1. What is a ‘G-protein’? How are they activated and deactivated? What are some
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Question
1. What is a ‘G-protein’? How are they activated and deactivated? What are some hallmarks of the proteins they are associated with?
2. All cell signaling pathways eventually end by activating ‘cellular response proteins’. What general types of proteins can these be? What effects do they have on cell function?
3.What effect can the length of a signal transduction pathway have on the eventual outcome? What might be a difference between a pathway with 4 separate steps, compared to a pathway with 7 separate steps?
Explanation / Answer
A) 1. G proteins, also known as guanine nucleotide-binding proteins, are a family of proteins that act as molecular switches inside cells, and are involved in transmitting signals from a variety of stimuli outside a cell to its interior.
2. Their activity is regulated by factors that control their ability to bind to and hydrolyze guanosine triphosphate (GTP) to guanosine triphosphate(GDP). When they are bound to GTP, they are 'on', and, when they are bound to GDP, they are 'off'. G proteins belong to the larger group of enzymes called GTPase.
There are two classes of G proteins.
1.first function as monomeric small GTPase,
2.second function as hetero trimeric G Protein complexes.
Latter class of complexes is made up of alpha (), beta () and gamma () subunits In addition, the beta and gamma subunits can form a stable dimeric complex referred to as the beta-gamma complex.
B) 1. G proteins located within the cell are activated by G proteins-coupled receptors (GPCRs) that span the cell membrane.
2. Signaling molecules bind to a domain of the GPCR located outside the cell, and an intracellular GPCR domain then in turn activates a particular G protein.
3.Whether Some inactive-state GPCRs have also been shown to be "pre-coupled" with G proteins. 4.The G protein activates a cascade of further signaling events that finally results in a change in cell function.
C) Activation of G proteins
1.When a ligand activates the G proteins-coupled receptors, it induces a Confirm change in the receptor that allows the receptor to function as guanine nucleotide exchange factor (GEF) that exchanges GDP for GTP thus turning the GPCR (G proteins-coupled receptors)"on".
2. The GTP (or GDP) is bound to the G subunit in the traditional view of hetero trimeric GPCR (G proteins-coupled receptors) activation. This exchange triggers the dissociation of the G subunit (which is bound to GTP) from the G dimer and the receptor as a whole.
3. molecular rearrangement, reorganization, and precomplexing of effector molecules are beginning to be accepted.Both G-GTP and G can then activate different signaling cascades and effector proteins, while the receptor is able to activate the next G protein
D) Deactivation/Termination of G proteins
1. The G subunit will eventually hydrolyze the attached GTP to GDP by its inherent enzymatic activity, allowing to re-associate with G and starting a new cycle.
2. A group of proteins called regulator of G Protein Singalling (RGSs), act as GTPase-activating protein (GAPs), are specific for G subunits. These proteins accelerate the hydrolysis of GTP to GDP, thus terminating the transduced signal.
3. In some cases, the effector itself may possess intrinsic GAP activity, which then can help deactivate the pathway. This is true in the case of phospholipase C-beta, which possesses GAP activity within its C-terminal region. This is an alternate form of regulation for the G subunit.
Noted that such G GAPs do not have catalytic residues (specific amino acid sequences) to activate the G protein. They work instead by lowering the required activation energy for the reaction to take place
E) Hallmarks of proteins
1. G protein-coupled receptor and G proteins working together transmit signals from many hormones neurotransmitter and other signaling factors.
2. They found that when adrenaline binds to a receptor, the receptor does not stimulate enzymes (inside the cell) directly. Instead, the receptor stimulates a G protein, which then stimulates an enzyme. An example is adenylate cyclase, which produces the second massanger cyclic AMP
3.G proteins regulate metabolic enzyes,ion,channels,transporter protein,and other parts of the cell machinery, controllingtrancription,motility,contractility and secretion, which in turn regulate diverse systemic functions such as Embryonic development, learning and memory, and homeostasis.
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