Question 1

Which reagent is commonly used in SDS-PAGE to reduce disulfide bonds in proteins, allowing for the separation of subunits?

Accepted Answer

β-mercaptoethanol (BME)

Question 2

In Native PAGE, proteins are separated based on which property?

Accepted Answer

Charge, shape, and size

Question 3

A protein sample runs as a single band at 180 kDa on Native PAGE, but as two bands at 90 kDa and 90 kDa on SDS-PAGE with β-mercaptoethanol. What does this indicate about the protein's structure?

Accepted Answer

It is a homodimer of 90 kDa subunits linked by disulfide bonds

Question 4

Why does SDS-PAGE separate proteins primarily by molecular weight?

Accepted Answer

SDS coats proteins with negative charge, masking their native charge and shape

Question 5

A protein runs as a single band at 400 kDa on Native PAGE. On SDS-PAGE with β-mercaptoethanol, it shows four bands at 180, 160, 90, and 60 kDa. What is the most likely subunit composition of this protein?

Accepted Answer

Protein X has 4 subunits with masses of 180, 160, 90 & 60 kDa.

Question 6

If a protein migrates as a single band at 120 kDa on Native PAGE and as a single band at 60 kDa on SDS-PAGE with β-mercaptoethanol, what does this suggest?

Accepted Answer

The protein is a homodimer of 60 kDa subunits

Question 7

Which of the following best explains why β-mercaptoethanol is used in SDS-PAGE analysis?

Accepted Answer

It reduces disulfide bonds, allowing subunits to separate

Question 8

A protein sample shows two bands at 100 kDa and 50 kDa on SDS-PAGE, but only one band at 150 kDa on Native PAGE. What is the most likely explanation?

Accepted Answer

The protein is a heterodimer of 100 kDa and 50 kDa subunits

Question 9

Which equation best describes the relationship between the migration distance ($d$) of a protein in SDS-PAGE and its molecular weight ($M$)?

Accepted Answer

$d \propto -\log(M)$

Question 10

A researcher suspects that a protein contains disulfide-linked subunits. Which experimental approach would best confirm this hypothesis?

Accepted Answer

Run SDS-PAGE with and without β-mercaptoethanol and compare band patterns

Question 11

If a protein migrates as a single band at 200 kDa on Native PAGE, but as two bands at 120 kDa and 80 kDa on SDS-PAGE with β-mercaptoethanol, what is the most likely quaternary structure?

Accepted Answer

Heterodimer of 120 kDa and 80 kDa subunits

Question 12

Which of the following statements about SDS-PAGE is TRUE?

Accepted Answer

SDS-PAGE separates proteins based on their molecular weight

Question 13

A protein complex is analyzed by Native PAGE and shows a band at 300 kDa. SDS-PAGE with β-mercaptoethanol reveals three bands at 120 kDa, 100 kDa, and 80 kDa. What is the most likely composition of the complex?

Accepted Answer

Heterotrimer of 120 kDa, 100 kDa, and 80 kDa subunits

Question 14

Which of the following would NOT be revealed by SDS-PAGE analysis?

Accepted Answer

The native quaternary structure of a protein

Question 15

A protein runs as a single band at 240 kDa on Native PAGE. On SDS-PAGE with β-mercaptoethanol, it shows two bands at 120 kDa each. What does this suggest about the protein?

Accepted Answer

It is a homodimer of 120 kDa subunits linked by disulfide bonds

Question 16

Which of the following best describes the effect of β-mercaptoethanol on protein structure during SDS-PAGE?

Accepted Answer

It reduces disulfide bonds, separating subunits

Question 17

A protein sample is analyzed by SDS-PAGE in the absence of β-mercaptoethanol and shows a single band at 200 kDa. When β-mercaptoethanol is added, two bands at 120 kDa and 80 kDa appear. What does this indicate?

Accepted Answer

The protein is a heterodimer of 120 kDa and 80 kDa subunits linked by disulfide bonds

Question 18

Which of the following is TRUE regarding the migration of proteins in SDS-PAGE?

Accepted Answer

Smaller proteins migrate further down the gel

Question 19

A protein complex is analyzed by Native PAGE and shows a band at 360 kDa. SDS-PAGE with β-mercaptoethanol reveals three bands at 180 kDa, 120 kDa, and 60 kDa. What is the most likely composition of the complex?

Accepted Answer

Heterotrimer of 180 kDa, 120 kDa, and 60 kDa subunits

SDS-PAGE Strategies and Protein Subunit Analysis

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