Amino Acids: Structure, Classification, and Nutritional Importance

Amino Acids Structure and Function

Introduction to Amino Acids

Amino acids are the fundamental building blocks of proteins, which are essential macromolecules in all living organisms. Each amino acid contains specific functional groups and structural features that determine its chemical properties and biological roles.

• Proteins are polymers made up of amino acids linked together by peptide bonds.

• Each amino acid contains both an amino group (-NH2) and a carboxylic acid group (-COOH).

• Attached to the central (α) carbon are a hydrogen atom and a variable R group (side chain), which determines the identity and properties of the amino acid.

• The α-carbon is typically a chiral center (except in glycine).

Example: The general structure of an amino acid can be represented as:

$\mathrm{H_2N{-}CHR{-}COOH}$

Zwitterions and Ionization

In aqueous solution, amino acids exist predominantly as zwitterions, which are dipolar ions carrying both positive and negative charges.

• At physiological pH, the amino group is protonated ($\mathrm{NH_3^+}$) and the carboxyl group is deprotonated ($\mathrm{COO^-}$).

• This dual charge allows amino acids to act as buffers in biological systems.

Example:

$\mathrm{H_2N{-}CHR{-}COOH} \rightarrow \mathrm{^+H_3N{-}CHR{-}COO^-}$

Classification of Amino Acids

Essential and Non-Essential Amino Acids

There are 20 standard amino acids that make up proteins. They are classified based on whether the human body can synthesize them.

• Essential amino acids: Cannot be synthesized by the body and must be obtained from the diet.

• Non-essential amino acids: Can be synthesized de novo in humans.

Non-Essential Amino Acids	Essential Amino Acids
Alanine	Histidine
Arginine	Isoleucine
Asparagine	Leucine
Aspartate	Lysine
Cystine	Methionine
Glutamic acid	Phenylalanine
Glycine	Threonine
Ornithine	Tryptophan
Proline	Valine
Serine
Tyrosine

Additional info: Arginine is considered semi-essential, especially in children.

Classification by R Group Properties

Amino acids can also be grouped based on the chemical nature of their side chains (R groups), which affects their behavior in proteins and metabolism.

• Nonpolar (hydrophobic): Side chains are mostly hydrocarbons (e.g., alanine, valine, leucine).

• Polar uncharged: Side chains contain groups that can form hydrogen bonds but are not charged (e.g., serine, threonine).

• Acidic (negatively charged): Side chains contain carboxyl groups (e.g., aspartate, glutamate).

• Basic (positively charged): Side chains contain amino groups (e.g., lysine, arginine, histidine).

Structures of Selected Amino Acids

The side chains of amino acids determine their chemical properties and roles in proteins. Below are examples of the structures of some common amino acids:

Amino Acid	Structure (R group)
Glycine	H
Alanine	CH3
Valine	CH(CH3)2
Leucine	CH2CH(CH3)2
Isoleucine	CH(CH3)CH2CH3
Phenylalanine	CH2C6H5 (benzyl group)
Tryptophan	CH2indole
Methionine	CH2CH2SCH3
Proline	Pyrrolidine ring

Additional info: The chemical structure of the R group determines the solubility, reactivity, and role of each amino acid in protein structure and function.