Content Category 1A: Structure/function of proteins and their constituent amino acids

The standard human genetic code encodes 20 amino acids. Ten of the amino acids are considered essential amino acids for humans because the human body cannot produce them and must be obtained through diet. However, knowing which amino acids are essential goes beyond what you need to know. You will need to know each amino acid's name, side chain structure, three letter abbreviation, and one letter abbreviation.

The fundamental structure of every amino acid is the same. An amino acid's central alpha (α) carbon functions as its glue. A free amino acid's alpha carbon is bonded to four different groups: a hydrogen atom, an amino group (-NH2), a carboxyl group (-COOH), and a distinct side chain (-R). Hopefully you took organic chemistry, because this is where concerns should be raised. An alpha carbon bound to four distinct substituents must be chiral according to the definition of chirality. Except for glycine, which defies the four unique substituent rule because its R-group is a second hydrogen atom, all amino acids are actually chiral.

There are structurally identical molecules called D and L enantiomers for each of the 19 chiral amino acids, which have opposite absolute configurations around the chiral carbon. The human body only uses L-amino acids for protein synthesis. The amino groups of L-amino acids are those whose Fischer projection (see below) points to the left. Except for cysteine, all 19 L-amino acids have an absolute configuration of S. The absolute configuration of cysteine's alpha carbon is R.