Q.30
Horseradish peroxidase and alkaline phosphatase are the two enzymes commonly utilized
as reagents in ELISA, because these enzymes
Options:
(A) are colored proteins
(B) are very small
(C) have high turnover number
(D) bind to ELISA plates
ELISA (Enzyme-Linked Immunosorbent Assay) relies on reporter enzymes that amplify signals through catalytic substrate turnover, enabling detection of picogram analyte levels. Horseradish peroxidase (HRP) and alkaline phosphatase (AP) dominate due to their exceptional enzymatic properties ideal for immunoassay sensitivity.
The correct answer is (C) have high turnover number, meaning thousands of substrate molecules converted per enzyme per minute, generating massive colorimetric/chemiluminescent signals from minimal enzyme.
Why (C) High Turnover Number Makes HRP & AP Ideal for ELISA
HRP exhibits k_cat ~4000 min⁻¹ (ABTS/TMB substrates); AP shows k_cat ~2000 min⁻¹ (pNPP). One enzyme molecule produces millions of detectable product molecules within assay timeframe (<1 hour), providing >10⁴ amplification over direct antigen labeling. This kinetic superiority enables sub-femtogram detection limits.
Explanation of All Options
Each property evaluated for ELISA suitability:
-
(A) Are colored proteins
Wrong. HRP/AP are colorless in native form. Color develops from substrate reaction products (HRP+TMB→blue; AP+pNPP→yellow). Native coloration would interfere with blank subtraction. -
(B) Are very small
Incorrect. HRP (44 kDa), AP (140 kDa) are large glycoproteins. Small size irrelevant—conjugation chemistry (SMCC, SATA) accommodates their size for stable Ab-enzyme conjugates. -
(C) Have high turnover number
Correct. Exceptional k_cat values provide signal amplification essential for ELISA sensitivity. Direct competitors (β-galactosidase) have lower turnover. -
(D) Bind to ELISA plates
No. Nonspecific plate binding would cause high background. HRP/AP conjugated via specific crosslinking to antibodies, relying on primary/secondary binding specificity.
Quick ELISA Enzyme Property Table
| Option | Property | True for HRP/AP? | ELISA Relevance |
|---|---|---|---|
| A | Colored proteins | No | Color from substrate reaction |
| B | Very small | No | 44-140 kDa, conjugation stable |
| C | High turnover | Yes | 10⁴-10⁶ amplification |
| D | Bind ELISA plates | No | Causes high background |
Biotech relevance: High turnover enables ELISA miniaturization (96→1536-well). AP preferred for automation (no H₂O₂), HRP for chemiluminescence sensitivity. Memory trick: “Turnover = money maker for ELISA signal.”
