Question 1

Why are detergents used to purify membrane proteins?

Accepted Answer

Detergents contain hydrophilic head groups and hydrophobic tails that form spherical micelles, allowing them to solubilize membrane proteins effectively. The hydrophobic portion replaces membrane phospholipids while keeping proteins in solution, enabling extraction and purification using techniques like affinity chromatography for downstream analysis.

Question 2

What are the three main types of detergents used in protein purification?

Accepted Answer

Ionic detergents like sodium dodecyl sulfate have charged head groups and denature proteins. Non-ionic detergents such as Triton X-100 are uncharged, while zwitterionic detergents like CHAPS have both positive and negative charges. Non-ionic and zwitterionic detergents are preferred because they preserve protein function.

Question 3

What is critical micelle concentration and why does it matter?

Accepted Answer

Critical micelle concentration (CMC) is the specific concentration at which detergent monomers form micelles in solution. For effective protein purification, detergent concentration must exceed the CMC, with at least one micelle per membrane protein. CMC varies by detergent structure and is affected by temperature and pH conditions.

Question 4

How does the detergent-to-lipid ratio affect membrane solubilization?

Accepted Answer

At ratios of 0.1:1 to 1:1, the lipid bilayer remains largely undisturbed. Ratios of 1:1 to 2:1 create mixed micelles containing lipid, protein, and detergent molecules. At 10:1 ratios, detergent completely replaces lipids in the protein complex. The optimal ratio is determined experimentally for each target protein.

Question 5

How does detergent structure influence its mildness and effectiveness?

Accepted Answer

Detergents with larger polar head groups or longer alkyl chains are milder and less likely to denature proteins. Membrane composition also affects detergent choice; for example, Triton X-100 solubilizes bacterial inner membranes but not outer membranes, while digitonin specifically targets eukaryotic plasma membranes.

Question 6

What are the three stages of integral membrane protein purification?

Accepted Answer

First, detergent micelles interact with the outer membrane bilayer. Next, the membrane solubilizes, forming lipid-detergent and protein-detergent complexes. Finally, after detergent removal, the protein is purified using chromatography techniques, with the choice of detergent determining whether the protein remains functionally active.

Question 7

How do membrane composition and detergent selection relate to protein extraction?

Accepted Answer

Different membranes require different detergents based on their lipid composition and protein structure. Triton X-100 acts on bacterial inner membranes but cannot solubilize outer membranes, while digitonin works specifically on eukaryotic plasma membranes. Selecting the correct detergent ensures efficient extraction while maintaining protein integrity.

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