G protein coupled-receptors can be classified into three categories: Gq, Gi, or Gs. Gq and Gs are stimulatory receptors whereas Gi is inhibitory. Gq activates the phospholipase C (PLC) pathway and Gs activates the cAMP and, subsequently, protein kinase C (PKC) pathway. Gi, on the other hand, inhibits several signaling cascades in the cells. One of the prominent effects of Gi receptor is that it inhibits the increase of calcium levels intracellularly. Recall that calcium levels are kept at a very low concentration inside the cell. Upon activation of certain pathways, calcium influx can occur from either the outside of the cell or from within the organelles (such as rough endoplasmic reticulum). This will lead to an increase in the cytoplasmic calcium levels. Increase in cytoplasmic calcium levels will initiate several pathways inside the cell. To prevent overactivity of these pathways, calcium levels are closely controlled within the cell. One way to regulate the calcium levels is by the activation of Gi receptor.

Insulin binds to receptor tyrosine kinases, G protein coupled receptors are found throughout the body (not just the central nervous system), and GPCR's respond to ligand binding, not voltage changes.

The alpha subunit of a G protein has intrinsic GTPase activity that, although slow, will automatically convert GTP back to GDP when the action of the G protein has finished.

When a G-protein is activated (in its ATP bound state), the alpha subunit dissociates from the beta and gamma subunits and binds to the effector enzyme for further activation and signal amplification downstream. For example, when adrenaline binds to the beta-andrenergic receptor, the alpha subnit dissociates from the beta+gamma subunits and activates adenylyl cyclase, which then produces cAMP, signaling for downstream protein targets to be phosphorylated.

G protein-coupled receptors contain nine seven transmembrane alpha helices. All other statements are true of G protein-coupled receptors.

These are examples of heterotrimeric G protein-dependent signaling. Glucagon and epinephrine hormones both cause GTP to bind to adenylate cyclase, which produces the second messenger cAMP.

The observed increase in diacylglycerol (DAG) is indicative of the activation of a GProtein-Coupled Receptor (GPCR).

Upon binding of a ligand to a GPCR, a conformational change in the receptor is transmitted to a G protein bound to the cell membrane within the cell. This subsequently causes the alpha-subunit of the G protein to lose its bound GDP, and in exchange it receives GTP. This, in turn, activates the G protein, causing the alpha subunit to dissociate from the beta-gamma subunit. This newly liberated alpha subunit-GTP complex then goes on to activate another component of the signal transduction cascade. There are two main types of GPCR signaling pathways, depending on the type of alpha subunit involved.

cAMP Pathway: When the alpha subunit is stimulatory, denoted as , it will activate an enzyme in the plama membrane called adenylyl cylcase. Activation of this enzyme results in the conversion of ATP into cAMP. cAMP, in turn, acts as a second messenger within the cell, activing Protein Kinase A (PKA). This protein kinase then goes on to phosphorylate several proteins within the cell, which leads to a response. Furthermore, the G protein may also be inhibitory and denoted as . This alpha subunit essentially does the opposite of what does. That is, it acts to inhibit adenylyl cyclase, with a subsequent decrease in intracellular levels of cAMP and a reduction in the activity of PKA.

Phosphatidylinositol Pathway: In this case, the G protein is denoted as . This particular G protein goes on to activate an enzyme called phospholipase C (PLC). PLC, in turn, cleaves a certain phospholipid within the plasma membrane called phosphatidylinositol-4,5-bisphosphate () into two products, inositol-1,4,5-trisphosphate () and diacylglycerol (DAG). dissociates from the membrane and binds to a receptor on the endoplasmic reticulum, stimulating the release of into the cytosol. Together, DAG and work together to activate Protein Kinase C (PKC), which then goes on to phosphorylate many proteins within the cell, leading to a cellular response.

And briefly, receptor tyrosine kinases (RTK) are receptors located in the plasma membrane. Upon binding its ligand, RTK's have two intracellular domains that phosphorylate each other, thus activating the receptor. The RTK then goes on to trigger a signal transduction cascade.

Ionotropic receptors are located in the plasma membrane, and they also serve as ion channels through which ions can flow. Generally, binding of ligand to ionotropic receptors induces a conformational change in the receptor that causes the ion channel to open.

The dihydropyridine receptor (DHP) is located in the plasma membrane and is generally associated with another receptor known as the ryanodine receptor, located in the membrane of the endoplasmic reticulum. Generally, the DHP receptor is activated by a change in membrane voltage, and upon stimulation causes: 1) an influx of from the extracellular fluid into the cytosol, and 2) is mechanically coupled to the ryanodine receptor, stimulating it to release from the endoplasmic reticulum into the cytoplasm.

And finally, as the name implies, intracellular receptors are not located in the plasma membrane, but instead located in either the cytosol or nucleus. For a ligand to bind this class of receptor, it must be able to diffuse across the plasma membrane to make its way into the cell. Generally, upon activation, intracellular receptor-ligand complexes act as transcription factors, directly modulating the activity of certain genes by altering their expression.

In its unactivated state, a G protein is present as a heterotrimer consisting of an alpha, a beta, and a gamma subunit. This heterotrimer is bound to GDP. Upon activation by conversion of GDP to GTP, the G protein will dissociate into an alpha subunit separated from the beta and gamma unit (these two are still connected).

The Gq Gs-alpha bound to GTP dissociates and stimulates adenlyate cyclase to produce cAMP. (Gq is involved in the phosphoinositide pathway, not the adenylate cyclase pathway.) All other answer choices are correct with regards to the adenylate cyclase signaling system.

, , and are not types of G proteins. Instead, they are types of subunits of a G-protein.

cAMP is a second messenger produced by the adenylate cyclase pathway (among other pathways).