This set of Cell Biology Multiple Choice Questions & Answers (MCQs) focuses on “Cell Signaling and Signal Transduction – G-protein Receptors and Messengers”.
1. G-protein coupled receptors are referred to as seven-transmembrane receptors.
a) True
b) False
View Answer
Explanation: G-protein coupled receptors (GCPRs) are called so because they interact with the GTP-binding proteins – G-proteins. These are also called 7TM because they consist of 7 transmembrane helices.
2. The G-protein coupled receptors have their _____________________ outside the cell.
a) amino terminus
b) carboxyl terminus
c) alpha helices
d) beta helices
View Answer
Explanation: The G-protein coupled receptors have their amino-terminus outside the cell and carboxyl-terminus inside the cell. The 7 transmembrane helices (alpha-helices) traverse the plasma membrane.
3. On the cytoplasmic site of GCPRs, the G-proteins bind to ___________________
a) amino-terminus
b) promoter region
c) second loop
d) third loop
View Answer
Explanation: There are 3 loops of G-protein coupled receptor located on the outside of the cell and 3 loops located on the cytoplasmic site. The G-proteins bind to the third intracellular loop while the loops located outside the cell form the ligand-binding active site.
4. Which of the following is a stimulus for rhodopsin?
a) light
b) acidic pH
c) basic pH
d) neurotoxins
View Answer
Explanation: Rhodopsin is a very stable G-protein coupled receptor. This receptor is always bound to a retinal group (the ligand). Rhodopsin is also a biological pigment in the retina and is very sensitive to light.
5. The inactive conformation of a G-protein coupled receptor is stabilized by ____________________
a) covalent interactions
b) hydrogen binding
c) non-covalent interactions
d) ionic interactions
View Answer
Explanation: The inactive conformation of the G-protein coupled receptor is stabilized by non-covalent interactions between the residues in the transmembrane alpha helices. Rotation or movements in these loops cause changes in the conformation of cytoplasmic loops.
6. The G-proteins bind only to _________________
a) cytosine
b) guanine
c) thymine
d) adenine
View Answer
Explanation: The G-proteins are called so because they bind only to the guanine nucleotides – GTP (guanosine triphosphate) and GDP (Guanosine diphosphate). The protein is heterotrimeric and consists of three different polypeptides subunits.
7. Desensitization is the process of blocking receptors from turning on ______________
a) G-proteins
b) Transcription factors
c) Promoters
d) Activators
View Answer
Explanation: Desensitization is the process of preventing the G-protein coupled receptors from turning on additional G-proteins to regain sensitivity for future stimulus. The activated receptors turn on G-proteins and the latter then turns effectors on, and all of them must remain inactive to be able to respond to future stimulus.
8. Arrestins compete with ________________ for binding with G-protein coupled receptors.
a) Effectors
b) Primary messengers
c) Secondary messengers
d) G-proteins
View Answer
Explanation: Arrestins are small group of proteins that bind to the G-protein coupled receptors. To bind to these receptors, the arrestins compete with heterotrimeric G-proteins. In the process of desensitization, arrestin binding prevents further activation of the G-proteins.
9. Retinitis pigmentosa is an inherited disorder caused due to mutation in the gene that encodes for __________________
a) G-proteins
b) Arrestin
c) Rhodopsin
d) G-protein receptor kinase
View Answer
Explanation: Retinitis pigmentosa (RP) is an inherited disorder characterized by progressive degeneration of the retina and eventual blindness. The disease is caused due to mutation in a gene that encodes the visual pigment of rods – rhodopsin.
10. Adenoma is caused by the over-secretion of ____________________
a) arrestin
b) rhodopsin
c) kinetin
d) thyroid hormone
View Answer
Explanation: Adenoma is a benign thyroid tumor that caused due to the secretion of large quantities of thyroid hormone without having to be stimulated by the pituitary hormone – thyroid stimulating hormone (TSH).
11. A somatic mutation is present in all of the individual’s body cells.
a) True
b) False
View Answer
Explanation: A somatic mutation is present in one cell type of an individual, such as a thyroid cell in case of an adenoma, while an inherited mutation is present in all the cells of an individual. Somatic mutations are the primary cause of human cancer.
12. The Kaposi’s sarcoma is caused by ____________________
a) Herpes virus
b) Adeno virus
c) Human immunodeficiency virus
d) Retro virus
View Answer
Explanation: The herpes virus, responsible for Kaposi’s sarcoma characterized by purplish skin lesions prevalent in AIDS patients, encodes a constitutively active receptor for interleukin-8.
13. The Cholera toxin produced by Vibrio cholera exerts its effect by _____________________
a) modifying G-protein
b) modifying primary messengers
c) modifying effectors
d) modifying receptors
View Answer
Explanation: The cholera toxin exerts its effect by inhibiting the GTPase activity of G-proteins in the cells of intestinal epithelium and modifying their G-alpha subunits. This causes the epithelial cells to secrete large volumes of fluid into the intestinal lumen.
14. Epinephrine causes an increase in ____________________
a) blood glucose levels
b) surface receptors
c) rhodopsin pigments
d) mast cells
View Answer
Explanation: Epinephrine is called the “fight-or-flight” hormone produced by the adrenal gland in situations of stress. It causes an increase in blood glucose levels to deal with stressful situations.
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