Q4.The pH at which net charge of protein becomes zero under an electrical field at room temperature is known as:

(A) Isoelectric potential
(B) Isothermal point
(C) Isomeric point
(D) Isoelectric point

Answer: (D) Isoelectric point

The isoelectric point (pI) is the pH where a protein’s net charge becomes zero in an electrical field at room temperature, causing no migration during electrophoresis.

Option Breakdown

Isoelectric Potential (A)

“Isoelectric potential” lacks standard biochemical definition; it vaguely suggests voltage differences but does not describe protein charge neutrality.
No established usage exists for protein pH-charge relationships.

Isothermal Point (B)

Isothermal point refers to constant temperature conditions in thermodynamics, unrelated to protein charge states or electrophoresis.
Temperature affects pI indirectly but defines no specific zero-charge pH.

Isomeric Point (C)

Isomeric point has no biochemical meaning; isomers share formulas but differ structurally without charge-pH implications.
Proteins exhibit conformational isomers, but this ignores net charge behavior.

Isoelectric Point (D)

At pI, protein ionizable groups (carboxyls deprotonated, amines protonated) balance, yielding zero net charge.
Proteins precipitate minimally and stop migrating in electric fields at pI.

The pH at which net charge of protein becomes zero under electrical field defines the isoelectric point, critical for electrophoresis and purification.

Isoelectric Point Mechanism

Proteins carry positive charge below pI (low pH protonates groups) and negative above pI (high pH deprotonates).
pI calculates as average pKa of ionizable residues: pI = (pKa1 + pKa2)/2 for simple amino acids.

Why Not Other Terms?

Isoelectric potential confuses with membrane potentials; isothermal relates to heat; isomeric ignores charge dynamics.

Practical Applications

Isoelectric focusing separates proteins by pI in pH gradients for 2D gels; adjusts buffers for solubility.
Therapeutic proteins engineered for specific pI enhance stability and targeting.

This confirms D as the precise biochemical term.