CHEM 143 CHAPTER 47

"Explain, from the perspective of energetics, how a catalyst affects reaction rates. How is it changedafter it affects reaction rates?"

"A catalyst is a substance that increases reaction rate by lowering activation energy. The catalyst isnot permanently changed in the reaction"

"In terms of potential energy, explain how a catalyst functions. Also, explain how the energydifference between reactants and products is affected by a catalyst."

"A catalyst lowers the activation energy barrier for a reaction. A greater fraction of the molecules in asample possess the minimum energy needed for reaction when a catalyst is introduced. The energydifference between reactants and products is unaffected by a catalyst."

"What is meant by saying that molecules undergo a conformational change when forming anenzyme-substrate complex? Describe what occurs when an enzyme-substrate complex forms."

"A conformational change is a change in the three-dimensional arrangement of the atoms that makeup a molecule. As an enzyme-substrate complex forms, groups of atoms in both molecules rotatearound single bonds to create the most energetically favorable intermolecular attractions possible"

"Distinguish between a reaction intermediate and a catalyst in a reaction mechanism."

"A reaction intermediate is produced in one elementary step of a reaction mechanism and consumedin a subsequent step. A catalyst is added as a reactant in an elementary step and then reappears inits original form as a product. Neither appears in the overall reaction stoichiometry"

"Distinguish among unimolecular,bimolecular, and termolecular elementarysteps in a reaction mechanism."

"A unimolecular elementary step is one where a single reactant decomposes to products. Abimolecular elementary step is the reaction of two particles, and a termolecular step is the reactionof three particles."

"The successful propagation of the human immunodeficiency viruses, HIV, depends on an enzymeknown as HIV-1 protease. Treatment of HIV includes administration of a class of drugs known asprotease inhibitors. Explain how such a drug would work."

"An enzyme inhibitor binds to the active site of an enzyme, preventing it from catalyzing the reactionof the substrate"

"What is an enzyme substrate?"

"An enzyme substrate is a reactant that the enzyme helps change to product in the enzyme-catalyzedreaction"

"What is an inhibitor? How does an inhibitor affect reaction rates?"

"An inhibitor is a substance that decreases reaction rate by raising activation energy"

"Many enzymes undergo feedback inhibition, in which a high concentration of enzyme-reactionproduct slows or stops enzyme activity. Compare the action of a furnace thermostat with enzymefeedback inhibition."

"Enzyme feedback inhibition is similar to a furnace thermostat because when a specified temperatureis reached (high concentration of reaction product), the furnace stops (the enzyme stops catalyzingthe reaction"

"What is enzyme specificity?"

"Enzyme specificity refers to the fact that only a specific substrate molecule can bind to the enzyme"

"How does an enzyme differ from an inorganic laboratory catalyst such as MnO2 in the followingreaction?2 KClO3(s)^MnO2⎯⎯⎯⎯⎯⎯⎯ 2 KCl(s) + 3 O2(g)"

"Enzymes and inorganic catalysts are both catalysts because they speed up a reaction withoutparticipating in the reaction itself. They differ in the mechanism by which they speed up thereaction"

"How does an enzyme affect the activation energy of a reaction?"

"Enzymes, like all catalysts, lower the activation energy of a reaction"

"Chemists have isolated, purified, studied, and reported on several thousand different enzymes. Whydoes a living system need so many enzymes?"

"Most enzymes catalyze only one reaction. Many enzymes are needed for the many chemicalreactions that occur in living systems."

"What is reaction order for the chemical changes that occur in a catalytic converter? Why does thisreaction order occur?"

"Reactions on a metal surface are typically zero-order. The availability of reaction sites on the metalcontrols the rate of the reaction."

"What exactly acts as the catalyst in an automotive catalytic converter?"

"The catalyst in a catalytic converter is the surface layer of a transition metal (usually platinum)."

"Ingestion of wood alcohol, CH3OH, can cause blindness or death when taken in sufficientquantities. The toxicity of wood alcohol is largely due to its reaction in the body from CH3OH toformaldehyde, HCHO. Formaldehyde is the poison. This reaction is catalyzed by an enzyme calledmethanol oxidase. Medical treatment of wood alcohol poisoning includes administration of ethanol,CH3CH2OH. What role does ethanol play in this treatment?"

"The enzyme methanol oxidase is inhibited by ethanol. The shape and polarity of CH3CH2OH aresimilar to that of CH3OH, and both molecules will fit into the active site of the enzyme"

"What is the induced-fit model of enzyme activity?"

"The induced-fit model depicts substrate and enzyme molecules as having flexible shapes that aremodified to produce an enzyme-substrate complex"

"How is a reaction energy profile for an enzyme-catalyzed biological reaction similar to the profilefor an inorganic reaction? How is it different?"

"The profiles are similar in that there is an activation energy barrier between reactant and productsthat is lower for the catalyzed reaction than for an uncatalyzed reaction. An inorganic reactiontypically has a single activation energy hump, whereas an enzyme-catalyzed biological reactionactivation energy barrier is typically comprised of three humps, one for the formation of theenzyme-substrate complex, one for the reaction of the substrate to products, and one for the releaseof products, leaving the unoccupied enzyme. Reaction energy profiles can have more than one ortwo humps when the reaction mechanism has multiple intermediates"

"What three substances undergo chemical change in an automobile's catalytic converter? What arethe product(s) of each of these chemical changes?"

"The three substances that undergo chemical change in a catalytic converter are nitrogen monoxide,carbon monoxide, and hydrocarbons. NO yields N2 and O2; CO reacts with oxygen to yield CO2;and hydrocarbons react with oxygen to yield CO2 and H2O"

"Identify the true statements and rewrite the false statements to make them true.(a) The active site of an enzyme is the portion of the enzyme where the catalytic properties occur.(b) The induced-fit model explains why one enzyme helps many reactions to occur faster.(c) An enzyme substrate is the material that is the product of the enzyme-catalyzed reaction.(d) An irreversible enzyme inhibitor cannot be removed from the enzyme's active site."

"True: a and d. (b) The induced-fit model explains why one enzyme helps a single reaction to occurfaster. (c) An enzyme substrate is the reactant in an enzyme-catalyzed reaction"

"Many enzymes work best at temperatures near 37°C. Normal body temperature is 37°C. What doyou suppose happens to the rates of enzyme-catalyzed reactions when you have a fever?"

"When you run a fever, it may make sense to assume that enzyme-catalyzed reactions would runfaster than at normal body temperature because a greater fraction of the reactant molecules havesufficient energy to undergo reaction-producing collisions. However, enzymes can lose theirstructural integrity at higher temperatures, which decreases the rate at which they can catalyzereactions."