Upon activation by a receptor, a G protein exchanges bound GDP forGTP, rather than phosphorylating GDP that is already bound. Similarly,the a subunit-GTP complex has a slow GTPase activity that hydrolyzesbound GTP, rather than exchanging it for GDP. Describe experimentalevidence that would be consistent with these conclusions.

The G-protein is heterotrimeric. It has alpha, beta, and gamma subunits. When GDP is bound to the…

Answered: Upon activation by a receptor, a G…

Biochemistry

9th Edition

ISBN:9781319114671

Author:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Publisher:Lubert Stryer, Jeremy M. Berg, John L. Tymoczko, Gregory J. Gatto Jr.

Chapter1: Biochemistry: An Evolving Science

Section: Chapter Questions

Problem 1P

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Signal-transducing heterotrimeric G proteins consist of three subunits designated α, β, and γ. The Gα subunit is a GTPase switch protein that cycles between active and inactive states depending on whether it is bound to GTP or to GDP. Review the steps for ligand-induced activation of effector proteins mediated by the heterotrimeric G proteins. Suppose that you have isolated a mutant Gα subunit that has an increased GTPase activity. What effect would this mutation have on the G protein and the effector protein?
Describe the general function and structural features of G-protein-coupled receptors (GPCRs). Be sure to include an explanation of GTPase activating proteins (GAPs) and GDP exchange factors (GEFs) in the context of the GTPase cycle. How are different classes of heterotrimeric G-proteins defined?
Signaling activity of G proteins is controlled by GTPase activating proteins (GAPs) and Guanine exchange factors (GEFs). What are the mechanisms by which GAPS and GEFs control G protein functions? GEFS inhibit G protein functions by stimulating GTP hydrolysis; GAPs activate G protein functions by stimulating GDP-GTP exchange. O GAPS inhibit G protein functions by stimulating GTP hydrolysis; GEFs activate G protein functions by stimulating GDP-GTP exchange. GEFS inhibit G protein functions by stimulating GTP hydrolysis; GAPS activate G protein functions by inhibiting GDP-GTP exchange. GAPs stimulate G protein functions by blocking GTP hydrolysis; GEFS inhibit G protein functions by stimulating GTP-GDP exchange. GAPS inhibit G protein functions by stimulating GDP-GTP exchange; GEFS activate G protein functions by stimulating GTP hydrolysis
GTP-binding proteins play critical roles in many signal transductions through G proteins. Describe two examples in which G proteins act and compare the role of the G proteins in each case.
Briefly describe the following properties of the Rab and Arf GTPases: a) Size, structure and cellular localization (for structure I want to know if they are lipidated and any other unique features) , b) How are they activated and inactivated (i.e. include the GEFs and GAPs), c). Give an example of downstream cellular effects.
Some strains of bacteria or microorganisms have developed toxins that can modify the activity of the alpha subunit of G proteins which results in disease. For ex. cholera toxin, produced by Vibrio cholerae, causes ADP ribosylation of the stimulatory Gαs subunit of G proteins. This modification abolishes the GTPase of Gαs, and results in an αs subunit that is always in the “on” or active state. It results in continuous stimulation of adenylyl cyclase (AC). The main cells affected by this are the epithelial cell in gastrointestinal tract. Knowing this altered activity of AC, explain why patients affected by this toxin experience severe diarrhea and dehydration that may result in death.
GTP is an important high-energy molecule that facilitates the activation of many cellular sig- nal transduction pathways. Certain genetic dysfunctions can inhibit the ability of a cell to synthesize GTP. Which of the following describes the most direct result of GTP synthesis inhibition? A B с D The cell would be able to carry out reception and transduction but would not be able to produce the cellular response in the relevant signal transduction pathway. The G protein-coupled receptor will not be able to bind corresponding ligands, inhibiting the reception components of the relevant signal transduction pathway. The cell will use ATP instead of GTP to activate the G protein on the intracellular region of the G protein-coupled receptor. The cell would not be able to activate G proteins on the intracellular regions of G pro- tein-coupled receptors.
Many cellular processes require the hydrolysis of GTP to GDP. However, unlike ATP, GTP hydrolysis is often not used as an energy source for the cell, but rather, is associated with a group of enzymes called GTPases. Which of the following statements about GTP and GTPases is correct? 1. GTPase activating protein (GAP) binds to GTPase when it is in the active state. 2. The exchange factor GEF binds GTP directly and delivers it to the GTPase. 3. The "switching" behaviour of GTPase is because GTP hydrolysis causes the enzyme to change shape. 4. Structural differences in the terminal phosphate group explains why ATP hydrolysis releases energy and GTP does not. O 1,2 and 3 O 1 and 3 O 2 and 4 O 4 only O All of 1,2.3 and 4 are correct
In the case of GPCR (G protein coupled receptor) signaling pathways, which of the following statements is INCORRECT? The gamma subunit of the trimeric G protein has a transmembrane domain whereas the alpha and beta subunits are peripheral proteins If G alpha was locked in a GTP bound state, it would be bound to the effector enzyme rather than to the beta and gamma subunits. In some but not all signaling pathways, when the beta and gamma subunits are separated from alpha - the beta/gamma pair can also stimulate the activation of effectors
RGS proteins accelerate the GAP activity of Gα subunits. What is the direct result of this GAP activity? - hydrolysis of GTP, resulting in a GDP-bound Gα - phosphorylation of GDP to GTP, resulting in a GTP-bound Gα - an exchange of GTP for GDP bound to Gα - an exchange of GDP for GTP bound to Gα
The G protein coupled receptor (GPCR) pathway elicits diverse intracellular responses in different cells. The basic steps of GPCR signaling are outlined in this diagram. Which of the following statements correctly describes the process of GPCR signaling? The GPCR activation is reversible after the signal of the ligand diminishes. The membrane-embedded enzyme uses GTP as a secondary messenger to initiate gene expression. The ligand attaches to both the GPCR and the membrane-embedded enzyme to activate the GPCR pathway. The ligand-bound GPCR sends a GTP molecule to an enzyme in the membrane and switches it into an active state.
Continuous exposure of a G protein-coupled receptor to its ligand leads to a phenomenon known as desensitization. Describe several molecule mechanisms for receptor desensitization

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