The correct answer is (3) Both X and Y are specific for the substrate, S.

Introduction

Substrate specificity is a key property for enzymes, defining their role in cellular metabolism and biotechnology. Assessing specificity can be approached by examining enzyme activity under different treatments—comparing controls, inhibitor-exposed, and heat-inactivated conditions. This article analyzes experimental data on two enzymes (proteins X and Y), guiding students through interpretation of activity curves to identify substrate specificity.

Experimental Setup Explained

• Proteins X and Y tested for activity on substrate S.

• Treatments:

  • Control (c): No inhibitor or heat; expects maximum activity.

  • Inhibitor (a): Protein exposed to an activity-specific inhibitor.

  • Heat treatment (b): Protein denatured by heat, expects no activity if protein is enzyme.

Analysis of Activity Curves

Protein X:

• Curve c: High activity at increasing substrate, represents normal enzyme kinetics (hyperbolic saturation curve).

• Curve a: Activity greatly reduced, inhibitor suppresses function—implies specific interaction with S.

• Curve b: Little to no activity, confirming protein is an enzyme inactivated by heat.

Protein Y:

• Curve c: Similar to protein X, high activity, typical enzyme kinetic profile.

• Curve a: Inhibitor reduces activity, indicating enzyme specificity for S.

• Curve b: Heat: no activity, again indicating protein’s enzymatic nature and specificity.

Interpretation

• Substrate specificity: For both X and Y, inhibitor decreases activity; heat abolishes activity, both indicating specific enzyme-substrate interaction.

• If protein was non-specific, inhibitor would not reduce activity sharply, and the saturation curve would be altered.

• Both proteins show classic response to both inhibitor and heat, hallmark of specific catalysis on S.

Explanation of Options

• Option 1 & 2: Only one protein is specific—contradicted by inhibitor effects in both cases.

• Option 3 (Correct): Both X and Y show substrate S specificity, reflected by dramatic inhibitor/heat effect.

• Option 4: Non-specific proteins would not react to inhibitor nor have distinct saturation curves.

Summary Table

Mechanistic and Practical Implications

• Specific enzyme-substrate relationships underpin accurate physiological regulation and biotechnological application.

• Inhibitor sensitivity and dramatic activity loss upon denaturation confirm specificity.

Conclusion

Both proteins X and Y are specific for substrate S, as evidenced by their inhibitor sensitivity and complete loss of activity upon heat treatment. This demonstrates classic enzyme specificity, distinguishing them from non-specific protein activity.