Amino acids
Amino acids are the building blocks of proteins — molecules containing an amino group, a carboxyl group, and a distinctive side chain attached to a central carbon. Twenty standard amino acids link together in chains to form all the body's proteins.
Amino acids are organic molecules built around a central alpha carbon bonded to four groups: an amino group (−NH2), a carboxyl group (−COOH), a hydrogen atom, and a variable side chain (R group) that gives each amino acid its identity. Twenty standard amino acids are encoded by the genetic code, and they polymerize through peptide bonds — formed by dehydration between one amino acid's carboxyl group and the next one's amino group — into the polypeptide chains that fold into proteins.
Side chains determine chemistry and classification. Nonpolar, aliphatic side chains (as in alanine, valine, leucine) are hydrophobic and tend to bury themselves inside folded proteins; polar side chains are hydrophilic; acidic side chains (aspartate, glutamate) carry negative charge; and basic side chains (lysine, arginine, histidine) carry positive charge, though histidine's side-chain pKa near 6 leaves it only partly protonated at physiological pH. At physiological pH, amino acids exist as dipolar ions (zwitterions) — the amino group protonated, the carboxyl group deprotonated — giving a net neutral molecule that can act as a buffer. Except for glycine, amino acids are chiral, and proteins use the L-configuration, often depicted with Fischer projections.
Nutritionally, amino acids divide into essential (must come from the diet) and nonessential (the body can synthesize them). Complete proteins supply all essential amino acids, a central concern in sports nutrition and protein intake planning.
Amino acids are foundational for several exams: the MCAT tests structure, side-chain classification, zwitterion behavior, and stereochemistry heavily; the USMLE Step 1 builds on them for translation and enzyme function; and the CSCS exam covers amino acids within macronutrients and dietary supplementation for athletes.
Key takeaways
- Amino acids have an alpha carbon bonded to an amino group, a carboxyl group, a hydrogen, and a variable side chain.
- Twenty standard amino acids join via peptide bonds to form proteins.
- Side chains classify amino acids as nonpolar (aliphatic), polar, acidic, or basic — driving how proteins fold.
- At physiological pH, amino acids exist as zwitterions: dipolar ions with no net charge.
- The MCAT, USMLE Step 1, and CSCS exams all test amino acids, from structure and stereochemistry to sports nutrition.