An amino acid is an organic molecule containing an amino group and a carboxyl group. Amino acids are central to biochemistry because they are the building blocks of proteins, but they also exist as free compounds with other biological functions. The twenty standard protein-forming amino acids share a common basic structure and differ in their side chains. Their chemical diversity enables proteins to adopt varied structures and perform functions ranging from mechanical support to enzyme activity. (openstax.org)
Molecular structure and stereochemistry
In an α-amino acid, the amino group is attached to the carbon immediately adjacent to the carboxyl carbon. For most standard amino acids, the neutral structural formula is H₂N–CH(R)–COOH, where R represents the side chain. Glycine has hydrogen as its side chain; alanine has a methyl group. Proline differs because its side chain connects back to the nitrogen, producing a ring and a secondary rather than primary amino group. The twenty standard amino acids are all α-amino acids, although the broader chemical category includes other arrangements. (openstax.org)
Except for glycine, the standard amino acids possess chirality at the α-carbon and can occur as enantiomers. Those ordinarily incorporated into proteins by ribosomes belong to the L series. The labels L and D describe configuration relative to a reference compound, not the direction in which a substance rotates polarized light. Glycine is achiral because its α-carbon carries two identical hydrogen substituents. (openstax.org)
Acid–base behavior
The familiar neutral formula does not depict the predominant form of most free amino acids in water near neutral pH. Their carboxyl group generally loses a proton, becoming –COO⁻, while the amino group gains one, becoming –NH₃⁺. When the side chain is uncharged, the resulting zwitterion carries both positive and negative charges but has zero net charge. These internal charges help explain why amino acids commonly form crystalline solids with relatively high melting points. (openstax.org)
Amino acids can accept or donate protons, depending on solution conditions. Acidic conditions favor more positively charged forms, whereas alkaline conditions favor more negatively charged forms. Ionizable side chains add further complexity. The isoelectric point, abbreviated pI, is the pH at which the average net electrical charge is zero; it does not mean that every functional group is uncharged. Different amino acids therefore have different pI values. (openstax.org)
Side chains and classification
Side chains provide a useful chemical classification. Nonpolar examples include alanine, valine, leucine, and isoleucine. Serine, threonine, asparagine, and glutamine have polar, ordinarily uncharged side chains. Aspartate and glutamate usually carry negative side-chain charges near physiological pH, while lysine and arginine usually carry positive ones. Histidine has a titratable ring whose charge depends particularly strongly on its local environment. Phenylalanine, tyrosine, and tryptophan contain aromatic rings; these categories overlap rather than forming a single exclusive classification. (openstax.org)
Within proteins, side-chain interactions contribute to protein folding. Nonpolar groups participate in the hydrophobic effect, while polar groups can form hydrogen bonds. Cysteine residues can form covalent disulfide links. The distribution of these interactions, together with backbone geometry, helps determine a protein’s three-dimensional organization and chemical behavior. (ncbi.nlm.nih.gov)
Incorporation into proteins
Amino acids are joined by peptide bonds between amino and carboxyl groups. Once incorporated, each unit is called an amino-acid residue. A chain has an amino-terminal, or N-terminal, end and a carboxyl-terminal, or C-terminal, end; amino-acid sequences are conventionally written from N to C. The sequence constitutes the protein’s primary structure. (old.goldbook.iupac.org)
During translation, a ribosome reads codons in messenger RNA according to the genetic code. Transfer RNAs act as adaptors carrying amino acids. Attachment to tRNA ordinarily involves an aminoacyl-tRNA synthetase and energy supplied by ATP. The ribosome transfers the growing chain to the incoming amino acid, extending it at the C-terminal end. This activated process is more specific than simply combining free amino acids in solution. (ncbi.nlm.nih.gov)
The conventional twenty-member set is not an exhaustive inventory of naturally encoded protein constituents. For example, pyrrolysine is a lysine derivative encoded by UAG in certain archaeal methyltransferase genes and incorporated through specialized translation machinery. Thus, the phrase “twenty amino acids” refers to the standard set, not every amino acid found in nature. (cir.nii.ac.jp)
Nutrition and metabolism
In human nutrition, nine standard amino acids are classified as essential amino acids: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. They must be supplied by the diet because endogenous synthesis is insufficient. “Nonessential” means that the body can synthesize an amino acid, not that it lacks biological importance. Conditional essentiality describes situations in which synthesis cannot meet physiological demand. (openstax.org)
Amino-acid metabolism connects nitrogen handling with carbon metabolism. In transamination, an amino group is transferred to a keto acid, often producing glutamate. Subsequent reactions can release ammonia; in humans, much waste nitrogen is converted to urea through the urea cycle. The remaining carbon skeletons enter metabolic pathways, where they can support energy production or contribute to the synthesis of other compounds. Each amino acid has its own degradation pathway, so amino-acid breakdown is not a single uniform reaction. (openstax.org)