26. Which one of the following graphs represents the kinetics of protein precipitation by addition of
ammonium sulphate? On the Y–axis, [Protein} represents the concentration of free protein in solution.

Introduction

Ammonium sulphate precipitation is a classic biochemical method used for protein purification based on solubility changes. When ammonium sulphate is added to a protein solution, proteins gradually come out of solution and form precipitates, meaning the concentration of free soluble protein decreases with time.

To determine the correct graph, we must track how free protein concentration changes after ammonium sulphate is added.

Understanding the Concept

• Initially: proteins are dissolved → [Protein] is high

• Ammonium sulphate is added → salt ions remove water from proteins

• Proteins aggregate and precipitate out

• Free soluble protein concentration declines over time

• Eventually reaches a low, stable value once precipitation completes

Therefore, the kinetic profile should be a rapid drop followed by a plateau near zero.

Option Analysis

(A) ✔ Correct graph

• Shows high initial free protein concentration

• Rapid decrease as precipitation occurs

• Levels off at low concentration once precipitation is complete

• Matches real precipitation kinetics
  Correct Answer: (A)

(B) ❌ Incorrect

• The graph rises initially before falling

• Free protein never increases upon adding ammonium sulphate

• Physically impossible—proteins will not dissolve more before precipitating

(C) ❌ Incorrect

• Shows increasing free protein concentration with time

• This would imply more proteins enter the solution, the opposite of salting-out behavior

• Does not represent precipitation kinetics at all

(D) ❌ Incorrect

• Shows free protein concentration increasing initially and then plateauing

• Suggests protein remains soluble, contradicting precipitation process

• Does not reflect salting-out effect

Why (A) is Correct

Ammonium sulphate precipitation follows first-order like decrease in soluble protein:

Thus Graph (A) best describes the kinetics.

Biological & Practical Importance

Protein precipitation is widely used for:

• Fractional purification of enzymes

• Concentration of dilute protein solutions

• Differential removal of contaminants

• Pre-chromatography purification

Understanding the kinetics helps optimize:

• Salt concentration

• Mixing time

• Recovery yield

Final Answer

👉 Option (A) correctly represents protein precipitation kinetics by ammonium sulphate.