Amino Acids and Peptides: Structure, Properties, and Biological Relevance

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Amino Acids: Structure and Classification

General Structure of Amino Acids

Amino acids are the fundamental building blocks of proteins. Each amino acid contains a central (α) carbon atom bonded to four different groups: an amino group (–NH3+), a carboxyl group (–COO−), a hydrogen atom, and a variable side chain (R group) that determines the amino acid's properties.

Chirality: With the exception of glycine, all amino acids are chiral, meaning they exist as L- and D-stereoisomers. Proteins in living organisms are composed almost exclusively of L-amino acids.
α-Carbon: The central carbon is a stereocenter, giving rise to optical activity.

General structure of an amino acid L- and D-alanine stereoisomers Chirality and mirror images in molecules

Classification of Amino Acids

The 20 standard amino acids are classified based on the properties of their side chains (R groups):

Nonpolar, aliphatic: Glycine, Alanine, Proline, Valine, Leucine, Isoleucine, Methionine
Aromatic: Phenylalanine, Tyrosine, Tryptophan
Polar, uncharged: Serine, Threonine, Cysteine, Asparagine, Glutamine
Negatively charged (acidic): Aspartate, Glutamate
Positively charged (basic): Lysine, Arginine, Histidine

Nonpolar, hydrophobic amino acids Structures of Leucine, Isoleucine, Methionine Aromatic amino acids: Phenylalanine, Tyrosine, Tryptophan Polar, uncharged amino acids: Serine, Threonine, Cysteine Negatively charged amino acids: Aspartate, Glutamate Positively charged amino acids: Lysine, Arginine, Histidine Histidine structure and properties Uncommon amino acids

Properties of Amino Acid Side Chains

The chemical nature of the side chain determines the solubility, reactivity, and role of each amino acid in proteins:

Hydrophobic (nonpolar) side chains tend to cluster in the interior of proteins, stabilizing structure via hydrophobic interactions.
Polar and charged side chains are often found on protein surfaces, interacting with water or forming salt bridges and hydrogen bonds.
Aromatic side chains can participate in stacking interactions and absorb UV light.
Sulfur-containing side chains (Cys, Met) can form disulfide bonds or participate in methyl group transfer.

Special Properties and Modifications

Disulfide Bonds

Cysteine residues can form covalent disulfide bonds (–S–S–) through oxidation, stabilizing protein tertiary and quaternary structures.

Disulfide bond formation between cysteine residues

Phosphorylation

Serine, threonine, and tyrosine residues can be phosphorylated, introducing a negative charge and altering protein function, especially in signaling pathways.

Uncommon Amino Acids

Some proteins contain amino acids derived from the standard 20 by post-translational modification (e.g., hydroxyproline, methyllysine, γ-carboxyglutamate, ornithine, citrulline).

Structures of uncommon amino acids

Peptides and Proteins

The Peptide Bond

Amino acids are linked by peptide bonds, formed via a condensation reaction between the α-carboxyl group of one amino acid and the α-amino group of another, releasing water.

Peptide bond: Has partial double-bond character, restricting rotation and conferring planarity.
Directionality: Peptides have an amino (N-) terminus and a carboxyl (C-) terminus.

Formation of the peptide bond Peptide chain highlighting peptide bonds

Levels of Protein Structure

Proteins exhibit hierarchical levels of structure:

Primary structure: Linear sequence of amino acids.
Secondary structure: Local folding patterns (α-helix, β-sheet).
Tertiary structure: Overall 3D shape of a single polypeptide chain.
Quaternary structure: Assembly of multiple polypeptide subunits.

Levels of protein structure

Diversity and Variability in Proteins

Proteins vary greatly in size, sequence, and function. Many proteins are polymorphic, meaning they exist in multiple sequence variants within a population. This variability can affect protein function and is relevant in health and disease (e.g., p53 polymorphisms and cancer risk).

Sequence alignment showing primary structure variability p53 protein polymorphisms and functional domains WRN protein sequence variation and functional domains

Examples of Peptides and Proteins

Insulin: A peptide hormone with two chains linked by disulfide bonds, showing sequence variation among species.
Aspartame: An artificial sweetener composed of two amino acids (aspartic acid and phenylalanine) in peptide linkage.

Insulin sequence comparison among species Aspartame structure

Protein Size and Complexity

Proteins range from small peptides to massive macromolecules. The number of amino acid residues and polypeptide chains varies widely among proteins.

Protein	Molecular Weight	Number of Residues	Number of Polypeptide Chains
Cytochrome c (human)	13,000	104	1
Hemoglobin (human)	64,500	574	4
Serum albumin (human)	68,500	609	1
Titin (human)	2,993,000	26,926	1

Table of protein molecular data

Summary Table: Amino Acid Classification

Group	Amino Acids	Key Properties
Nonpolar, aliphatic	Gly, Ala, Pro, Val, Leu, Ile, Met	Hydrophobic, interior of proteins
Aromatic	Phe, Tyr, Trp	Hydrophobic, absorb UV, stacking interactions
Polar, uncharged	Ser, Thr, Cys, Asn, Gln	Hydrophilic, H-bonding, reactive groups
Negatively charged (acidic)	Asp, Glu	Hydrophilic, acidic, form salt bridges
Positively charged (basic)	Lys, Arg, His	Hydrophilic, basic, form salt bridges

Key Equations

Peptide bond formation:
Disulfide bond formation:

Additional info: This guide covers the structure, classification, and properties of amino acids, peptide bond formation, protein structure levels, and the biological significance of sequence variability and post-translational modifications. For further details on uncommon amino acids and advanced protein techniques, consult your textbook.