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Chapter 10: Problem 53

Ligand binding to some growth-factor receptors triggers kinase cascades and also leads to activation of enzymes that convert \(\mathrm{O}_{2}\) to hydrogen peroxide \(\left(\mathrm{H}_{2} \mathrm{O}_{2}\right)\), which acts as a second messenger. Describe the likely effect of \(\mathrm{H}_{2} \mathrm{O}_{2}\) on the activity of cellular phosphatases.

Short Answer

Expert verified
H2O2 likely inhibits phosphatases, leading to prolonged protein phosphorylation and enhanced kinase signaling.

Step by step solution

01

Understanding H2O2 as a Second Messenger

Hydrogen peroxide (H_2O_2) is generated in response to growth-factor receptor activation and functions as a second messenger in signaling pathways. As a second messenger, its role is to relay and amplify the signal within the cell, impacting various cellular processes.
02

Role of Cellular Phosphatases

Phosphatases are enzymes responsible for dephosphorylating proteins, meaning they remove phosphate groups from protein molecules, likely turning off protein functions, including kinase signaling pathways.
03

Impact of H2O2 on Phosphatases

Hydrogen peroxide is known to oxidize and inactivate certain cysteine residues in the active sites of phosphatases. This oxidation leads to the temporary inhibition of phosphatase activity, which prolongs the phosphorylation state of proteins, thus enhancing the signaling cascade initiated by kinases.
04

Conclusion Regarding H2O2's Effect

The presence of H_2O_2 likely results in the inhibition of phosphatases, causing sustained phosphorylation of proteins. This enhances and prolongs the signal relayed by kinases, potentially leading to a stronger or more prolonged cellular response.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Kinase Signaling Pathways
Kinase signaling pathways are like cellular communication highways. These pathways start from a signal outside the cell, such as a growth factor binding to its receptor, which then triggers a series of events inside the cell.
After ligand binding, the receptor is often activated and behaves like a switch, setting off a chain reaction of phosphorylation events. Phosphorylation is where kinases, a type of enzyme, add phosphate groups to proteins, changing their function and activity.

This cascade of events allows the initial signal to be amplified and reach different parts of the cell, ensuring a coordinated response.
  • Phosphorylation by kinases usually activates proteins, promoting cellular actions like division or growth.
  • This can be thought of as flipping a switch, turning on a range of cellular mechanisms.
  • Kinase cascades allow for fine control by rapidly transmitting signals from the cell surface to target molecules.
Cellular Phosphatases
Cellular phosphatases act in opposition to kinases, carefully balancing the activities within the cell. These enzymes remove phosphate groups from proteins, a process known as dephosphorylation.
This action is critical for turning off signaling pathways or returning the cell to a resting state. By removing phosphates, phosphatases down-regulate or inactivate proteins, halting the signals initiated by kinases.

  • Phosphatases ensure that signaling does not run unchecked by resetting switches turned on by kinases.
  • This balance between phosphorylation and dephosphorylation is essential for cell regulation.
  • Without phosphatases, cells could not properly control growth or response to stress, potentially leading to diseases.
Oxidation of Cysteine Residues
In the context of cellular signaling, oxidation is a pivotal process, especially when discussing hydrogen peroxide ( H _2O_2 ) acting as a second messenger. Certain amino acids like cysteine, found in the active sites of proteins, are highly susceptible to oxidation.
When H _2O_2 is present, it uniquely targets cysteine residues within phosphatases, modifying and inactivating them.

  • This oxidation doesn't permanently damage the protein but temporarily inhibits its function.
  • By oxidizing the cysteine residues, H _2O_2 effectively prolongs the active state of phosphorylated proteins.
  • This adjustment allows the signaling cascade to continue without interruption, reinforcing the cell's response to external signals.
  • The reversible nature of cysteine oxidation serves as a regulatory mechanism, finely tuning cellular responses.

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Most popular questions from this chapter

Some \(\mathrm{G}\) protein-linked receptors are associated with \(\mathrm{G}\) proteins that inhibit rather than stimulate the activity of adenylate cyclase. A toxin secreted by the bacterium Bordetella pertussis (the causative agent of whooping cough) catalyzes the covalent attachment of an ADPribose group to the \(\alpha\) subunit of the inhibitory \(\mathrm{G}\) protein, preventing it from carrying out its normal function. How does this affect the activity of adenylate cyclase? How are intracellular levels of cAMP affected?

Stimulation of the insulin receptor by ligand binding and autophosphorylation eventually leads to the activation of both protein kinase B (Akt) and protein kinase C. Protein kinase B phosphorylates glycogen synthase kinase 3 (GSK3) and inactivates it. (Active GSK3 inactivates glycogen synthase by phosphorylating it.) Glycogen synthase catalyzes synthesis of glycogen from glucose. In the presence of insulin, GSK3 is inactivated, so glycogen synthase is not phosphorylated and is active. Protein kinase \(\mathrm{C}\) stimulates the translocation of glucose transporters to the plasma membrane by a mechanism not currently understood. One strategy for treating diabetes is to develop drugs that act as inhibitors of the phosphatases that remove phosphate groups from the phosphorylated tyrosines on the insulin receptor. Why might this be an effective treatment for diabetes?

a. Draw the reaction that shows the protein kinase A-catalyzed phosphorylation of a threonine residue on a target protein. b. Draw the reaction that shows the phosphatase-catalyzed hydrolysis of the phosphorylated threonine. c. Some bacterial signaling systems involve kinases that transfer a phosphoryl group to a His side chain. Draw the structure of the phospho-His side chain.

The bacterium Yersinia pestis, the pathogen responsible for bubonic plague, caused the deaths of about a third of the population of Europe in the fourteenth century. The bacterium produces a phosphatase called YopH, which hydrolyzes phosphorylated tyrosines and is much more catalytically active than mammalian phosphatases. a. What happens when the Yersinia bacterium injects YopH into a mammalian cell? b. Why is the bacterium itself not affected by YopH? c. Scientists are interested in developing YopH inhibitors in order to treat Yersinia infection, a re-emerging disease. What are some important considerations in the development of a YopH inhibitor?

A sample of cells has a total receptor concentration of \(25 \mathrm{mM}\). Ninety percent of the receptors have bound ligand and the concentration of free ligand is \(125 \mu \mathrm{M}\). What is the \(K_{\mathrm{d}}\) for the receptor-ligand interaction?
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