Biochemistry : Tyrosine Kinase Pathway

Study concepts, example questions & explanations for Biochemistry

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Example Questions

Example Question #17 : Biochemical Signaling

Receptor tyrosine kinases (RTKs) transduce extracellular signals into intracellular signaling cascades. This is possible because RTKs have an extracellular ligand binding domain to sense ligands outside of the cell, a transmembrane domain that spans the cell membrane, and an intracellular domain that activates pathways within the cell.

Which of the following best describes the mechanism by which the cytosolic domains of RTKs activate downstream signaling cascades?

Possible Answers:

Ligand binding to the extracellular RTK domain triggers an influx of calcium through calcium-channels, activating second messengers within the cell. 

Ligand binding to the extracellular RTK domain stimulates dimerization of RTKs, and the cytosolic domains cross-phosphorylate one another to activate the kinase domains. 

Ligand binding to the extracellular RTK domain triggers cleavage of the intracellular domains, which then act as soluble second messengers to activate protein kinases. 

Ligand binding to the extracellular RTK domain facilitates phosphorylation of the RTK intracellular domain by protein kinase A, which is required for all downstream signaling cascades to be activated. 

Ligand binding to the extracellular RTK domain permits the RTK to interact with other receptors on other cells, triggering bidirectional signaling cascades. 

Correct answer:

Ligand binding to the extracellular RTK domain stimulates dimerization of RTKs, and the cytosolic domains cross-phosphorylate one another to activate the kinase domains. 

Explanation:

Ligand binding stabilizes the dimerization of cytosolic domains of RTKs, and the intracellular domains can trans-phosphorylate one another (autophosphorylate) to activate the kinase domains. While there are other mechanisms of activation for RTKs, none of the other answers provide a correct mechanism for this activation. 

Example Question #18 : Biochemical Signaling

Protein-tyrosine kinase activation can result in the activation of two classical second messengers, inositol triphosphate (IP3) and diacylglycerol (DAG). These molecules are produced as a result of a hydrolysis reaction that is stimulated by protein-tyrosine kinase activation.

What is the enzyme that catalyzes this hydrolysis reaction, and what molecule is cleaved into IP3 and DAG? 

Possible Answers:

Protein kinase B, phosphatidylcholine 

Protein kinase C, 

Phospholipase C, 

Phospholipase C, 

Protein kinase A, diglyceride 

Correct answer:

Phospholipase C, 

Explanation:

In this specific pathway, protein-tyrosine kinase phosphorylation activates phospholipase C (PLC), which then catalyzes the hydrolysis of , a membrane phospholipid, into IP3 and DAG. IP3 and DAG then go on to activate second messenger cascades. Protein kinases can be activated by tyrosine kinases, but PLC is the enzyme specifically required for the IP3/DAG cascade. 

Example Question #19 : Biochemical Signaling

1. Conformational change brings protein tyrosine kinases close together

2. Receptor dimerization

3. Autophosphorylation activates receptor tyrosine kinases

4. Hormone/ligand binds to extracellular subunits

Which of the following correctly places the steps of the receptor tyrosine kinase in order?

Possible Answers:

Correct answer:

Explanation:

The first step in all signaling pathways is ligand binding. This causes the extracellular domains to dimerize, inducing a conformational change in transmembrane segments. The cytoplasmic protein tyrosine kinase domains are then then brought close so they can cross-phosphorylate leading to receptor tyrosine kinase activation. Insulin signaling is a primary example of receptor tyrosine kinase signal transduction, and its receptor is already dimerized.

Example Question #4 : Tyrosine Kinase Pathway

Which of the following correctly matches the SH2 (Src Homology 2) and SH3 (Src Homology 3) domains with their residues?

Possible Answers:

SH2 domains bind phosphotyrosine, SH3 domains bind proline-rich sequences

SH2 domains bind phosphoserine, SH3 domains bind phosphothreonine

SH2 domains bind phosphotyrosine, SH3 domains bind phosphoserine

SH2 domains bind phosphothreonine, SH3 domains bind phosphotyrosine

Correct answer:

SH2 domains bind phosphotyrosine, SH3 domains bind proline-rich sequences

Explanation:

SH2 and SH3 domains are Src homology domains that interact with insulin receptor tyrosine kinase substrates. They are especially important in the Ras-activated MAP kinase cascade. SH2 domains, because of their deep positive arginine pockets, tightly bind phosphotyrosine residues but not phosphoserine and phosphothreonine residues.

Example Question #1 : Tyrosine Kinase Pathway

Where do protein kinases most commonly add phosphate groups from ATP for signaling purposes?

Possible Answers:

To the sulfur on methionine

To the carboxyl group on aspartic and glutamic acids

To the hydroxyl group of tyrosine, threonine, or serine

To the nitrogen on arginine, histidine, and lysine

To the  group on cysteine

Correct answer:

To the hydroxyl group of tyrosine, threonine, or serine

Explanation:

The most common point of transfer of an ATP phosphate group is to the hydroxyl of tyrosine, serine, and threonine. Tyrosine and serine/threonine-specific kinases, in particular, help regulate signal transduction. As for the other amino acids, another kinase that sometimes appears is a histidine kinase, mostly in prokaryotes. Sulfur, nitrogen, and carboxyl groups are not typical targets for phosphorylation within proteins for signaling purposes.

Example Question #2 : Tyrosine Kinase Pathway

What are some hormones that exert their actions by activating receptor tyrosine activity?

I. Insulin

II. Epidermal growth factor 

III. Platelet-derived growth factor 

IV. Epinephrine

Possible Answers:

II, III, and IV

I, II, III, and IV

I and II

I and III

I, II, and III

Correct answer:

I, II, and III

Explanation:

Insulin, platelet-derived growth factor and, epidermal growth factor all use receptors that have intrinsic tyrosine activity. The hormones bind to the receptor which activates the tyrosine kinase. This in turn induces receptor transautophosphorylation and activation of -domain downstream molecules. Epinephrine is a hormone that when bound, activates a G protein-coupled receptor and leads to activation of adenyl cyclase.

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