that in the solid state and neutral solutions, amino acids exist as zwitterions—species that contain both a negatively charged group (usually the carboxylate group, -COO⁻) and a positively charged group (typically the ammonium group, -NH₃⁺). The zwitterionic form is a key feature of amino acids, contributing to their unique physical and chemical properties.

2.0Structure of Amino Acids

Each amino acid consists of a central carbon atom known as the α-carbon (the carbon atom adjacent to the carboxyl group, -COOH). Attached to the α-carbon are four distinct groups:

1. An amino group (-NH₂).
2. A carboxyl group (-COOH).
3. A hydrogen atom (-H).
4. A variable side chain (R-group) is unique to each amino acid and determines its specific properties.

At physiological pH (approximately 7.4), the following ionisation occurs:

• The carboxyl group (-COOH) dissociates to form a negatively charged carboxylate ion (-COO⁻).
• The amino group (-NH₂) is protonated, resulting in a positively charged ammonium ion (-NH₃⁺).

This results in the amino acid as a zwitterion, a molecule that carries both positive and negative charges but is electrically neutral overall.

The zwitterionic structure is a key characteristic of amino acids and plays an important role in their solubility, behaviour in solution, and interactions within biological systems.

3.0Formation of Peptide Bonds

Proteins are formed when amino acids, the building blocks of proteins, link together in a specific sequence via the formation of peptide bonds.

1. Peptide Bond Formation: The amino group −NH₂​ of one amino acid reacts with the carboxylic acid group −COOH of another. This reaction releases a water molecule (H₂O) in condensation or dehydration synthesis.
2. Dipeptides and Peptides: When two amino acids are linked, they form a dipeptide. This process repeats, with additional amino acids joining the chain, forming a polypeptide.

3. Protein Formation: A protein comprises one or more long chains of polypeptides folded into a specific three-dimensional structure necessary for biological function.

This sequential linkage of amino acids and the folding of polypeptides are crucial for forming functional proteins in living organisms.

4.0Properties of Amino Acids

1. Physical Appearance: Amino acids are crystalline, colourless compounds.
2. Melting Point: Due to their zwitterionic nature, they have exceptionally high melting points, often exceeding 200°C.
3. Solubility: The solubility of amino acids in water is influenced by the nature of their side chains (R-groups), with polar and charged side chains increasing solubility and nonpolar side chains decreasing it.
4. Amphoteric Nature: Amino acids are amphoteric, meaning they can react as both acids and bases depending on the pH of the environment.
5. Optical Activity: All amino acids have an asymmetric carbon atom except glycine (which lacks a chiral carbon). This asymmetry causes them to rotate plane-polarised light, a property known as optical activity.

5.0Classification of Amino Acids

1. Classification Based on the Position of the Amino Group

Amino acids contain an amino group (-NH₂) and a carboxylic acid group (-COOH). Amino acids are classified based on the position of the amino group relative to the carboxylic acid group.α-, β-, γ-, etc.:

• α-amino acids: The amino group is attached to the carbon atom directly adjacent to the carboxyl group (-COOH).
• β-amino acids: The amino group is attached to the second carbon from the carboxyl group.
• γ-amino acids: The amino group is attached to the third carbon from the carboxyl group.

Note: The side chain (R group) can be alkyl, aryl, or other groups, but it never contains unstable, strained cycles or highly reactive functional groups.

2. Classification of α-Amino Acids Based on Nutritional Requirements

3. Classification Based on Chemical Structure

• Acidic Amino Acids: Contain more carboxylic groups (-COOH) than amino groups (-NH₂).
  Examples:
• • Aspartic acid
  • Glutamic acid
• Basic Amino Acids: Contain more amino groups (-NH₂) than carboxylic groups (-COOH).
  Examples:
• • Lysine
  • Arginine
  • Histidine
• Neutral Amino Acids: Have an equal number of amino groups and carboxylic groups.
  Examples:
• • Alanine
  • Glycine
  • Serine

4. Classification of Amino Acids Based on Polarity of the Side Chain (R)

6.0Functions  of Amino Acids in the Body

Amino acids perform essential functions by serving as building blocks for physiologically active molecules and participating in vital metabolic processes:

1. Hormone and Pigment Formation:

• Tyrosine: Converts into hormones like thyroxine (regulates metabolism), adrenaline (fight-or-flight response), and the skin pigment melanin.

2. Heme and Metabolite Synthesis:

• Glycine: Involved in synthesising heme, a component of haemoglobin, and in producing the vitamin nicotinamide and the plant hormone indole acetic acid.

3. Urea Cycle Regulation:

• Citrulline and ornithine play a key role in the liver's urea cycle, essential for detoxifying ammonia and regulating safe ammonia levels in the blood.