Why does the charge on a protein vary with change in ph

As the pH of a solution increases, deprotonation of the acidic and basic groups on proteins occur,
so that carboxyl groups are converted to carboxylate anions (R-COOH to R-COO-) and ammonium groups are converted to amino groups (R-NH3+ to R-NH2).
In proteins the iso-electric point (pI) is defined as the pH at which a protein has no net charge.
When the pH > pI, a protein has a net negative charge and when the pH < pI, a protein has a net positive charge.
The pI varies for different proteins.