Another important feature of amino acids is the existence of both a basic and an acidic group at the α-carbon. Compounds such as amino acids that can act as either an acid or a base are called amphoteric. The basic amino group typically has a pKa between 9 and 10, while the acidic α-carboxyl group has a pKa that is usually close to 2 (a very low value for carboxyls). The pKa of a group is the pH value at which the concentration of the protonated group equals that of the unprotonated group.
pKa is the negative base-10 logarithm of the acid dissociation constant (Ka) of a solution.
pKa = -log10Ka
A lower pKa value shows a stronger acid. Example, the pKa of acetic acid is 4.8, while the pKa of lactic acid is 3.8. Using the pKa values, lactic acid is a stronger acid than acetic acid.
The reason pKa is used is because it describes acid dissociation using small decimal numbers.
Meaning, at physiological pH (about 7–7.4), the free amino acids exist largely as dipolar ions or “zwitterions” (German for “hybrid ions”; a zwitterion carries an equal number of positively and negatively charged groups). Any free amino acid and likewise any protein will, at some specific pH, exist in the form of a zwitterion.
All amino acids and all proteins, when subjected to changes in pH, pass through a state at which there is an equal number of positive and negative charges on the molecule. The pH at which this occurs is known as the isoelectric point (or isoelectric pH) and is denoted as (pI, pH(I), IEP).
When dissolved in water, all amino acids and all proteins are present predominantly in their isoelectric form. Stated another way, there is a pH (the isoelectric point) at which the molecule has a net zero charge (equal number of positive and negative charges), but there is no pH at which the molecule has an absolute zero charge (complete absence of positive and negative charges). Amino acids and proteins are always in the form of ions; they always carry charged groups. This fact is vitally important in considering further the biochemistry of amino acids and proteins.
Amino acids can be linked by a condensation reaction in which a −OH is lost from the carboxyl group of one amino acid along with a hydrogen from the amino group of a second, forming a molecule of water and leaving the two amino acids linked via an amide—called, in this case, a peptide bond. When individual amino acids are combined to form proteins, their carboxyl and amino groups are no longer able to act as acids or bases, since they have reacted to form the peptide bond. The acid-base properties of proteins are dependent upon the overall ionization characteristics of the individual R groups of the component amino acids.
Amino acids joined by a series of peptide bonds constitute a peptide. Small polymers of amino acids (fewer than 50) are called oligopeptides, larger ones (more than 50) are referred to as polypeptides.
In summary, it is the sequence of amino acids that determines the shape and biological function of a protein as well as its physical and chemical properties. The functional diversity of proteins arises because proteins are polymers of 20 different kinds of amino acids. The hormone insulin, which has 51 amino acids. With 20 different amino acids to choose from at each of these 51 positions, a total of 2051, or about 1066, different proteins could theoretically be made.