Biochemical Signaling - Biochemistry

When insulin acts on its receptor, it has the overarching function to begin storing energy. Insulin is released when the level of glucose in the blood is high. Thus, more glucose is taken into cells, as is fat. So breakdown of glycogen and breakdown into fatty acids would not occur in the presence of insulin - these processes imply that the body is in need of energy. Moreover, the GLUT4 transporters are the main method by which glucose is taken into cells when insulin is active, so there would be increased activity of these transporters.

Dipeptidyl peptidase-4 (DPP4) curbs the amount of insulin that is released in response to increased glucose. Activating DPP4 would not increase the insulin secretion. Increased ATP concentration would increase the amount of insulin released because it would activate active transporters. Inhibiting potassium channels would slow the termination of the action potential, allowing insulin to be secreted for longer. Increasing glucose in the bloodstream would activate more pancreatic cells to release insulin.

Type I diabetes occurs when the body is incapable of producing insulin, so after a meal it is necessary to inject it. Because insulin isn't being produced, the signal cascade following insulin secretion never occurs. Without insulin, when blood glucose is high it isn't taken up by the muscle cells. Glucagon release is still occurring, so fatty acids are being oxidized to provide energy. This depletes the supply of triacylglycerol. Ketone production is in fact too high in people with type I diabetes, leading to ketoacidosis, an acidification of the blood from excess ketone bodies. Glycogen synthesis would not be triggered, as glucagon would still be triggering glycogen breakdown.

In this question, we're asked to determine a set of metabolic pathways that would be activated by insulin.

Firstly, let's recall what the primary function of insulin is. After consuming a meal, digestion and absorption allows for the increase in blood glucose levels and fatty acid levels. To help regulate this, insulin is secreted from the pancreas.

Insulin's function is to reduce blood sugar levels by allowing cells to absorb glucose through their plasma membranes via glucose transporters. Because cells are now taking up more glucose and thus now have a surplus of it, some of that glucose is used to produce energy via glycolysis. Additionally, excess glucose absorbed in the liver can be converted into glycogen via glycogenesis.

As for fats, insulin helps in the formation of fatty acids as opposed to their degradation. This is because after consuming a meal, a large amount of energy is available in the form of macromolecules absorbed via the breakdown of food. So rather than using stored fatty acids to supply energy, the body takes this opportunity to synthesize new fatty acids for storage.

The correct answer is glucagon. Epinephrine does stimulate the breakdown of glycogen and lipids, but it is an amino acid derivative, not a polypeptide. The rest are all polypeptide hormones, but with different functions. Somatostatin inhibits the release of insulin and glucagon from the pancreas. Insulin has the opposite effect of glucagon, reducing blood sugar levels by stimulating the synthesis of glycogen and fat. Ghrelin stimulates appetite.

All steroid hormones are derived from cholesterol, which is a lipid molecule with three six-membered rings and one one-membered ring; it is thus tetracyclic.

Since steroid hormones are derived from cholesterol, they are all lipid-soluble and diffuse across the plasma membrane of both their target and their secretory cells. Since they are able to diffuse through the phospholipid bilayer, their receptors are either cytoplasmic or nuclear. Also, they must be synthesized on-demand since they can't be stored in vesicles; the membrane would be unable to contain them. All hormones travel to their target tissues via the blood. Neurotransmitters are the signal molecules that are are released into the synaptic cleft.

Steroid hormones are nonpolar and hydrophobic, whereas peptide hormones are polar and hydrophilic. This means that the steroid hormones cannot dissolve in water but peptide hormones can dissolve in water. Since they are minimally soluble in water, steroid hormones are carried by special transporters in the blood.

Steroid hormones are nonpolar molecules that are synthesized from a cholesterol molecule. Recall that cholesterol is a four membered hydrocarbon ring structure; therefore, steroid hormones are synthesized from a molecule with four rings. Gonads, or sex organs, and adrenal glands are the two main sources of steroid hormones. Gonads produce several sex hormones (such as estrogen, progesterone, and testosterone) that are involved in male and female reproduction. Adrenal glands produce aldosterone, cortisol, and a few inactive sex hormones that are activated in the gonads. Aldosterone is involved in regulation of sodium reabsorption in kidneys and cortisol is involved in metabolism. Recall that steroid hormones can traverse the hydrophobic interior of membranes. This applies for both plasma and nuclear membranes; therefore, steroid hormones can have receptors inside the cytoplasm or nucleoplasm (inside the nucleus).

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.

signals inhibition to adenylyl cyclase, thus decreasing the amount of cAMP produced. has the opposite stimulatory effect on adenylyl cyclase. None of the other answers directly target adenylyl cyclase.

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

The G protein-coupled receptor (GPCR) is a signaling receptor found in many cells throughout the body. It utilizes a second messenger system to convey signals to the cell. This means that, upon activation, the GPCR will activate second messenger molecules such as cAMP that will cause biochemical changes inside the cell. One of the downstream molecules cAMP acts on is called protein kinase C (PKC). Recall that kinases are enzymes that facilitate the phosphorylation of molecules. PKC will phosphorylate several molecules that activate different signaling pathways.

Note that ion transport (such as sodium ion transport) occurs when an ion channel is activated. G protein-coupled receptors are not ion channels; therefore, they do not facilitate the movement of ions across membranes.

There are several types of receptors found on a cell membrane. These receptors function to transduce an external signal (in the form of a ligand or voltage changes) into an intracellular signal. The question states that the receptor autophosphorylates itself. This means that the receptor must have kinase activity. Recall that, upon activation (by ligand binding), receptor tyrosine kinases self phosphorylate their tyrosine residues; therefore, the receptor stated in this question is a receptor tyrosine kinase. After auto-phosphorylation, the receptor tyrosine kinase will phosphorylate other molecules (kinase) that will lead to a signaling cascade.

As mentioned, tyrosine residues are phosphorylated, not aspartic acid residues.

The question states that the synthesized hormone is soluble in water; therefore, this must be a polar molecule. The hormone involved in regulation of sodium reabsorption in kidneys is aldosterone. This is a steroid hormone that is synthesized by the adrenal glands. Since the hormone in the question is polar, it cannot be aldosterone. Recall that structure determines function. Thus, if the structures of these two hormones differ significantly, their functions will also differ significantly. Note that steroid hormones are nonpolar and will not dissolve in water.

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.

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

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.

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

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.