Isoelectric point (aminoacids and proteins)

Amino acids are, as mentioned in other posts, molecules that have an amino group and a carboxylic group. These two groups are ionizable, ie, they may undergo protonation/deprotonation. Moreover, the side chains of various amino acids may present additional ionizable groups. This means that amino acids are molecules that can display positive, negative, or neutral charge. What influences the overall charge of each amino acid in a given context is:
1. Chemical composition of the amino acid and the molecule where it is inserted (if that is the case ...). The presence of certain atoms/functional groups in a molecule alters the distribution of its electron cloud, making some covalent bonds stronger and other weaker. The weakening of the bonds involving hydrogen atoms turn easier the occurrence of deprotonation.
2. The pH of the solution in which the amino acid is inserted. As is logical, functional groups will present a state of protonation that is influenced by the pH, that means, if the pH is lower than its pKa, the functional group tends to be protonated, and if it is greater than the pKa, it tends to be deprotonated.
Therefore, based on the characteristics of each amino acid, and the environment where it is, it is possible to obtain different total charges.
The isoelectric point is defined as the pH value for which the total charge of the amino acid is zero. Note that this does not mean that there are no charges on the amino acid, because in reality there are charges, indeed. This means is that when subjected to this pH, total positive charges equal the total negative charges. At this point, the amino acid solubility decreases. When an amino acid is placed in a solution with a pH below its isoelectric point, it acquires positive charge as the functional groups tend to be protonated (gain H+). If the pH is above the isoelectric point, total charge is negative, because the functional groups tend to be predominantly deprotonated (lose H+).

In the case of proteins, it applies exactly the same concept. However, in this case one must consider the total of ionizable groups present in the molecule, and the isoelectric point is defined as the pH value for which the total charge of the protein is zero. 

Again, at this value the solubility of the protein is zero, and it tends to precipitate. This is explored in the laboratory by means of a technique called isoelectric focusing.

.