GATE 2019 BT Q.25: solution and answer

GATE 2019 · BTQ.25 · 1 mark · NAT

Protein concentration of a crude enzyme preparation was 10 mg mL $^{- 1}$ . 10 μL of this sample gave an activity of 5 μmol min $^{- 1}$ under standard assay conditions. The specific activity of this crude enzyme preparation is ___________ units mg $^{- 1}$ .

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Answer and solution

ANSWER

The specific activity is 50 units mg⁻¹.

Official answer: 50Source: Official answer key, GATE 2019 BT (IIT Madras)

Definition: one unit (U) of enzyme activity is the amount of enzyme that converts 1 μmol of substrate per minute under standard assay conditions. The assay on the 10 μL aliquot gave 5 μmol min $^{- 1}$ , so that aliquot contains 5 units of activity.
Definition: specific activity = enzyme activity (in units) per mg of total protein present in that same aliquot.
Protein mass in the 10 μL aliquot: $10 mg mL^{- 1} \times 10 μ L = 10 mg mL^{- 1} \times 0.01 mL = 0.1 mg$ .
Specific activity $= \frac{5 units}{0.1 mg} = 50 units mg^{- 1}$ .

KEY IDEA

One enzyme unit (U) is defined as the amount of enzyme that converts 1 μmol of substrate (or produces 1 μmol of product) per minute under the stated assay conditions; specific activity is that activity expressed per mg of total protein in the same sample.

WATCH OUT FOR

Using the protein concentration (10 mg mL⁻¹) directly as "mg protein" instead of first converting it to the mass of protein actually present in the 10 μL aliquot that was assayed (0.1 mg), which would wrongly give 5/10 = 0.5 units mg⁻¹.

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GATE 2019 BT Q.25: solution and answer

Answer and solution

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