At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge. ( information from Wikipedia) why is this happen?? Can anyone explains the theory?? If talk about glutamine acid??

Consider a solution of glutamate at its pI (which I seem to recall is about 3.25). At this pH, the ammonium group has a one full (+) charge. The side chain carboxylic acid is mostly neutral, but a small percentage of the time it loses a proton. Therefore it has roughly a charge of 0.1(-). The carboxylic acid that is directly attached to the alpha carbon has a charge of roughly 0.9(-). Thus the positive and negative charges cancel out at this pH. These numbers all depend on one's choice of pK_a values.