Amino Acid
Quiz

There are 20 standard amino acids used to build proteins in humans. Of these, 9 are essential amino acids — they cannot be synthesized by the human body and must be obtained from food. They are: histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. The remaining 11 are non-essential (the body can make them), though some become "conditionally essential" during illness or stress.

Amino acids are small organic molecules that serve as the monomers (building blocks) of proteins. A protein is a polymer chain of amino acids linked together by peptide bonds. When you eat dietary protein, digestive enzymes break it down into individual amino acids, which are then absorbed and used to build the body's own proteins. Different proteins have different amino acid sequences that determine their shape and function.

A codon is a sequence of three nucleotide bases in mRNA (messenger RNA) that codes for a specific amino acid. There are 64 possible codons in the genetic code. Three of these are "stop codons" that signal the end of protein synthesis, while the remaining 61 code for the 20 amino acids. Since there are more codons than amino acids, most amino acids are coded by multiple codons — this redundancy is called degeneracy of the genetic code.

The chemical nature of an amino acid depends on its R group (side chain). Acidic amino acids (aspartate and glutamate) have carboxyl groups in their side chains that can donate protons at physiological pH. Basic amino acids (lysine, arginine, and histidine) have side chains that can accept protons. Neutral amino acids have non-ionizable side chains. The charge of amino acids at different pH values is important in protein folding, enzyme function, and electrophoresis.

Glycine (Gly, G) is the simplest amino acid — its R group is just a hydrogen atom. This simplicity gives it great conformational flexibility and it's commonly found in structurally flexible regions of proteins and abundantly in collagen. Tryptophan (Trp, W) has the most complex structure with a large bicyclic indole ring as its side chain. It's also the rarest amino acid in most proteins and the precursor for neurotransmitter serotonin.