Amino Acid

What Is an Amino Acid?

Amino acids are organic compounds that serve as the building blocks of proteins. Each amino acid consists of a central carbon atom bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable side chain (R group) that determines the amino acid's unique chemical properties. Proteins are formed when amino acids are linked together by peptide bonds into long chains called polypeptides.

The human body uses 20 standard amino acids to build proteins. These are divided into three categories based on the body's ability to synthesize them:

Classification of Amino Acids

• Essential amino acids (9): cannot be synthesized by the body and must be obtained from diet — histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine
• Non-essential amino acids (11): the body can synthesize these in adequate amounts under normal conditions — alanine, arginine, asparagine, aspartate, cysteine, glutamate, glutamine, glycine, proline, serine, and tyrosine
• Conditionally essential amino acids: non-essential under healthy conditions but become essential during illness, rapid growth, or physiological stress (e.g., arginine, glutamine)

Functions and Dietary Relevance

Beyond protein synthesis, individual amino acids serve specialized roles:

• Leucine: key activator of muscle protein synthesis via the mTOR pathway
• Tryptophan: precursor to serotonin and melatonin
• Phenylalanine and tyrosine: precursors to dopamine, epinephrine, and thyroid hormones
• Glycine: component of collagen and the antioxidant glutathione

Consuming complete proteins or varied combinations of plant proteins ensures adequate intake of all essential amino acids. People following plant-based diets should be attentive to lysine (lower in grains) and methionine (lower in legumes).