This set of Protein Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Structure Function Relationships – Lipid Anchored Membrane Proteins”.
1. Which of the following proteins are bound covalently to one or more lipid molecules in the plasma membrane?
a) Peripheral membrane proteins
b) Cytosolic proteins
c) Intrinsic proteins
d) Lipid-anchored proteins
View Answer
Explanation: Lipid-anchored proteins are bound covalently to one or more lipid molecules in the plasma membrane. Peripheral membrane proteins, cytosolic proteins, and intrinsic proteins are not bound covalently to one or more lipid molecules in the plasma membrane.
2. The polypeptide chain of lipid anchored proteins itself enter the phospholipid bilayer.
a) True
b) False
View Answer
Explanation: The above statement is false. The hydrophobic tail of the attached lipid is embedded in one leaflet of the membrane and anchors the protein to the membrane. The polypeptide chain itself does not enter the phospholipid bilayer.
3. Which class of proteins is shown in the figure below?
a) Cytosolic proteins
b) Peripheral membrane proteins
c) Integral membrane proteins
d) Lipid-anchored proteins
View Answer
Explanation: The protein is shown in the figure above belong to the class of lipid-anchored proteins. These proteins are bound covalently to one or more lipid molecules in the plasma membrane. They can be attached to either the cytosolic leaflet or the exoplasmic leaflet.
4. A group of cytosolic proteins is anchored to the cytoplasmic face of a membrane by a fatty acyl group attached to an amino acid at the N-terminal. Which of the following is that amino acid?
a) Tryptophan
b) Alanine
c) Methionine
d) Glycine
View Answer
Explanation: A group of cytosolic proteins is anchored to the cytoplasmic face of a membrane by a fatty acyl group attached to an N-terminal glycine residue. This modification of N-terminal glycine residue is called acylation.
5. The lipid anchors used to anchor proteins to the cytosolic face are not used for the exoplasmic face, and vice versa.
a) False
b) True
View Answer
Explanation: The above statement is true. The lipid anchors used to anchor proteins to the cytosolic face are not used for the exoplasmic face, and vice versa. In lipid anchored proteins, the hydrocarbon chains of the lipid anchor are embedded in the bilayer, but the protein itself does not enter the bilayer.
6. Which of the following process is not associated with lipid-anchored proteins?
a) Acylation
b) Prenylation
c) GPI anchor linking
d) Methylation
View Answer
Explanation: Methylation is not associated with lipid-anchored proteins. Acylation, Prenylation, and GPI anchor linking are the ways by which a cytosolic protein can be linked to the plasma membrane. In acylation a fatty acyl group is attached to the protein, and in prenylation a prenyl group is attached.
7. Which of the following fatty acyl groups can be covalently attached to a protein so that it can get anchored to the plasma membrane?
a) Acyl carnitine
b) Stearic acid
c) Linoleic acid
d) Palmitate
View Answer
Explanation: Palmitate/myristate are the fatty acyl groups that can be covalently attached to a protein so that it can get anchored to the plasma membrane. These membrane-anchored proteins are also called as lipid anchored proteins.
8. Which of the following is necessary and sufficient for the binding of proteins to the membrane?
a) N-terminal methionine
b) Hydrophobic amino acid residues
c) Several sugar groups
d) GPI anchors
View Answer
Explanation: GPI anchors are necessary and sufficient for the binding of proteins to the membrane. For instance, the treatment of cells with phospholipase C releases GPI anchored proteins such as Thy-1 and PLAP from the cell surface. Phospholipase C cleaves the phosphate-glycerol bond in phospholipids and GPI anchors.
9. What is the modification in which a hydrocarbon chain is attached to an amino acid residue on a protein which helps it anchor onto the plasma membrane?
a) Ubiquitination
b) Acetylation
c) Methylation
d) Prenylation
View Answer
Explanation: Prenylation is the modification in which a hydrocarbon chain is attached to an amino acid residue on a protein that helps it anchor onto the plasma membrane. Prenyl anchors are built from 5-carbon isoprene units, which are also used in the synthesis of cholesterol.
10. Some specialized proteins with distinctive covalently attached polysaccharides called proteoglycans are bound to the exoplasmic face of the plasma membrane by which of the following?
a) Fatty acyl groups
b) Prenyl groups
c) Methyl groups
d) GPI anchors
View Answer
Explanation: Some specialized proteins with distinctive covalently attached polysaccharides called proteoglycans are bound to the exoplasmic face of the plasma membrane by GPI (Glycosylphosphatidylinositol) anchors. The fatty acyl chains of GPI anchor extend into the lipid bilayer, just like those of typical membrane phospholipids.
11. A group of cytosolic proteins is anchored to the membrane by a hydrocarbon chain. This hydrocarbon chain is attached to which amino acid residue?
a) Lysine
b) Tyrosine
c) Alanine
d) Cysteine
View Answer
Explanation: A group of cytosolic proteins is anchored to the membrane by a hydrocarbon chain. This hydrocarbon chain is attached to a cysteine residue at or near the C-terminus.
12. Which of the following is not a lipid-anchored protein?
a) Ras proteins
b) Thy-1
c) PLAP
d) Glycophorin
View Answer
Explanation: Glycophorin is not a lipid-anchored protein. Ras proteins, Thy-1, and PLAP are lipid-anchored proteins. These proteins are attached to the plasma membrane with the help of GPI anchors.
13. GPI anchors are glycolipids.
a) False
b) True
View Answer
Explanation: The above statement is true. The exact structure of GPI anchors varies greatly among different cell types, but they always contain phosphatidylinositol, whose two fatty acyl chains extend into the lipid bilayer; phosphoethanolamine, which covalently links the anchor to the C-terminus of a protein; and several sugar residues. Therefore, GPI anchors are glycolipids.
14. Which of the following anchors help in the attachment of proteins to the plasma membrane, specifically on the cytosolic leaflet?
a) GPI anchors
b) Acylation and GPI anchors
c) Prenylation and GPI anchors
d) Acylation and prenylation
View Answer
Explanation: Acylation and prenylation are the processes of the addition of anchors that help in the attachment of proteins to the plasma membrane, specifically on the cytosolic leaflet. GPI anchors attach proteins on the outer leaflet of the plasma membrane.
15. Ras proteins (a GTPase superfamily) are anchored to the plasma membrane by which of the following?
a) GPI anchors
b) Methylation
c) Acylation
d) Prenylation
View Answer
Explanation: Ras proteins (a GTPase superfamily) are anchored to the plasma membrane by prenylation. In prenylation a hydrocarbon chain s attached to a cysteine residue at or near the C-terminus of the protein.
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