A GTP-binding protein is an enzyme that acts to start a reaction going by catalyzing the addition of a phosphate group, which is a high-energy bond, onto another molecule (Landers, n.d.).
The search for GTP-binding proteins was initiated in the early 1960s with the aim of identifying the regulatory step between the membrane and the inside of the cell. Even though proteins that bind to GTP were isolated relatively early, it took almost a decade before it was understood that they were involved in lowering levels of cAMP inside cells.
1. a) Binding of ATP to the nucleotide binding domain
GTP binding proteins have a seven α-helix bundle structure. The three helices on the top surface of the protein are connected to a nucleotide binding pocket, one of which is bound by GTP or GDP. In this configuration, the GTP binding site is exposed to the exterior of the cell, while the GDP binding site is internalized.
When a signal molecule binds to an extracellular receptor, it activates GTP hydrolysis and releases GDP from its binding pocket on the protein surface.
This causes structural changes that make GTP more accessible to the interior of the cell, where it can diffuse into the nucleotide binding pocket and bind to the protein. The structural change also exposes other sites on the protein’s surface for interaction with cytoplasmic proteins that initiate various cellular responses.
2. b) Hydrolysis of GTP bound to the nucleotide binding domain
The binding of GTP to the nucleotide-binding domain of an alpha subunit initiates the activation process. The alpha subunit and its associated beta subunits then detach from the GDP-bound gamma subunit, forming a heterotrimeric G protein.
GTPase activity occurs in two steps. First, the gamma phosphate group is hydrolyzed by the beta and gamma subunits to produce GDP, which remains bound to both units. This process is known as prenylation. Second, the alpha subunit hydrolyzes the beta phosphate group to produce GTP, which remains bound until it is hydrolyzed in response to another signal.
This hydrolysis of GTP bound to the nucleotide-binding domain occurs when a second messenger binds to the receptor and activates alpha subunit. In other words, it activates the alpha subunit when it is no longer needed.
3. c) Dephosphorylation of the phosphate binding domain
The GTP-binding protein (G protein) cycle is a crucial component of many signaling pathways. The cycle begins when the GTPase activity of an inactive G protein, bound to GDP, is stimulated by a GTPase activating protein (GAP), causing the release of GDP and the binding of GTP.
This active G protein complex then interacts with one or more effectors, either directly or via an exchange factor (GEF). The effector proteins can stimulate or inhibit a wide variety of enzymes, ion channels and transporters.
The active phase of the cycle is terminated with the hydrolysis of GTP to GDP by the intrinsic guanine nucleotide exchange factor (GEF) activity of the G protein. This leads to detachment from its effector, which then allows for deactivation and reassociation with membrane-bound receptors.
Allosteric feedback control during signal transduction can be provided by G proteins that activate other G proteins or their GEFs. For example, in retinal rod cells, activation of transducin (a heterotrimeric G protein) stimulates its GEF activity toward rhodopsin kinase, which phosphorylates rhodopsin and thereby reduces its activity as a light-activated ion channel
4. d) Binding of an effector molecule
GTP-binding proteins, also known as G proteins, are a family of proteins that bind guanine triphosphate (GTP).
The three types of these proteins are heterotrimeric G proteins, which contain alpha (), beta () and gamma () subunits; RAS, which is a monomeric protein; and small GTPases, which contain five subunits.
These proteins are activated by ligands such as hormones or neurotransmitters that bind to the receptor. As a result of the binding of a ligand to the receptor, the G protein is activated by releasing the bound GDP and replacing it with GTP. This activates the protein.
The effector molecule normally binds to the activated protein and causes conformational changes in it so that it can interact with another molecule on the effector side.
5. e) Binding of Mg2+ ions to the nucleotide binding domain
The GTP binding protein, or G protein, is a type of protein that binds to guanosine triphosphate (GTP). These proteins are involved in a wide range of biological processes and act as signal transducers by helping to relay chemical signals from the inside of a cell to the outside.
G proteins are composed of three subunits—alpha, beta, and gamma. When the alpha subunit binds with GTP, it becomes activated so that it can regulate an enzyme or ion channel.
Typically, this occurs when a hormone binds to a receptor on the surface of the cell. The receptor then activates the alpha subunit and the alpha subunit activates an enzyme or ion channel within the cell through its interaction with GDP (guanosine diphosphate).
The alpha subunit can then be deactivated by hydrolyzing its bound GTP into GDP through the action of another protein called RGS (regulator of G-protein signaling).
Conclusion
A very key regulatory system made up of things such as GTP-binding protein and others is known to be responsible for the formulation of vesicles that delivers other proteins not just within the cell but even throughout the body. Another instance where these things might be activated is during the apoptosis which normally accompanies tissue death and this depends on either a hormone or some kind of mutation in the genes.