Signal transduction - Essay Example

Signal transduction a) Compare and Contrast the signal transduction mechanisms utilized by the hormones, Adrenaline and Insulin Signal transduction is the mechanism by which cells respond to growth factors, hormones and cytokines. Both adrenaline and insulin are small hydrophilic molecules attached on the receptor molecules in the cell surface membrane. The hormones have an effect on their targeted cells by a mechanism known as signal-transduction mechanism. To initiate their actions, they bind to a specific receptor on the plasma membrane.

Insulin is the major protein hormone produced and secreted in the pancreas. Its main functions are to regulate the production of storage macromolecules such as glycogen, protein and triglycerides, and the uptake of nutrients like fatty acids, amino acids and glucose by liver, muscle and adipose tissue. Insulin starts its action by binding to a receptor on the outer surface membrane of the cell membrane of its many target cells. This triggers the activation of insulin receptor, which signals the target cells on the regulation of chemical mediators.

There are two types of subunits in the insulin receptor - α-subunit and the β-subunit. The α-subunit is where insulin binds, while the β-subunit cross the membrane and jut into the cytosol. The α-subunit activates the protein tyrosine kinase, which is associated with the β-subunit. The β-subunit subsequently phosphorylates tyrosine into a number of residues like Tyr-960 and to a number of enzymes found inside the cytosol. The phosphorylation of the tyrosine residues stimulates an interaction between the isoforms and receptor of the insulin receptor substrate 1 (IRS-1).

This substrate then recognizes and attaches to various signal transduction proteins in sections known as SH2 domains. Many of the physiological responses to insulin engage the IRS-1 through complex mechanisms involving intensive investigation. The action of adrenaline on liver cells and muscles was the first process to be used to describe the signal-transduction mechanism. Adrenaline is attached on receptor molecules in the cell membrane. It speeds the breakdown of glycogen in liver cells. Its molecules connect to the receptor molecules in the cell membrane.

The guanine nucleotide-binding protein (G-protein) found in the membrane is activated by the formation of a hormone-receptor complex. The protein acts as a relay between an enzyme called adenyl cyclase and the hormone-receptor complex. The enzyme helps in catalyzing the conversation between ATP and cyclic adenine monophosphate(cAMP) in the liver cell. cAMP is a small molecule that acts as an intracellular mediator or a second messanger, the hormone being the first messenger. cAMP initiates a complex chain reaction after its diffusion into the liver cells.

The activation glycogen phosphorylase marks the end of the chain reaction. Glycogen phosphorylase in turn facilitates the breakdown of glycogen to glucose phosphate. This complex series of reaction generates a cascade effect that increases the hormones response. Throughout the chain reaction, cAMP activates an enzyme molecule, which activates many other molecules of its substrate becoming the next enzyme in the chain of reaction. b) How the cellular concentration of ATP regulates the secretion of insulin from the β cells of the pancreasThe beta cells of the pancreas respond to blood glucose concentration by changing the rate in which insulin is synthesized and secreted.

This process involves a complex mechanism. The entry of glucose into β cells is mediated by GLUT2 (type 2 glucose transporters). Glucose is phosphorylated by enzyme glucokinase making it effectively trapped within the β cells. The modified glucose is further metabolized to form ATP, which is the central energy molecule. When the ATP: ADP ratio increases, the ATP-gated K channels in the cellular membrane shut up. This prevents the K+ from being pushed across the cell membrane. The increased concentration of potassium ions leads to a rise in positive charge inside the cell, which causes depolarization of the cell.

The net consequence is the activation of voltage- gated Ca channels that transports calcium ions into the cell. The rapid increase in intracellular calcium concentrations activates the process of exocytosis, which facilitates the export of the insulin-storing granules. Eventually, insulin becomes exported from beta cells and it diffuses into nearby blood vessels. Broad vascular capacity of neighboring pancreatic islets leads to the brisk diffusion of insulin between blood vessels and beta cells.

The available amounts of stored insulin determine the initial amount of insulin to be released upon glucose absorption. After insulin is depleted, a second phase is initiated to release it.