amino acid
Meaning of amino acid: Twenty of them, ordered by instructions copied from DNA, are enough to build every protein in the human body.
Definition of amino acid
An amino acid is a small molecule built around a central carbon that carries an amino group, an acid group, a hydrogen atom and a variable side chain. The side chain is the part that differs from one amino acid to the next, and it gives each its own size, charge and behavior in water. Joined in chains by peptide bonds, amino acids form every protein the body makes.
How do amino acids become proteins?
The order of amino acids in a protein is written in DNA. Each group of three DNA letters specifies one amino acid, the message is copied into RNA, and a ribosome reads it and links the matching amino acids one at a time. That amino acid sequence then determines everything else. Side chains that repel water turn inward, charged ones face outward, and the chain folds into a specific three-dimensional shape. Shape is function: an enzyme's pocket, an antibody's binding site and the fibers of collagen are all consequences of the sequence. Change a single amino acid in the wrong place and the folded shape, and therefore the protein, can stop working.
Which ones have to come from food?
Cells can build some amino acids from simpler starting materials and cannot build others at all. The ones they cannot make have to arrive in the diet, which is what essential means in this context. It is not a comment on importance, since every one of the twenty is required to build a protein.
| Group | Members | Source |
|---|---|---|
| Essential | Histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, valine | Diet only |
| Nonessential | Alanine, aspartic acid, asparagine, glutamic acid, serine and others | Made by the body |
| Conditionally essential | Arginine, cysteine, glutamine, glycine, proline, tyrosine | Usually made, but needed from food during illness, injury or rapid growth |
What else do they do?
Several serve as raw material for other molecules entirely. Tryptophan is converted into serotonin, tyrosine into thyroid hormones and adrenaline, and glutamate acts directly as a signal between nerve cells. Amino acids can also be burned for energy or turned into glucose when intake is short. Inherited disorders of this chemistry exist: in phenylketonuria the enzyme that handles phenylalanine is missing, so the amino acid accumulates, which is why newborns are screened for it.
Used in a sentence
Laboratory analysis measured the levels of several amino acids in the blood sample.
Good to know
Amino acids the body cannot make must come from food, which is what essential means in this context; the label describes the source rather than the importance of the amino acid.
Word origin
Where “amino acid” comes from
Formed from amino, indicating a chemical group derived from ammonia, and acid, from Latin acidus, meaning sour.