Question 1

Which of the following statements best explains why integral membrane proteins require detergents for extraction from the membrane, while peripheral proteins do not?

Accepted Answer

Integral proteins are embedded in the bilayer through hydrophobic interactions, which detergents disrupt, whereas peripheral proteins interact via polar interactions that can be disrupted by changes in salt concentration or pH.

Question 2

A hydropathy plot for a membrane protein reveals seven regions of high hydropathy, each about 20 residues long. What structural feature is most likely present in this protein?

Accepted Answer

Seven transmembrane $$\alpha-$$helices, as seen in bacteriorhodopsin.

Question 3

Which of the following amino acids would most likely be found in the hydrocarbon core of a transmembrane $$\alpha-$$helix, based on transfer free energy values?

Accepted Answer

Leucine ($$\Delta G_{transfer} = 13.0 kJ/$$mol)

Question 4

Why are tyrosine and tryptophan residues often found at the boundary between the membrane and aqueous environment in integral membrane proteins?

Accepted Answer

Their amphipathic nature allows them to interact with both lipid and aqueous phases, stabilizing the interface.

Question 5

Which of the following best describes a polytopic integral membrane protein?

Accepted Answer

It crosses the membrane multiple times, with several hydrophobic sequences of about 20 residues each.

Question 6

A mutation replaces a hydrophobic residue in the transmembrane region of a $$\alpha-$$helix with a charged residue. What is the most likely effect on the protein?

Accepted Answer

The protein may misfold or fail to insert properly into the membrane due to unfavorable energetics.

Question 7

Which of the following is a function of membrane proteins that is directly related to energy conversion in cells?

Accepted Answer

Organization of enzymes for sequential reactions such as photosynthesis and oxidative phosphorylation.

Question 8

Why is it more difficult to predict the topology of $$\beta-$$barrel membrane proteins from their primary sequence compared to $$\alpha-$$helical membrane proteins?

Accepted Answer

Hydropathy plots do not reveal obvious transmembrane regions for $$\beta-$$barrels due to alternating hydrophobic and hydrophilic residues.

Question 9

Which of the following best describes the role of SNARE proteins in neurotransmission?

Accepted Answer

They mediate vesicle fusion with the plasma membrane, allowing neurotransmitter release.

Question 10

A protein is found to be covalently attached to a lipid, which anchors it to the membrane. What type of membrane protein is this?

Accepted Answer

Lipid-anchored (lipid-linked) membrane protein.

Question 11

Which of the following is TRUE regarding caveolae?

Accepted Answer

They are local regions of the plasma membrane enriched in cholesterol and caveolin, increasing surface area for membrane receptors.

Question 12

Which of the following statements about monotopic integral proteins is correct?

Accepted Answer

They interact with only one leaflet of the membrane via small hydrophobic domains.

Question 13

Which property of membrane proteins allows them to organize sequential reactions for energy conversion?

Accepted Answer

Ordered arrays of enzymes embedded in the membrane.

Question 14

What is the main reason that membrane proteins are more difficult to study structurally than soluble proteins?

Accepted Answer

They require artificial membranes to maintain their structure and activity, making purification and crystallization challenging.

Question 15

Which of the following best describes the topology of $$\beta-$$strands within a $$\beta-$$barrel membrane protein such as porin?

Accepted Answer

Successive amino acids in a strand alternately point into the barrel interior and outward toward the exterior.

Question 16

Which of the following is a key function of membrane microdomains (rafts)?

Accepted Answer

They facilitate clustering of specific proteins, such as receptors, to enhance signaling.

Question 17

Which of the following is NOT a way that proteins can induce membrane curvature?

Accepted Answer

Covalent attachment of carbohydrates to the protein.

Question 18

Which of the following is TRUE about the spike protein of SARS-CoV-2 and the ACE2 receptor on host cells?

Accepted Answer

Both are integral membrane proteins.

Question 19

Which of the following best explains why $$\beta-$$barrels are specifically found in membrane-spanning domains rather than any kind of $$\beta-$$sheet?

Accepted Answer

$$\beta-$$barrels form closed structures that allow the hydrophobic exterior to interact with the membrane and the hydrophilic interior to form channels.

Membrane Proteins: Structure, Function, and Classification

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