Second Messengers

With an inhibitory G protein, the binding of a ligand and stimulation of the receptor will activate the alpha subunit of the G protein, however since it is an inhibitory G protein, it will not go on to activate adenyl cyclase. With no activation of Adenyl cyclase it will lead to decrease cAMP and other secondary messengers.

In this question, we're asked to identify a statement that is not connected with intracellular signal transduction pathways (STP). To do so, we'll need to look at each answer choice individually.

Upon binding of a ligand to a G protein-coupled receptor (GPCR), the conformational change of this receptor is transmitted to a G-protein that is on the inner leaflet of the plasma membrane. This causes the individual sub-units of the G-protein to dissociate from each other, which then goes on to activate other components of the signal transduction pathway.

Activation of GPCR can also result in a signal transduction pathway in which a particular intracellular enzyme is activated. This enzyme is responsible for cleaving a specific fatty acid off of certain phospholipids from the plasma membrane. The fatty acid cleaved off is called phosphatidylinositol bisphosphate, which acts as a second messenger in STP's.

Another consequence of the activation of certain GPCR's is the activation of an enzyme called protein kinase A (PKA). This enzyme then goes on to phosphorylate other kinase enzymes. The end result is amplification of the entire signal.

One of the common second messengers in STP's is cyclic AMP (cAMP) and cyclic GMP (cGMP). One of the mechanisms in place to turn STP's off is to degrade these cyclic nucleotides. The class of enzymes responsible for this is called phosphodiesterases.

Beta-hydroxybutyrate is a ketone body that forms when excess acetyl-CoA is present. This molecule is not involved in signal transduction pathways.

The binding of four cAMP molecules to PKA dissociates it into two regulatory subunits and two catalytic subunits. The actual sites that the cAMP binds to, however, are allosteric sites - they are not directly on the regulatory sites or the catalytic sites.

There are many types of second messengers including diacylglycerol, cAMP, cGMP, calcium, and inositol trisphosphate. However, a G-protein is part of a pathway that utilizes second messengers, but is not one itself.

cAMP is a second messenger molecule that activates several molecules. Second messenger molecules often amplify the original signal, allowing for the signal to travel all across the cell. One of the molecules activated by cAMP is protein kinase C (PKC). This molecule, as the name implies, is a kinase; therefore, it phosphorylates other molecules. Note that this is a function of protein kinase C, not a direct function of cAMP.

Often following the activation of a G protein, ATP is converted to the second messenger, cAMP, by adenylate cyclase. This propagates the amplification of the signal transduction.

Phospholipase C catalyses the formation of DAG (diacylglycerol) and IP3 (inositol triphosphate) from PIP2 (phosphatidylinositol-4,5-bisphosphate). IP3 promotes the influx of calcium ions into the cytoplasm while DAG stimulates protein kinase C.

When a ligand binds to its associated receptor, the signal is passed into the cell and on to a distinct final molecule (often DNA transcription factors). Second messengers allow for significant amplification of a single ligand/receptor signal in order to cause mass change within a cell, and therefore within the body.

Receptor tyrosine kinase pathway utilizes second messenger molecules to activate molecules in the cell that, subsequently, activate cellular mechanisms. Ion channels allow for flow of ions between membranes; they do not directly activate second messenger molecules.

The function of phospholipase C is to cleave phosphatidylinositol biphosphate (PIP2) into the two second messenger molecules, diacylglycerol (DAG) and inositol triphosphate (IP3). These can then act within signal transduction pathways to amplify ligand/receptor signals.