Enzyme with Highest Turnover Number kcat: Catalase Tops Catalase, Carbonic Anhydrase, Fumarase, Acetylcholinesterase

The enzyme with highest turnover number kcat is catalase, a must-know for GATE Life Sciences and biochemistry students. Its kcat reaches 4 × 107 s-1, outpacing carbonic anhydrase, fumarase, and acetylcholinesterase.

✅ Correct Answer

(A) Catalase

Catalase has the highest turnover number (kcat) among the options at up to 4 × 107 s-1, far exceeding carbonic anhydrase (~106 s-1), acetylcholinesterase (~105 s-1), and fumarase (~103 s-1).

kcat Definition

The turnover number, or kcat, measures substrate molecules converted to product per enzyme active site per second at saturation (Vmax/[E]total). Higher kcat indicates faster catalysis; diffusion-limited enzymes approach ~108-109 s-1. Catalase’s heme-based mechanism decomposes H2O2 explosively, yielding the highest value.

Option Breakdown

Enzyme kcat (s-1) Key Fact
(A) Catalase (H2O2) 4 × 107 Fastest known; diffusion-limited.
(B) Carbonic anhydrase 105 – 106 Rapid CO2 hydration; high but lower than catalase.
(C) Fumarase (fumarate) ~800 TCA cycle enzyme; moderate speed.
(D) Acetylcholinesterase 1.4 × 104 Hydrolyzes ACh; efficient but not top.
  • (A) Correct: Tops lists at 40 million s-1; breaks down toxic H2O2: 2 H2O2 → 2 H2O + O2.
  • (B) Incorrect: ~400,000-1,000,000 s-1 for CO2/HCO3–; fast but catalase exceeds by 40-100x.
  • (C) Incorrect: Low ~800 s-1; slower hydration in Krebs cycle.
  • (D) Incorrect: ~14,000-140,000 s-1 for neurotransmitter; diffusion-near but trails leaders.

Factors Influencing kcat

Evolution favors high kcat for critical roles like detoxification (catalase). Lower for precise control (fumarase). All listed are efficient, but catalase’s compound I intermediate enables burst kinetics.

Understanding Turnover Number (kcat)

kcat, or turnover number, quantifies catalytic speed: substrate molecules processed per second per active site. Calculated as kcat = Vmax / [E]total, it peaks in diffusion-limited enzymes near 108 s-1. Catalase exemplifies this by rapidly neutralizing H2O2.

kcat Comparison Table

Enzyme Substrate kcat (s-1) Role
Catalase H2O2 4 × 107 Antioxidant defense
Carbonic Anhydrase CO2/HCO3– 105–106 pH/blood CO2 balance
Acetylcholinesterase Acetylcholine 1.4 × 104 Nerve signaling
Fumarase Fumarate 800 TCA cycle

Why Catalase Excels

Catalase’s four subunits and channel-like active site enable massive throughput, essential for peroxide detox in peroxisomes. Carbonic anhydrase is rapid for respiration but zinc-based limits it below catalase. Fumarase and acetylcholinesterase prioritize specificity over raw speed.

GATE Exam Insights

Questions like “Among catalase, carbonic anhydrase, fumarase, acetylcholinesterase, which has highest kcat?” test enzyme kinetics. Memorize: catalase > CA > AChE > fumarase. Use for MCQs on catalytic efficiency (kcat/Km).

Master the enzyme with highest turnover number kcat—catalase—for top scores in molecular biology sections.