Amino acids

Last revised by Daniel J Bell on 27 Jul 2022

Amino acids are the monomeric building blocks of proteins. Amino acids concatenate into chains, which are arbitrarily divided into peptides, polypeptides, or proteins according to chain length. Generally, chains of 10 or fewer amino acids are deemed to be peptides, chain length from 10-100 are polypeptides, and greater than ~100 amino acids are called proteins. 

All amino acids (except proline) consist of a carboxyl (-COOH) group and an amino (-NH2) group, with varying side chains.

Proline is chemically strictly an imino acid, which has an imino (>C=NH) group, instead of an amino group, although for biological usage it is counted as an amino acid.

Essential amino acids cannot be synthesized by the cells, and therefore must be obtained from the diet.
Conversely the non-essential amino acids can be made by the body and therefore are not mandatory constituents of a balanced diet.

The list of amino acids has the full name of the amino acid and in parentheses the standard three letter code and single letter code.

  • valine (Val, V)
  • leucine (Leu, L)
  • isoleucine (Ile, I)
  • threonine (Thr, T)
  • methionine (Met, M)
  • phenylalanine (Phe, F)
  • tryptophan (Trp, W) 
  • lysine (Lys, K)
  • histidine (His, H)
  • glycine (Gly, G)
  • alanine (Ala, A)
  • serine (Ser, S)
  • cysteine (Cys, C)
  • tyrosine (Tyr, Y)
  • aspartic acid (Asp, D)
  • asparagine (Asn, N)
  • glutamine (Gln, Q)
  • glutamic acid (Glu, E)
  • proline (Pro, P)
  • arginine (Arg, R)
  • selenocysteine (Sec, U)

Some amino acids can be labeled with radioactive isotopes for use in nuclear medicine 3,4. Methionine, for example, labeled with carbon-11 can be used in the study of tumors of the central nervous system 5-7. One more example: 18F-DOPA, an intermediate amino acid in the biosynthetic pathway of dopamine, labeled with fluorine-18 is used in the study of Parkinson disease 8-10.

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