Biochemistry : Second Messengers

Study concepts, example questions & explanations for Biochemistry

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Example Questions

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Example Question #34 : Signal Transduction Pathways

Which of the following is true regarding second messengers?

Possible Answers:

They are activated by ligand gated ion channels

They are activated by voltage gated ion channels

They are activated by ligand gated and voltage gated ion channels

None of these

Correct answer:

None of these

Explanation:

Signaling receptors can be divided into two categories: ion channels and second-messenger utilizing receptors. Ion channels are activated by voltage changes (voltage-gated) or ligand binding (ligand-gated) and they tend to increase the flow of ions into and outside of cell. Receptors, such as G protein-coupled receptors, are activated by ligand binding; however, they signal the cell by activating second messenger molecules such as cAMP.

Example Question #35 : Biochemical Signaling

Second messengers are __________ by receptor tyrosine kinase pathway and are __________ by voltage gated ion channels.

Possible Answers:

not activated . . . activated

not activated . . . not activated

activated . . . activated

activated . . . not activated

Correct answer:

activated . . . not activated

Explanation:

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.

Example Question #36 : Biochemical Signaling

Which of the following is not a direct function of cAMP?

I. Amplification of signal

II. Phosphorylation of molecules

III. Activation of kinases

Possible Answers:

II only

I only

I, II, and III

I and III

Correct answer:

II only

Explanation:

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.

Example Question #1 : Second Messengers

Phosphatidylinositol bisphosphate (PIP2) can be cleaved by phospholipase C to produce lipid-derived second messengers. Which of the following are the two second messengers derived from PIP2? 

Possible Answers:

Diacylglycerol (DAG) and inositol trisphosphate (IP3)

Diacylglycerol (DAG) and protein kinase A (PKA)

Inositol triphosphate (IP3) and protein kinase C (PKC)

Phosphatidylcholine and cyclic AMP (cAMP)

Protein kinase C (PKC) and nitric oxide (NO)

Correct answer:

Diacylglycerol (DAG) and inositol trisphosphate (IP3)

Explanation:

PIP2 gets cleaved into two smaller molecules by phospholipase C, and these two molecules are DAG and IP3. The protein kinases are not produced from this reaction, nor is cAMP or phosphatidylcholine. This is simply a matter of knowing that DAG and IP3 are the two most important lipid-derived second messengers. 

Example Question #3 : Second Messengers

Cyclic GMP (cGMP) is produced when the enzyme __________ converts the precursor GTP into cGMP. The reaction involves the removal of __________ from the GTP precursor.

Possible Answers:

guanylyl cyclase . . . two phosphate groups

cGMP protein kinase . . . two phosphate groups

adenylyl cyclase . . . two phosphate groups

guanylyl cyclase . . . one phosphate group

cGMP protein kinase . . . one phosphate group

Correct answer:

guanylyl cyclase . . . two phosphate groups

Explanation:

Guanylyl cyclase is the enzyme responsible for catalyzing this reaction, and the reaction involves removing two phosphate groups from guanosine triphosphate to generate cyclic guanosine monophosphate. Adenylyl cyclase performs a similar reaction but the substrate is adenosine triphosphate and the product is cyclic adenosine monophosphate. cGMP protein kinase is a target that is activated by cGMP, but is not involved in this reaction. 

Example Question #2 : Second Messengers

Second messenger cascades are frequently initiated by activation of a G protein-coupled receptor (GPCR). Ligand binding to the extracellular domain of the GPCR triggers a conformation change in the GPCR that permits activation and dissociation of the G protein to which it is associated. What is the biochemical change catalyzed by the activated GPCR that permits activation of its associated G protein?

Possible Answers:

The GPCR opens ion channels, and influx of calcium activates the G protein

The GPCR exchanges the G protein's bound GDP for a GTP

The GPCR phosphorylates protein kinases, which phosphorylate and activate the G protein

The GPCR breaks covalent bonds between the intracellular domain and the G protein

The GPCR exchanges the G protein's bound GDP for a GMP

Correct answer:

The GPCR exchanges the G protein's bound GDP for a GTP

Explanation:

Once the conformation change has been induced by ligand binding, the GPCR can act as a guanine exchange factor (GEF) which exchanges out a bound GDP (lower energy) on the G-protein for a GTP (higher energy). This triggers the dissociation of the G-protein, and it goes on to activate various second messenger cascades within the cell. Generally, addition of phosphate groups in biochemistry signals "activation," and removal triggers "deactivation."

Example Question #3 : Second Messengers

Which of the following hormone/target-tissue combinations is not activated by cyclic AMP?

Possible Answers:

Luteinizing hormone/heart

Vasopressin/kidney

Adrenaline/muscle

Thyroid-stimulating hormone/thyroid

Glucagon/liver

Correct answer:

Luteinizing hormone/heart

Explanation:

All of the hormones, and tissues, listed, have responses which can be mediated by cyclic AMP. Luteinizing hormone, however, does not target the heart. Rather, it targets organs of the reproductive system.

Example Question #6 : Second Messengers

Which of the following is not a second messenger?

Possible Answers:

cGMP

G-protein

Calcium

cAMP

diacylglycerol

Correct answer:

G-protein

Explanation:

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.

Example Question #7 : Second Messengers

How does cAMP regulate the action of Protein kinase A (PKA)?

Possible Answers:

cAMP phosphorylates PKA which sets it into action.

Four molecules of cAMP bind only to the catalytic subunits of PKA which allows them to function

cAMP does not affect the action of PKA

cAMP is initially bound to PKA to prevent its action, and when it dissociates PKA is able to function

Four molecules of cAMP bind to PKA and dissociate it into 2 catalytic subunits and 2 regulatory subunits

Correct answer:

Four molecules of cAMP bind to PKA and dissociate it into 2 catalytic subunits and 2 regulatory subunits

Explanation:

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.

Example Question #41 : Biochemical Signaling

cAMP is one of the most fundamentally important 2nd degree messengers in the cell, released by a variety of receptors.

In a phosphorylation system, what is the direct purpose of cyclic AMP, what does protein does this secondary messenger activate?

Possible Answers:

Phosphorylase kinase B

Glycogen synthase

Glycogen phosphorylase

Protein kinase A

Phosphoprotein phosphatase inhibitor

Correct answer:

Protein kinase A

Explanation:

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.

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