Tutor profile: Sanjana M.
There are four types of Reinforcement or Punishment in Operant Conditioning. Provide a conceptual definition of operant processes and the types of reinforcement/punishment.
Shaping describes the reinforcement of successive approximations for some target behavior. If you’ve ever tried to teach a dog to sit on command, you know that you can’t expect the dog to sit the first time it’s told to do so. Rather, you shape the dog’s behavior gradually for doing things that are similar to sitting until you have it sitting regularly on command. Extinction is the elimination of a behavior that has previously been acquired. Extinction occurs when a behavior results in no consequences. Reinforcement is defined as anything that makes a behavior more likely to occur in the future. Positive reinforcement is the presentation of a stimulus in order to reinforce a behavior. For instance, if you give a child a piece of candy for cleaning their room, you have positively reinforced the “room cleaning” behavior. Negative reinforcement is the removal of a stimulus in order to reinforce a behavior. For instance, when you buckle your seat belt in your car, the annoying seat belt alarm turns off, which reinforces your seatbelt buckling behavior. Punishment is defined as anything that makes a behavior less likely to occur. Positive punishment involves presenting something to result in punishment. Parents who spank their children to try to make bad behavior less common are attempting to use positive punishment. Negative punishment involves removing something to make behavior less likely to occur in the future. Parents who punish children by taking away phone privileges are attempting to use negative punishment to modify their children’s behaviors. A conditioned reinforcer has no reinforcing value, in and of itself, but rather it takes on reinforcing value because it has been associated with a primary reinforcer. So in a way, principles of classical conditioning come into play with conditioned reinforcement. The green papers with pictures of dead presidents on them that we carry around in our wallets have no intrinsic value. However, because, in the past, paper money has been paired with food, water, shelter, and other rewards, the paper takes on a reinforcing value by association. Without that association, green slips of paper would not provide any reinforcement.
What are real life examples of all the types of enzyme inhibition? Competitive, Uncompetitive, Non-Competitive, Mixed and Irreversible
Competitive inhibition: the inhibitor resembles the substrate and fits into the active site reversibly, competing for active site occupancy with the substrate. An example of competitive inhibition is statin drugs. These drugs are HMG-CoA reductase inhibitors used to lower cholesterol levels by inhibiting the enzyme’s function of producing cholesterol in the liver. The statin drugs inhibit HMG-CoA reductase by occupying the enzyme’s active site, prohibiting HMG-CoA to enter and preventing the enzyme from converting HMG-CoA to mevalonic acid, which is an irreversible step in the cholesterol synthesis pathway. Because statins bind the active site reversibly, they are competitive inhibitors and not irreversible inhibitors. Uncompetitive inhibition – the inhibitor binds the enzyme only after the enzyme-substrate complex is formed and does not bind the active site. Uncompetitive inhibition is incredibly uncommon, possibly because of the way this inhibition works. If the inhibitor can bind only after the enzyme-substrate complex forms (but must act before substrate is converted to product), then depending on the rate of the enzyme, the time window for inhibitor action is very limited. Lithium, a drug used to treat manic depression, has been shown to act as an uncompetitive inhibitor in the phosphoinositide synthesis pathway, inhibiting inositol monophosphatase and thus preventing inositol recycling in the brain. Non-competitive inhibition – the inhibitor binds the enzyme whether or not the substrate is bound and does not bind the active site. Alanine acts as a non-competitive inhibitor for the enzyme pyruvate kinase. Pyruvate kinase transfers one phosphate group from PEP (phosphoenolpyruvate) to ADP, creating pyruvate and one ATP. Alanine can bind to pyruvate kinase (whether or not PEP is bound) to inhibit activity, creating a negative feedback loop (because alanine synthesis derives from pyruvate). Mixed inhibition – the inhibitor can bind the enzyme whether or not the substrate is bound, but has a preference for one state over the other. This means it is a mix of non-competitive and uncompetitive inhibition. The metal palladium is a mixed inhibitor for xanthine oxidase, an enzyme that converts xanthine to uric acid. Irreversible inhibition – the inhibitor binds the enzyme irreversibly, rendering the enzyme non-functional. The inhibitor binds the active site where it is modified in some way to irreversibly bind such that a substrate can never enter the active site. Aspirin is an example of irreversible inhibition. It binds the active sites of cyclooxygenase 1 and 2 irreversibly to prevent their inflammatory responses.
What is the general flow of a G-Protein cascade?
First, a hormone or signal molecule binds to an integral protein on one of its extracellular domains—this protein is called a G-protein-coupled receptor or GPCR. This causes a conformational change that activates a cytosolic domain of that same integral protein. Along the cytosolic face of the membrane, is a G protein made up of an alpha, beta, and gamma subunit. The alpha subunit binds both GTP and GDP. When GDP is bound, the protein is “off” and when GTP is bound it is “on.” Usually, but not always, the activated receptor protein acts as a catalyst for the replacement of GDP by GTP, activating the alpha subunit of the G protein. The activated alpha subunit then separates from the beta and gamma subunits. The activated alpha subunit acts as an agonist for another enzyme, often adenylyl cyclase. Adenylyl cyclase is an enzyme that catalyzes the conversion of ATP to cAMP plus two molecules of inorganic phosphate. Cyclic AMP just happens to be an agonist for Protein Kinase A, which phosphorylates proteins—usually enzymes. Many enzymes are turned on or off through being phosphorylated or dephosphorylated. The cascade can be shut down in various ways. Often the beta and gamma subunits re-bind with the alpha subunit, deactivating them. In other cases GPCR is phosphorylated one or more times, which leads to deactivation.
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