Question

What is pI? How the knowledge of the pI of a protein enables characterisation of the protein?

Answer 1

At a pH below their pI, proteins carry a net positive charge; above their pI they carry a net negative charge.
Proteins can be characterized according to their isoelectric point on a polyacrylamide gel using isoelectric focusing, which uses a pH gradient to separate proteins. Isoelectric focusing is also the first step in 2-D gel polyacrylamide gel electrophoresis.