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ProblemSet8_9AK - Chem 114A Practice Problems Chapter 11 1...

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Chem 114A Practice Problems Chapter 11: 1. Explain why enzymes are stereospecific? The protein enzyme is a chiral molecule whose binding clefts and catalytic residues are arranged in a specific three-dimensional asymmetric array. Hence, only substrates with the appropriate stereochemistry can bind to the enzyme, and the enzyme transforms the substrate to product according to the spatial arrangement of interacting functional groups. 2. What is an apoenzyme and how does it differ from a holoenzyme? Which form is active? An appoenzyme is the protein portion of an enzyme that has lost its cofactor (a metal ion or coenzyme). A holoenzyme is an active anzyme containing both the protein and the cofactor. 3. What is the relationship between vitamins and coenzymes? Many enzymes are synthesized from precursors that are vitamins (substances that an animal cannot synthesize and must obtain from its diet). However, not all coenzymes have vitamin precursors and not all vitamins are precursors of coenzymes. 4. What is the rate-determining step of an enzyme-catalyzed reaction? The rate-determining step is the slowest of the steps in the reaction mechanism, the step with the greatest free energy of activation. 5. Answer yes or no to the following questions and explain your answer. a) Can the absolute value of G for a reaction be larger than + G (free energy of activation)? b) Can + G for an enzyme-catalyzed reaction be greater than + G for the nonenzymatic reaction? c) In a two step reaction, such as the one diagrammed in Fig.11-6, must the intermediate (I) have less free energy than the reactant (A)? d) In a multistep reaction, does the transition state with the highest free energy always correspond to the rate-determining step? a) Yes. G is the difference in free energy between reactants and products, whereas + G is the difference in energy between the reactants and transition state. b) No. By definition, a catalyst decreases + G of a reaction. c) No. The free energy of the intermediate may be greater than that of the
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reactant. The reaction will proceed as long as the G of the overall reaction A P is negative. d) No. The rate-determining step is the one whose + G greatest. This does not always correspond to the step with the highest free energy, since + G depends on the difference in free energies between a reactant or a
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