In isoelectric focusing, where does a protein migrate relative to its pI?

In isoelectric focusing, where does a protein migrate relative to its pI?

• A. To the site where the pH is the same as its pI
• B. To the site where the molecular weight correlates with the pI
• C. To the site where the protein's net charge exceeds the pI
• D. To the site where the protein's net charge is less than the pI

Answer: (A)

In isoelectric focusing, proteins move in a pH gradient until they reach the position where their net charge is zero. That occurs at the pH equal to the protein’s isoelectric point (pI). At this point, the protein has no net charge and its electrophoretic mobility becomes zero, so it stops migrating and focuses there. Below the pI, the protein is positively charged and would migrate toward the negative electrode; above the pI, it is negatively charged and would migrate toward the positive electrode. Since the gradient guides them to the pH that matches their pI, the correct description is the site where the pH equals the protein’s pI. Separation in IEF depends on charge, not molecular weight, which is why weight does not determine the stopping point.

In isoelectric focusing, proteins move in a pH gradient until they reach the position where their net charge is zero. That occurs at the pH equal to the protein’s isoelectric point (pI). At this point, the protein has no net charge and its electrophoretic mobility becomes zero, so it stops migrating and focuses there. Below the pI, the protein is positively charged and would migrate toward the negative electrode; above the pI, it is negatively charged and would migrate toward the positive electrode. Since the gradient guides them to the pH that matches their pI, the correct description is the site where the pH equals the protein’s pI. Separation in IEF depends on charge, not molecular weight, which is why weight does not determine the stopping point.