Substrate Substrate Factors Influencing Enzyme Action Active site Active site Enzyme-substrate complex Enzyme-substrate complex Enzyme Enzyme
Enzyme reactions enzyme + substrate enzyme + products enzyme-substrate complex
Factors that affect enzyme rates of reaction • An enzyme’s rate of reaction is measured by the amount of substrate converted to a product per minute. • An enzyme’s activity can be affected by: • General environmental factors, such as temperature and pH. • Cofactors and coenzymes. • Enzyme and substrate concentration which affects the saturation of the active site. • Inhibitors.
Optimal temperature for typical human enzyme Optimal temperature for enzyme of thermophilic (heat-tolerant bacteria) Rate of reaction 40 0 20 60 80 100 Temperature (°C) Optimal temperature for two enzymes Optimal pH for pepsin (stomach enzyme) Optimal pH for trypsin (intestinal enzyme) Rate of reaction 2 3 6 7 9 10 0 1 4 5 8 pH Optimal pH for two enzymes • What is the effect of temperature on enzyme function? • What is the effect of pH on enzyme function?
1. Environmental Factors • Each enzyme has an optimal temperature in which it can function. • Each enzyme has an optimal pH in which it can function. • The wrong temperature of pH can denature an enzyme, which means the enzyme proteins shape is destroyed.
2. Cofactors and Coenzymes • Cofactors are nonproteininorganic enzyme helpers. • For example, metal ions like iron in hemoglobin hold oxygen. • Coenzymes are organic cofactors. • For example, vitamins like niacin which serves as an electron acceptor.
Example of Cofactor Enzyme Activation:Binding of one oxygen to hemoglobin activates other sites to enhance oxygen binding at the three other sites.
3a. Enzyme Concentration • What does the graph tell us about the effect of increasing [enzyme]? Rate of Reaction • Increasing the [enzyme] means the rate of reaction keeps increasing. Enzyme Concentration
3b. Substrate Concentration • What does the graph show us about [substrate]? • Increasing [substrate] only increases the reaction rate until all the enzyme active sites are in use.
4. Enzyme Inhibitors • Competitive inhibitors bind to the active site of an enzyme, competing with the substrate. • Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective. • Noncompetitive inhibition is also called allosteric regulation.
LE 8-19 A substrate can normally bind to the active site of an enzyme. Substrate Active site Enzyme Normal binding A competitive Inhibitor mimics the substrate, competing for the active site. Competitive inhibitor Competitive inhibition A noncompetitive inhibitor binds to the enzyme away from the active site, altering the shape of the enzyme so that its active site no longer functions. Noncompetitive inhibitor Noncompetitive inhibition
AZT and AIDS AZT is the first retroviral drug approved to fight the HIV virus. It is a competitive inhibitor for the enzyme reverse transcriptase.
Allosteric Regulation of Enzymes • Allosteric regulation is the term used to describe cases where an enzyme’s function is affected by the binding of a regulatory molecule at a different site than the active site. • Allosteric regulation may either inhibit or stimulate an enzyme’s activity. • Usually the end product of a long series of reactions that make a metabolic pathway inhibits an enzyme near the beginning of the pathway. • This way when the [product] is high the chemical reactions leading to the product are slowed down.
Homework • Read MHR p 41 – 50 • Do p 54 # 1 – 11, 13 • Adapt and redraw the diagram on p43 to illustrate an anabolic reaction instead of a catabolic reaction. • Note: anabolic = synthesis catabolic = hydrolysis or breakdown
Feedback Inhibition Initial substrate (threonine) Active site available Threonine in active site Enzyme 1 (threonine deaminase) Isoleucine used up by cell Intermediate A Feedback inhibition Enzyme 2 Active site of enzyme 1 can’t bind theonine pathway off Intermediate B Enzyme 3 Intermediate C Isoleucine binds to allosteric site Enzyme 4 Intermediate D Enzyme 5 End product (isoleucine)
Substrates enter active site; enzyme changes shape so its active site Holds the substrates (induced fit). Substrates held in active site by weak interactions, such as hydrogen bonds and ionic bonds. Quick Review of Induced Fit Model of Enzyme Action Active site acts on substrates Substrates Enzyme-substrate complex Active site is available for two new substrate molecules. Enzyme Products are released. Substrates are converted into products. Products