G-PROTEIN COUPLED RECEPTORS

G proteins are membrane proteins that act as transducers by passing the signal conveyed to a receptor, via a hormone, to an enzyme that is located inside the membrane. This enzyme then alters the concentration of a second messenger, which induces the required intracellular response. There are many different types of G proteins; the primary difference among them is found in the a subunit, rather than the or the y subunits. G proteins play an important role in signal amplification because one receptor can activate several G proteins, which in turn effects the activity of many enzymes, such as adenylate cyclase or phospholipase C. These G proteins are divided into three major groups defined by their effect. Gs proteins activate adenylate cyclase, raising cyclic adenosine monophosphate (cAMP) levels; and Gp proteins activate phosphalipase C, liberating inositol triphosphate and diacylgylcerol.

Breakdown of GTP to GDP [Ref- Ganong 22/e p41]

This question is an indirect repetition from May 08 (q no. 22 of biochemistry). a–subunit of G protein has inherent GTPase activity which converts GTP into GDP.

proteins

• G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three different subunits)
• The three subunits are:

– Ga, which carries the binding site for the nucleotide. At least 21 different kinds of Ga molecules are found in mammalian cells. – G(3

– GY

• How They Work
• In the inactive state G protein has GDP bound to its Ga subunit.
• When a hormone or other ligand binds to the associated G protein coupled receptor (GPCR) , the GDP is exchanged for GTP
• GTP activates Ga causing it to dissociate from Gi3Gy (which remain linked as a dimer).
• Activated Ga in turn activates an effector molecule (adenylyl cyclase- an enzyme in the inner face of the plasma membrane which catalyzes the conversion of ATP into the “second messenger” cyclic AMP).
• The beta and gamma subunit do not separate from each other, and Gf3Gy dimer also activates a variety of effectors.
• The actions of the Gu-GTP complex are short lived because the a–subunit has an inherent GTPase activity, resulting in the rapid hydrolysis of GTP to GDP. This leads to reassociation of the Ga unit with the GI3Gy dimer. This inactivates the G protein.

G proteins act as inhibitory and excitatory because of difference in alpha subunit [Ref: Harper 26/e p458; Lippincott Biochem 3/e p93; Ganong 22/e p41]

• G-protein coupled receptors (GPCR) are the largest superfamily of cell surface receptors.
• They typically have seven helices that traverse the membrane.
• These receptors are integral membrane proteins characterized by an extracellular ligand-binding region, seven transmembrane helices, and an intracellular domain that interacts with G-proteins.
• The function of GPCR is to transduce signals that induce a cellular response to the environment.
• Mechanism:
• The ligand binds to a site on the extracellular portion of the receptor.
• Binding of the ligand to the receptor

*  activates a G protein associated with the cytoplasmic C-terminal.

• This initiates the production of a “second messenger”. The most common of these are

*  cyclic AMP, (cAMP) which is produced by adenylyl cyclase from ATP and

* inositol 1,4,5-trisphosphate (IP3)

• The second messenger, in turn, initiates a series of intracellular events such as

*  phosphorylation and activation of enzymes

*  release of Ca²⁺ into the cytosol from stores within the endoplasmic reticulum

G proteins

• G proteins are so-called because they bind the guanine nucleotides GDP and GTP. They are heterotrimers (i.e., made of three different subunits)
• The three subunits are:

* Ga, which carries the binding site for the nucleotide. At least 21 different kinds of Ga molecules are found in mammalian cells.

*  GP

*  Gy

• How They Work
• In the inactive state G protein has GDP bound to its Ga. subunit.
• When a hormone or other ligand binds to the associated GPCR the

GDP is exchanged for GTP

• GTP activates Ga causing it to dissociate from GI3Gy (which remain linked as a dimer).
• Activated Ga in turn activates an effector molecule (adenylyl cyclase- an enzyme in the inner_. sface of the plasma membrane which catalyzes the conversion of ATP into the “second messenger” cyclic AMP).
• The beta and gamma subunit do not separate from each other, and Gi3Gy dimer also activates a variety of effectors.
• The actions of the Ga-GTP complex are short lived because the G-protein has an inherent GTPase activity, resulting in the rapid hydrolysis of GTP to GDP. This leads to reassociation of the Ga unit with the GI3Gy dimer. This inactivates the G protein.
• The ability of a ligand to stimulate or inhibit the second messenger depends on the type of G-protein that is linked to the receptor. One family of G-proteins, designated Gs, is specific for stimulation of adenylyl cyclase; another family, designated Gi, causes inhibition of the enzyme. These different actions of G proteins are attributed to different alpha subunits. Gs contains a, and Gi contains

Some Types of Ga Subunits

• Ga_s_This type stimulates (s = “stimulatory”) adenylyl cyclase. Ga_s is the target of the toxin liberated by Vibrio cholerae, the bacterium that causes cholera. Binding of cholera toxin to Ga_s keeps it turned “on”. The resulting continuous high levels of cAMP causes a massive loss of salts from the cells of the intestinal epithelium. Massive amounts of water follow by osmosis causing a diarrhea that can be fatal if the salts and water are not quickly replaced.
• Ga;-This inhibits (i = “inhibitory”) adenylyl cyclase lowering the level of cAMP in the cell.
• Ga_g_This activates phospholipase C (PLC) which generates the second messengers:

* inositol trisphosphate (IP3)

*  diacylglycerol (DAG)

• Ga_t_The “t” is for transducin, the molecule responsible for generating a signal in the rods of the retina in response to light.

Heterotrimeric G proteins are made up of three subunits designated , , and . Both the and the  subunits have lipid modifications that anchor these proteins to the plasma membrane.

The  subunit is bound to GDP. When a ligand binds to a G protein-coupled receptor, this GDP is exchanged for GTP and the  subunit separates from the combined  and  subunits.

The separated subunit brings about many biologic effects.
The intrinsic GTPase activity of the subunit then converts GTP to GDP, and this leads to reassociation of the  with the  subunit and termination of effector activation. The GTPase activity of the  subunit can be accelerated by a family of regulators of G protein signaling (RGS).

Ref: Barrett K.E., Barman S.M., Boitano S., Brooks H.L. (2012). Chapter 2. Overview of Cellular Physiology in Medical Physiology. In K.E. Barrett, S.M. Barman, S. Boitano, H.L. Brooks (Eds), Ganong’s Review of Medical Physiology, 24e.

G Protein have  trimeric structure with alpha,beta and gamma subunits.But activated form has only GTP bound alpha subunit-GTP-alpha.

The action is terminated when the GTPase activity of alpha subunit hydrolyses GTP to GDP.ADP ribosylation of alpha subunit leads to inhibition of GTPase activity.

GnRH receptors are a part of G protein coupled receptor superfamily and it functions primarily through the phospholipase C induced diacylglycerol and inositol triphosphate second messenger system.

Vasopressin uses calcium or phosphatidylinositol (PIP2) as second messengers.

C i.e. Made up of 4 units

G protein is made up of 3 subunits – a, /3, and 7Q. And the intrinsic GTPase activity of a subunit converts GTP to GDPQ.
G- Protein is nucleotide regulatory protein that bind GTP and translate a signal to a biological effect inside the cells.

Small G Protein Involved in many cellular function. They are related to product of ras proto-oncogeneQ

– Rab family : regulates rate of vesicle traffic between endoplasmic reticulum, golgi apparatus, lysosome, endosomes & cell membrane

–  Rho / Rac family : mediates interaction b/w cytoskeleton & cell membrane

–  Ras family : regulates growth by transmitting signals from the cell membrane to nucleus

Large Heterotrimeric G proteins

• These couple cell surface receptors to catalytic unit, that catalyze the intracellular formation of second messengers or couple the receptor to ion channels.
• Are made up of 3 subunits – a, 13, and y Q. The a unit is bound to GDP and it separates from (3y subunit, when a ligand binds to G protein coupled receptor. The intrinsic GTPase activity of a subunit converts GTP to GDPQ and this leads to reassociation of a with the By subunit.
• All the hetrotrimeric G-protein coupled receptors are proteins that span the cell membrane seven times (serpentine receptors)Q
• Can be divided into 5 families – Gs, Gi, Gt, Gq and G13
• Cholera toxin stimulates binds Gs a subunit and adenyl cyclase; whereas pertusis toxin bind Gi a subunit and inhibit adenyl cyclase.
• There are three major effector pathway through which G- protein coupled receptor function:

i)      Adenyl cyclase : c-AMP pathway (Via Gs & Gi) Q

ii)       Phospholipase C : 1P₃ – DAG pathway (Via Gq)Q

iii)      Channel regulation:

A i.e. It binds to ligands at the cell surfac

G protein coupled (linked) receptor (GPCRs)

• These cell surface receptors bind to the ligand at the cell surfaceQ. And have 7 trans membrane segments that loop in and out of the cell membraneQ. So the receptor is also k/a seven transmembrane domain (7TM) receptors, heptahelical or serpentine receptor.
• Part of receptor that protudes into cytoplasm (cytoplasmic tail of receptor) is coupled to G protein that has 3 different subunits (hetrotrimeric) – a, y. This G protein has an ability to bind guanosine nucleotides.

–                   In resting (inactive) state the a, 13, y subunits form a complex that binds GDP on

a subunitQ.

When receptor gets activated (after ligand binding) it causes this GDP bound trimeric G protein complex to associate with cytoplasmic tail of receptor and to exchange GDP by GTP. This exchange causes a subunit to dissociate

from combined fl & r subunits. This separated a subunit brings about biological effectQ. The 0 y subunits do not separate from each other and activates variety of effectors. (So dissociation of subunits 1/t action).

When ligand is removed from receptor, the intrinsic GTP ase activity of a subunit inactivates itself by converting its bound GTP into GDPQ.

In the absence of hormone, the heterotrimeric (a,13,y) G protein complex is in an inactive GDP bound form anchored to plasma membrane through prenylated groups on 13 y subunits and by myristoylated groups on a-subunit , but not associated with the receoptor. Binding of hormone (H) to receptor causes conformational change of receptor and activation of G protein complex d/t exchange of GDP with GTP on a subunit.

Tadenyl cyclase, Ca”/Na⁺ /C1- channels for a-s

–         TIC* channels for al and ao

–         T Phospholipase C[It for a_q and phospholipase CI3₂ for an

–         T C GMP phosphodiesterases for a_t and CI channel for cti2

• Depending on the coupling of ligand (hormone) to inhibitory G protein(Gi) or stimulatory G protein (Gs) receptors, these can either decrease or increase the activity of intracellular enzyme. (so it is not dependent on a subunit).
• Mechanism of action: Ligand (photon, protein, amine hormone, & neurotransmitter) binding —> conformation change & activation of G protein —> exchange of GDP for GTP and dissociation between a- and 13 y subunits which interact with effectors.
• a – subunit act on Na, Ca, K- channels and phospholipse – C (PLC). p y subunit acts on adenyl cyclase, phospholipase A2, DLC, Ca – ATPase etc.

Adenyl cyclase – cAMP pathway activation (i.e. adenyl cyclase activation lit T cAMP) causes stimulatin of cAMP dependent prtein kinase (protein kinase A) which phosphorylates the receptorQ.

G protein coupled receptor kinase are protein kinase that phosphorylates only active GPCRs Phosphorylation of receptor 1/t translocation & arrest linking.

• GPCRs are found only in eukaryotes, including yeast, plants & choanoflagellates. These are grouped in 6 classes: A (rhodopsin like); B (secretin receptor family); C (pheromone/ metabotropic glutamate); D (Fungal mating pheromone receptor); E (cyclic AMP receptor); and F (frizzled or smoothened).

Ans. is ‘c’ i.e., M₂

M₂ muscarinic receptors are G-protein coupled receptors and act through cAMP.