This set of Molecular Endocrinology Multiple Choice Questions & Answers (MCQs) focuses on “G Protein-Coupled Receptors”.
1. What is the common structural feature of the GPCR superfamily?
a) Seven membrane-spanning helices
b) DNA binding domain
c) Ligand binding site
d) Receptor binding
View Answer
Explanation: A common structural trait which is the seven membrane-spanning helices, is shared by all GPCR superfamily members. In their primary amino acid sequence and in the domains that serve in ligand binding, separate subfamilies diverge.
2. Which among the following is the alpha subunit gene which couples many GPCRs to the stimulation of adenylyl cyclase?
a) Gsα
b) Gtlα
c) Glα
d) G7b
View Answer
Explanation: In mammals, there are at least 16 separate alpha-subunit genes. In range of expression, these differ greatly. Some are universal, such as Gsα, which combines several GPCRs with adenylyl cyclase stimulation.
3. Which among the following is the alpha subunit gene which couples the GPCR rhodopsin to cyclic guanosine monophosphate phosphodiesterase?
a) Gsα
b) Gtlα
c) Glα
d) G7b
View Answer
Explanation: In mammals, there are at least 16 different alpha-subunit genes. In retinal rod photoreceptor cells, Gtlα, which integrates GPCR rhodopsin with cyclic guanosine monophosphate phosphodiesterase, is extremely localized.
4. All GPCRs act as guanine nucleotide exchange factors.
a) True
b) False
View Answer
Explanation: All GPCRs function as exchange factors for guanine nucleotides. They catalyze the exchange of GDP strongly bound to the alpha-subunit of heterotrimeric G proteins for GTP in their activated (agonist-bound) conformation.
5. RGS domains with additional roles have been found in modular proteins also.
a) True
b) False
View Answer
Explanation: RGS domains were also discovered in modular proteins with additional features. In some cases, the role of low-molecular-weight GTP-binding proteins in the RAS superfamily is related to heterotrimeric G protein signalling.
6. What are the three separable components in a G protein coupled signal transduction?
a) Receptor, G protein, and ligand
b) Receptor, G protein, and effector
c) C domain, G protein, and effector
d) Substrate, G protein, and effector
View Answer
Explanation: There are three separable components (i.e., receptor, G protein, and effector) involved in G protein-mediated signal transduction. Within a cell, several GPCRs, G proteins, and effectors are expressed.
7. Where do the regulators of G protein signaling (RGS) bind to accelerate the GTPase activity of GPCR?
a) G protein alpha subunit
b) G protein beta subunit
c) G protein gamma subunit
d) G protein DNA binding domain
View Answer
Explanation: A broad gene family has been identified whose members are G protein signalling (RGS) regulators. RGS proteins bind to the transition state of the GTP-activated G protein alpha-subunit and accelerate GTPase behavior.
8. Who had discovered that a family of GPCR kinases and arrestin proteins is involved in GPCR desensitization?
a) Mcleod
b) Murphy
c) Ostwald
d) Lefkowitz
View Answer
Explanation: Lefkowitz has shown that, after agonist binding, a family of GPCR kinases and arrestin proteins is involved in GPCR desensitization. In comparison to G proteins, GPCRs interact specifically with a variety of other proteins.
9. How are cysteines seen in the C terminal of GPCR structure?
a) Palmitoylated
b) Acetylated
c) Dephosphorylated
d) Phosphorylated
View Answer
Explanation: In the C terminal of G-Protein coupled receptors, one or more cysteines are palmitoylated. This creates a fourth intracellular loop in the structure which has major functional roles. Two highly conserved cysteine residues are present near the extracellular surface in most rhodopsin-like GPCRs, forming a disulfide bond that has been shown to be necessary for the structure and activation of the receptor.
10. What process is important for proper folding and trafficking to the plasma membrane in the GPCRs?
a) Acetylation
b) Glycosylation
c) Phosphorylation
d) Palmitoylation
View Answer
Explanation: Instead of ligand binding, glycosylation tends to be essential for proper folding and trafficking of the plasma membrane. Glycosylation is an important and highly regulated secondary protein processing mechanism within cells.
11. Which is the largest group of G protein coupled receptors?
a) Family 1
b) Family 2
c) Family 3
d) Family 4
View Answer
Explanation: Family 1 is the largest group of G protein coupled receptors. This family itself can be subdivided into many groups which have their own particular functions. The largest category of plasma membrane receptors are G-protein coupled receptors (GPCRs), of which rhodopsin and adrenergic receptors are the most widely recognized.
12. Which among the following is an example of the largest subset of Family 1 of GPCR?
a) Opsins
b) Chemokine
c) Morphine
d) Leptins
View Answer
Explanation: Opsins, odorant receptors, and monoamine, purinergic, and opiate receptors constitute the main subset of family 1. These are distinguished by a short N-terminus end which defines their uniqueness.
13. Which among the following is an example of the second subset of Family 1 of GPCR?
a) Chemokine
b) Monoamines
c) Purinergic
d) Opsins
View Answer
Explanation: Chemokine and activated protease, forms the second subset of Family 1 of GPCR. Certain peptide hormone receptors, characterized by a slightly longer N-terminus, also comprise this subset. Chemokines are a family of small cytokines, or cell-secreted signalling proteins.
14. Which among the following is an example of the third subset of Family 1 of GPCR?
a) Follicle Stimulating Hormone
b) Leptin
c) Opsins
d) Chemokines
View Answer
Explanation: Receptors for large glycoprotein receptors, TSH, luteinizing hormone, and follicle-stimulating hormone are found in the third subset of family 1 of GPCR. They have an extracellular N-terminus of around 400-residues.
15. Parathyroid Hormone belongs to which family of G Protein Coupled Receptors?
a) Family 2
b) Family 1
c) Family 3
d) Family 4
View Answer
Explanation: Even inside the transmembrane helices, Family 2 displays virtually no sequence homology to Family 1, and is distinguished by an N-terminus of around 100 residues. Receptors for many peptide hormones such as parathyroid hormone (PTH), calcitonin, vasoactive intestinal peptide, and hormone releasing corticotropin are included in the membership.
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