activator

Examples of activator in the following topics:

• Plasma Membrane Hormone Receptors

  • Binding of these hormones to a cell surface receptor results in activation of a signaling pathway; this triggers intracellular activity to carry out the specific effects associated with the hormone.
  • The activated G protein in turn activates a membrane-bound enzyme called adenylyl cyclase.
  • These activated molecules can then mediate changes in cellular processes.
  • The binding of a hormone at a single receptor causes the activation of many G-proteins, which activates adenylyl cyclase.
  • Hormone binding to receptor activates a G protein, which in turn activates adenylyl cyclase, converting ATP to cAMP. cAMP is a second messenger that mediates a cell-specific response.

• Secondary Active Transport

  • In secondary active transport, a molecule is moved down its electrochemical gradient as another is moved up its concentration gradient.
  • Unlike in primary active transport, in secondary active transport, ATP is not directly coupled to the molecule of interest.
  • Both antiporters and symporters are used in secondary active transport.
  • Secondary active transport brings sodium ions, and possibly other compounds, into the cell.
  • An electrochemical gradient, created by primary active transport, can move other substances against their concentration gradients, a process called co-transport or secondary active transport.

• Cancer and Transcriptional Control

  • Increased transcriptional activation of genes result in alterations of cell growth leading to abnormal gene expression, as seen in cancer.
  • This could lead to increased transcriptional activation of that gene that results in modified cell growth.
  • Researchers have been investigating how to control the transcriptional activation of gene expression in cancer.
  • The EGFR pathway activates many protein kinases that, in turn, activate many transcription factors that control genes involved in cell growth.
  • New drugs that prevent the activation of EGFR have been developed and are used to treat these cancers.

• Activation Energy

  • This small amount of energy input necessary for all chemical reactions to occur is called the activation energy (or free energy of activation) and is abbreviated EA.
  • However, the measure of the activation energy is independent of the reaction's ΔG.
  • The activation energy of a particular reaction determines the rate at which it will proceed.
  • The higher the activation energy, the slower the chemical reaction will be.
  • This figure implies that the activation energy is in the form of heat energy.

• Control of Metabolism Through Enzyme Regulation

  • This prevents the enzyme from lowering the activation energy of the reaction, and the reaction rate is reduced.
  • Allosteric activators can increase reaction rates.
  • Cells have evolved to use feedback inhibition to regulate enzyme activity in metabolism, by using the products of the enzymatic reactions to inhibit further enzyme activity.
  • However, while ATP is an inhibitor, ADP is an allosteric activator.
  • In contrast, allosteric activators modify the active site of the enzyme so that the affinity for the substrate increases.

• Cell Signaling and Cellular Metabolism

  • Metabolic regulation also allows organisms to respond to signals and interact actively with their environments.
  • Firstly, the regulation of an enzyme in a pathway is how its activity is increased and decreased in response to signals.
  • Secondly, the control exerted by this enzyme is the effect that these changes in its activity have on the overall rate of the pathway.
  • Cyclic AMP activates PKA (protein kinase A), which in turn phosphorylates two enzymes.
  • The first enzyme promotes the degradation of glycogen by activating intermediate glycogen phosphorylase kinase (GPK) that in turn activates glycogen phosphorylase (GP), which catabolizes glycogen into glucose.

• Enzyme Active Site and Substrate Specificity

  • Enzymes catalyze chemical reactions by lowering activation energy barriers and converting substrate molecules to products.
  • The enzyme's active site binds to the substrate.
  • The positions, sequences, structures, and properties of these residues create a very specific chemical environment within the active site.
  • Environmental conditions can affect an enzyme's active site and, therefore, the rate at which a chemical reaction can proceed.
  • If the enzyme changes shape, the active site may no longer bind to the appropriate substrate and the rate of reaction will decrease.

• Steroids

  • Steroids, like cholesterol, play roles in reproduction, absorption, metabolism regulation, and brain activity.
  • It has also been discovered that steroids can be active in the brain where they affect the nervous system, These neurosteroids alter electrical activity in the brain.
  • They can either activate or tone down receptors that communicate messages from neurotransmitters.
  • Since these neurosteroids can tone down receptors and decrease brain activity, steroids are often used in anesthetic medicines.

• Transport of Electrolytes across Cell Membranes

  • Ions cannot diffuse passively through membranes; instead, their concentrations are regulated by facilitated diffusion and active transport.
  • For this reason, athletes are encouraged to replace electrolytes and fluids during periods of increased activity and perspiration.
  • The mechanisms that transport ions across membranes are facilitated diffusion and active transport.
  • All movement can be classified as passive or active.

• Platelets and Coagulation Factors

  • The inner surface of blood vessels is lined with a thin layer of cells (endothelial cells) that under normal situations produce chemical messengers that inhibit platelet activation.
  • When the platelets are activated, they clump together to form a platelet plug (fibrin clot) ( b), releasing their contents.
  • The released contents of the platelets activate other platelets and also interact with other coagulation factors.
  • Outside of the body, platelets can also be activated by a negatively-charged surface, such as glass.
  • Non-physiological flow conditions (especially high values of shear stress) caused by arterial stenosis or artificial devices (e.g. mechanical heart valves or blood pumps) can also lead to platelet activation.