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.

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.

A phosphorylation cascade, involves many different steps and complicated interactions between kinases, phosphorylases, and phosphatases. In this case, the enzymes mentioned relate to the phosphorylation and dephosphorylation cascade involved with glycogen synthesis and degradation.

When a beta-adrenergic receptor or glucagon receptor is activated, two types of G-protein couple receptors, a G-protein is phosphorylated and disassociates GTP to act upon the enzyme, Adenylate cyclase, to synthesize cylic AMP (cAMP) from ATP.

This first step following the release of cAMP is that it acts upon protein kinase A by attaching to its two R subunits (requiring 4 cAMP) while releasing two C subunits. The C subunits function as other chemical messengers in the cell, acting upon multiple different enzymes to ultimately increase the rate of glycogen degradation and decrease the rate of glycogen synthesis.

Second messengers are molecules that act within cells to either increase or decrease activity or amount of a final molecule. All of the answer choices are second messengers in various pathways.

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.

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.

After adenylate cyclase converts ATP to cAMP, this second messenger goes on to bind to protein kinase A. Unactivated protein kinase A requires cAMP in order to become activated, at which point it can phosphorylate certain threonine and serine residues on target molecules.

In order for protein kinase A to become activated, cAMP must bind to it. PKA has a structure composed of two regulatory subunits and two catalytic subunits all bound together. The catalytic units are active on their own, so in order to work they must simply become dissociated from the regulatory subunits. Thus, cAMP will bind to only the regulatory subunits of PKA which then allows dissociation of the already catalytic subunits.

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.

Atrial natriuretic factor (ANF) and nitric oxide use cGMP as a second messenger to exert their effects. The ANF has guanylyl cyclase activity which converts GTP (guanosine-5'-triphosphate) to cGMP (cyclic guanosine monophosphate). This in turn activates protein kinase G and leads to relaxation of smooth muscle. IP3, calcium and DAG are second messengers in activation pathways of G protein-coupled receptors, as is the case of the epinephrine receptor.