nounBeginner
/əˈmiː.noʊ ˈæs.ɪd/
Latin amino- “containing NH₂” + acidum “acid” · Asparagine first isolated 1806
Simple Definition
An amino acid is an organic molecule containing an amino group (–NH₂) and a carboxyl group (–COOH), with a unique side chain (R group) attached to its central carbon. Amino acids are the individual building blocks of all peptides and proteins.

Detailed Explanation

There are 20 standard amino acids encoded by the human genetic code. Each shares the same core structure — a central alpha-carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain (R group). The general formula is: H₂N–CH(R)–COOH.

Amino acids are grouped by their R group properties: nonpolar (Gly, Ala, Val, Leu, Ile, Pro, Phe, Trp, Met), polar (Ser, Thr, Cys, Tyr, Asn, Gln), positively charged (Lys, Arg, His), and negatively charged (Asp, Glu).

9 amino acids are essential — the body cannot make them: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The remaining 11 are nonessential. Six become conditionally essential during illness or stress.

Beyond building peptides and proteins, amino acids serve as neurotransmitter precursors (tryptophan → serotonin, tyrosine → dopamine), energy sources (branched-chain amino acids), antioxidant components (cysteine in glutathione), and signaling molecules (leucine activates mTOR for muscle protein synthesis).

Key Facts

  • General formula: H₂N–CH(R)–COOH
  • 20 standard amino acids in the human genetic code
  • 9 essential + 11 nonessential (6 conditionally essential)
  • Glycine is the simplest (R = H); tryptophan is the largest
  • Molecular weight range: 75 Da (glycine) to 204 Da (tryptophan); average ~128 Da
  • All 20 exist as L- or D-isomers; biology uses only L-amino acids
  • First isolated: asparagine from asparagus juice, 1806

Biological Function

Amino acids are the monomers of all peptides and proteins. When linked by peptide bonds, they form chains that fold into enzymes, hormones, antibodies, and structural molecules. Individual amino acids also serve critical metabolic functions as neurotransmitter precursors, energy sources, and signaling intermediates.

Example in Context

Scientific“The amino acid sequence of a peptide — its primary structure — is determined by the mRNA codon sequence read by ribosomes during translation.”
Everyday“Eggs are a complete protein because they contain all 9 essential amino acids your body cannot make on its own.”

Frequently Asked Questions

Amino acids are organic molecules that serve as building blocks for peptides and proteins. There are 20 types in the human body, each with a unique side chain that determines its properties.
Nine amino acids are essential, meaning your body cannot make them and they must come from food: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine.
The R group (side chain) is the variable part of each amino acid that makes it unique. It determines whether the amino acid is hydrophobic, hydrophilic, charged, or neutral.
Related Terms Peptide Peptide Bond Protein Collagen

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