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GATE 2026 BT Q.35: solution and answer

GATE 2026 · BTQ.35 · 1 mark · NAT

You have purified an enzyme using a series of chromatographic methods. It was observed that a 10 µg mL⁻¹ of this purified enzyme converted 10 mM substrate per hour at 25°C and pH 7. Its specific activity is _______ IU µg⁻¹. (rounded off to three decimal places)

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Answer and solution

ANSWER

The purified enzyme's specific activity is 0.017 IU µg⁻¹.

Official answer: 0.016 to 0.017Source: Official answer key, GATE 2026 BT (IIT Guwahati)

  1. Definition: 1 IU (International Unit) of enzyme = the amount of enzyme that converts 1 µmol of substrate per minute under the given assay conditions.

  2. Choose a convenient basis, e.g. 1 mL of the enzyme solution. Enzyme mass present = 10 µg mL⁻¹ × 1 mL = 10 µg.

  3. Substrate converted in that 1 mL, per hour = 10 mM × 1 mL = 10 mmol L⁻¹ × 0.001 L = 0.01 mmol = 10 µmol per hour.

  4. Convert the rate to a per-minute basis (as required by the IU definition): 10 µmol hour⁻¹ ÷ 60 min hour⁻¹ = 0.16667 µmol min⁻¹, i.e. 0.16667 IU of total activity present in the 1 mL sample.

  5. Specific activity = total activity (IU) ÷ total enzyme mass (µg) = 0.16667 IU ÷ 10 µg = 0.016667 IU µg⁻¹.

  6. Rounded to three decimal places: specific activity = 0.017 IU µg⁻¹.

KEY IDEA

One International Unit (IU) of enzyme activity is defined as the amount of enzyme that converts 1 µmol of substrate per minute under the specified assay conditions. Specific activity is total enzyme activity (IU) divided by the total mass of enzyme protein (µg) present in the sample, so it is independent of the assay volume chosen for the calculation — any consistent volume basis (1 mL, 1 L, etc.) gives the same ratio.

WATCH OUT FOR

A common error is to divide by 1000 twice (once converting mM to M and again converting mmol to µmol) or to forget the hour-to-minute conversion (dividing/multiplying by 60 incorrectly), which changes the final specific activity by one or two orders of magnitude. Another common slip is reporting activity in IU/mL (total activity concentration) instead of IU/µg (specific activity, i.e., normalized to enzyme mass).

From the GATE 2026 BT paper. Every question of this paper · Download the full paper · All 9 Assays & electrophoresis questions, with notes

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