Structure of amino acids Properties of amino acids:
ionizationhydrophobicity
Amino acids are building blocks of proteins;Proteins are most
abundant macromolecules in living cells;Proteins are assembled from
20 amino acids and the sequence determines the properties of
individual proteins;
Chiral carbonThe side chain determines the properties of amino
acids (hydrophobicity etc.)
AAs exist as stereoisomers compounds with the same molecular
formula that differ in the arrangement (configuration). Two
stereoisomers that are nonsuperimposable mirror images exist for
each chiral amino acid (enantiomers).19 out of 20 common amino
acids are stereoisomers (exception: glycine)
The mirror-image pairs of AAs:D (dextro; in Latin dexter ~
right)L (levo; in Latin laevus ~ left)In Fischer projections
horizontal bonds extend toward the viewer, and the vertical bonds
extend away
Ionization of amino acids:pKa of a COOH group: 1.8-2.5pKa of a
NH2 group: 8.7-10.7At physiological pH AAs are zwitterions (dipolar
ions with both positive and negative charge).The physical
properties of amino acids are influenced by the ionic states of the
a-carboxyl and a-amino groups and by ionizable groups in the side
chains.Each amino acid has either 2 or 3 pKa values, and these
values differ among the amino acids. Consequently, at a given pH,
AAs have different net charges.
AAs differ in the R group.
The AAs can be grouped into classes based on the properties of
the R groups, in particular the polarity or tendency to interact
with water at physiologiocal pH.
Hydropathy measure of hydrophobicity of amino acids.
