Life Sciences · Biochemistry
Amino
Acids
The molecular alphabet of life — 21 building blocks that link together to form every protein in your body, from your hair to your enzymes to the antibodies in your blood.
3D model: Edumol on Sketchfab
Structure
Anatomy of an amino acid
Every amino acid shares the same core backbone — the unique R group (side chain) is what makes each one different.
Explorer
The 21 proteinogenic amino acids
Tap any card for details. The standard genetic code encodes 20; selenocysteine (Sec/U) is the 21st.
These side chains carry a net charge at physiological pH 7.4. Charged amino acids are often found on protein surfaces, where they interact with water and other charged molecules.
These side chains form hydrogen bonds with water but carry no net charge. They are often found in enzyme active sites or on protein surfaces.
Each of these breaks a rule: glycine has no chirality, proline forms a ring with the backbone, cysteine forms disulfide bridges, and selenocysteine uses the rare element selenium.
These side chains avoid water and cluster together in a protein's core, driving folding. They are the "greasy" glue that holds protein 3-D structure together.
Bonding
Peptide bonds: linking amino acids
A peptide bond forms when the carboxyl group of one amino acid reacts with the amino group of another, releasing a water molecule (condensation reaction).
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1
Two amino acids align: the –COOH of amino acid 1 faces the –NH₂ of amino acid 2.
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2
A condensation reaction occurs — the –OH from the carboxyl and an –H from the amino group combine and leave as water (H₂O).
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3
A covalent C–N peptide bond forms between the two residues. The linked pair is called a dipeptide.
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4
The process repeats at both ends, growing the chain into a polypeptide. Hundreds or thousands of amino acids can link this way.
Interactive Lab
Build a peptide chain
Click amino acids to add them to your chain. Peptide bonds form automatically between each residue.
Nutrition
The 9 essential amino acids
Your body cannot synthesize these — they must come from food. Complete protein sources (meat, fish, eggs, soy, quinoa) contain all nine.
Beyond Structure
What amino acids do beyond proteins
Tryptophan → serotonin & melatonin. Tyrosine → dopamine & norepinephrine. Glutamate itself is the brain's main excitatory neurotransmitter.
Amino acids can be broken down (catabolized) for energy when carbohydrates are scarce. Alanine shuttles nitrogen from muscle to liver during fasting.
Serine and threonine are phosphorylated by kinases to switch proteins on/off. Tyrosine is targeted in major receptor pathways (insulin, growth factors).
Cysteine is a key component of glutathione — the cell's primary antioxidant. Selenocysteine in glutathione peroxidase destroys reactive oxygen species.
Glycine is ~35% of collagen by mass. Proline creates structural "kinks" in collagen's triple helix. Cysteine disulfide bridges stabilize keratin (hair, nails).
Methionine is the universal methyl group donor via S-adenosylmethionine (SAM). DNA methylation — a key epigenetic mark — depends on adequate methionine intake.
Reflect
Discussion questions
- All 21 amino acids share the same backbone. What does it tell us about evolution that such a universal structure was selected?
- Why would charged amino acids tend to end up on the outside of a folded protein rather than buried inside?
- Phenylketonuria (PKU) is a disorder where the enzyme that converts phenylalanine to tyrosine is missing. Based on what you know, predict two consequences of untreated PKU.
- Serotonin is made from tryptophan. What dietary or lifestyle changes might affect serotonin levels, and why?
- Selenocysteine uses selenium, a trace element. What might go wrong if someone has a selenium-deficient diet?