Amino Acids: The Building Blocks of Peptides

Amino acids are the monomeric units of peptides and proteins. Each amino acid consists of a central carbon (α-carbon) bonded to an amino group (−NH₂), a carboxyl group (−COOH), a hydrogen atom, and a variable side chain (R group). The side chain determines the amino acid's chemical properties and, by extension, the behavior of the peptide or protein containing it.

The 20 standard amino acids

The genetic code encodes 20 standard amino acids. They are classified by their side-chain properties:

Nonpolar (hydrophobic)

• Glycine (Gly, G): the simplest amino acid — no side chain, maximum backbone flexibility. Present in collagen's characteristic Gly-X-Y repeats and in GHK-Cu (Gly-His-Lys)
• Alanine (Ala, A): small methyl side chain. Common in alpha-helices
• Valine (Val, V), Leucine (Leu, L), Isoleucine (Ile, I): branched-chain amino acids (BCAAs). Hydrophobic core residues in folded proteins
• Proline (Pro, P): cyclic side chain constrains the backbone, introducing kinks. Found in collagen (Gly-Pro-Hyp) and in BPC-157's sequence (GEPPPGKPADDAGLV — note the triple proline)
• Methionine (Met, M): thioether side chain. Susceptible to oxidation, which can inactivate peptides during storage
• Phenylalanine (Phe, F), Tryptophan (Trp, W): aromatic rings. Trp absorbs UV light and is prone to photo-oxidation — a key reason peptides must be stored away from light

Polar, uncharged

• Serine (Ser, S), Threonine (Thr, T): hydroxyl groups. Sites for phosphorylation in signaling cascades
• Asparagine (Asn, N), Glutamine (Gln, Q): amide groups. Asparagine is a common deamidation site — a major peptide degradation pathway
• Tyrosine (Tyr, Y): phenolic hydroxyl. Absorbs UV light; phosphorylation target in kinase signaling
• Cysteine (Cys, C): thiol group. Forms disulfide bonds (−S−S−) that stabilize tertiary structure. Present in oxytocin's cyclizing disulfide bridge (Cys1−Cys6)

Positively charged (basic)

• Lysine (Lys, K): primary amine side chain (+1 charge at pH 7). The 'K' in GHK-Cu; common PEGylation and lipidation site
• Arginine (Arg, R): guanidinium group (+1 charge). Strong hydrogen-bond donor; present in cell-penetrating peptides
• Histidine (His, H): imidazole ring. The only amino acid that titrates near physiological pH (pKa ~6.0), making it critical for pH-dependent binding. The 'H' in GHK-Cu — it coordinates the copper ion

Negatively charged (acidic)

• Aspartate (Asp, D), Glutamate (Glu, E): carboxylate side chains (−1 charge at pH 7). Common in metal-binding sites and enzyme active sites

Side-chain properties that matter for peptide pharmacology

Stability vulnerabilities

Certain amino acids create weak points that limit peptide shelf life:

• Asparagine (N): undergoes deamidation to aspartate, especially at Asn-Gly sequences. This is the most common chemical degradation pathway for peptides
• Methionine (M): oxidizes to methionine sulfoxide in the presence of oxygen or peroxides
• Cysteine (C): forms unwanted disulfide bonds or oxidizes to cysteic acid
• Tryptophan (W): photo-oxidizes under UV light exposure

This is why peptide storage conditions matter: cold, dark, dry, and oxygen-free environments protect these vulnerable residues.

Protease susceptibility

Most proteases cleave at specific amino acid motifs. For example, DPP-4 (dipeptidyl peptidase-4) cleaves after the second amino acid when the penultimate residue is alanine or proline — this is why native GLP-1 is rapidly destroyed. Semaglutide substitutes Aib (α-aminoisobutyric acid, a non-standard amino acid) at position 8 to block this cleavage site.

Receptor binding

The side chains of specific amino acids form the contact surfaces with receptors. In GHK-Cu, the histidine imidazole coordinates Cu²⁺ while the glycine and lysine contribute to overall binding geometry for the copper transport mechanism. Mutating any of the three residues eliminates activity.

Non-standard amino acids in peptide therapeutics

Peptide engineering frequently uses non-standard amino acids to improve pharmacological properties:

• Aib (α-aminoisobutyric acid): backbone-constrained; blocks DPP-4 cleavage in semaglutide
• D-amino acids: mirror-image stereochemistry resists protease cleavage. Used in FOXO4-DRI (full D-retro-inverso)
• Hydroxyproline (Hyp): hydroxylated proline found in collagen. Essential for collagen triple-helix stability
• Norleucine (Nle): oxidation-resistant methionine replacement. Used in some research peptide analogs

Amino acids in peptide naming

Peptide names sometimes directly reflect the amino acid sequence. Argireline = Acetyl Hexapeptide-3, named for the arginine residues in its sequence. GHK-Cu is named for its three amino acids: Glycine-Histidine-Lysine, complexed with copper. Understanding amino acid one-letter codes (G, H, K, etc.) makes it possible to read peptide sequences directly — a useful skill when evaluating research papers and product labels.