Question

Which of the following enzymes of Glycolytic pathway has achieved the catalytic perfection in the sense that any increase in its catalytic efficiency would not increase the rate of reaction catalysed by it?

• A. Hexokinase
• B. Pyruvate kinase
• C. Triose phosphate isomerase
• D. Phosphofructokinase

Hint:

A "catalytically perfect" enzyme has its reaction rate limited solely by substrate diffusion, which typically lies in the range of \(10^8 - 10^9 \text{ M}^{-1}\text{s}^{-1}\).
TPI is the classic biochemistry textbook example of this phenomenon.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
An enzyme is considered to have achieved "catalytic perfection" when its rate of catalysis is limited only by the rate at which its substrate diffuses into the active site.

Step 2: Detailed Explanation:
Triose phosphate isomerase (TPI or TIM) catalyzes the rapid, reversible interconversion of the triose phosphate isomers: dihydroxyacetone phosphate (DHAP) and D-glyceraldehyde 3-phosphate (GAP) in glycolysis.
The value of the specificity constant (\(k_{\text{cat}}/K_M\)) for TPI is in the range of \(10^8\) to \(10^9 \text{ M}^{-1}\text{s}^{-1}\).
This rate is at the theoretical limit imposed by molecular diffusion in an aqueous medium.
This means that almost every collision between the enzyme and the substrate results in product formation, and any further mutation increasing the chemical reaction rate would not speed up the overall catalysis.

Step 3: Final Answer:
Thus, Triose phosphate isomerase is the catalytically perfect enzyme in this pathway, matching option (C).