Question 42:
The step in an enzymatic reaction of a pathway that needs greatest amount of activation energy is called:
The step requiring the greatest activation energy in an enzymatic reaction pathway is the rate-limiting step, as it acts as the slowest bottleneck determining the overall pathway speed.
Option Analysis
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(A) Rate constant: This is a kinetic parameter (k) measuring reaction speed, not the step with highest energy barrier; it quantifies velocity after activation energy is overcome.
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(B) Rate limiting: Correct answer. This slowest step demands the highest activation energy, creating a “bottleneck” that controls flux in pathways like glycolysis; regulation often targets it.
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(C) Rate enhancing: No such term exists; it might confuse with catalysis reducing activation energy, but doesn’t describe the high-energy step.
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(D) Kcat: Turnover number measuring catalytic cycles per enzyme per second; relates to efficiency post-activation, not the energy-demanding step itself.
The rate limiting step in enzymatic reactions with the highest activation energy controls metabolic pathway speed, making it essential for biochemistry and GATE Life Sciences preparation.
What Defines Rate Limiting Step?
In multi-step enzymatic pathways, the rate limiting step is the slowest reaction requiring the greatest activation energy to reach its transition state. This bottleneck dictates overall flux, as seen in glycolysis where phosphofructokinase catalyzes the key step. Faster steps upstream accumulate substrates, while downstream ones await products.
Why Highest Activation Energy?
Slower reactions climb a higher “energy hill,” per transition state theory; enthalpy doesn’t dictate speed—activation energy does. Enzymes lower this barrier overall, but the rate limiting step retains the tallest remaining hurdle, enabling regulation via allosteric effectors or phosphorylation.
Role in Metabolic Control
Pathways like gluconeogenesis or the TCA cycle feature dedicated rate limiting enzymes (e.g., fructose-1,6-bisphosphatase) with high Ea for fine-tuned control. Altering this step via inhibitors (e.g., statins on HMG-CoA reductase) impacts flux dramatically.
Exam Relevance for GATE
For competitive exams, identify it as the highest Ea step, not kcat or Km alone—though low substrate saturation amplifies limitation. Practice: In a pathway, the step with largest ΔG‡ is rate limiting.
| Term | Role | Relation to Ea |
|---|---|---|
| Rate Limiting | Slowest step, pathway pacemaker | Highest required |
| Rate Constant | Speed measure (k) | Inversely tied post-Ea |
| Kcat | Enzyme efficiency | Post-transition metric |
| Km | Substrate affinity | Influences near-Ea speed |
