2. Which of the following are nucleophiles and which are electrophiles?

Transcription

1 Life Sciences 1a ractice roblems 7 1. a) ow many intermediates are there in the reaction? b) ow many transition states are there? c) What is the fastest step in the reaction? d) Which is more stable, A or D? 2. Which of the following are nucleophiles and which are electrophiles? a) + b) - c) 2 0 d) 3

2 3. The first step in breaking down glucose is the conversion of glucose to glucose-6- phosphate in the following reaction: i + glucose glucose The ΔG of this reaction is 13.8 kj/mol. a) Is this reaction favorable? AT can be hydrolyzed to form AD and i in the following reaction: AT + 2 AD + i The ΔG of this reaction is kj/mol. b) Is this reaction favorable? c) If these two reactions are coupled, what is the overall ΔG for both reactions? d) If these two reactions are coupled, then overall are they favorable? e) What is the reaction equation for the overall coupled reaction? f) Explain why AT is so important for living systems (in terms of ΔG). 4. Explain why the rate of a chemical reaction is dependent on concentration of reactants in the reaction solution. ow can an enzyme affect the concentration of substrates and thereby alter the rate?

3 5. A chemist mixes equal numbers of molecules A, B, and C in a flask. Molecule B can react with molecule A to make AB, or it can react with molecule C to make BC. The reaction energy diagram for both reactions is below. This reaction energy diagram assumes standard conditions. Free Energy Course of the Reaction (Reaction Coordinate) a) After a small amount of reaction time (when only a tiny amount of products have been generated) does the chemist have more AB or BC? Briefly Explain. b) After one hour of reaction time the chemist would like to have more of the other product. Give two ways the chemist could accomplish this. c) After equilibrium has been reached does the chemist have more AB or BC? Briefly explain.

4 6. In lecture you saw how proteins can be broken down by hydrolysis, and how acid and base catalysis can help this process occur. RA is also broken down in the presence of a base. The mechanism for the cleavage of RA is drawn below a) While this reaction can proceed somewhat efficiently in the presence of a base, an enzyme called Rase A uses acid-base catalysis to make this reaction proceed even faster in a way that is similar to that used by IV protease. The active site of this enzyme has catalytic is residues instead of the Asp residues found in IV protease. ne of these is residues is unprotonated (a base) while the other is protonated (an acid). Explain how these is residues can catalyze this reaction. ropose a transition state for the reaction. + -

5 b) Draw the reaction energy diagram for this reaction with and without the participation of the enzyme Rase A. c) Do you think that Rase A would also be able to accelerate the hydrolysis of DA? Explain your answer.

6 1. a) 2, B and C b) 3 c) C D d) D 2. Which of the following are nucleophiles and which are electrophiles? a) + electrophile-likes electrons, can accept electrons b) - nucleophile-likes nuclei, has electrons to share c) 2 0 nucleophile d) 3 nucleophile 3. Breaking down glucose a) ot in the forward direction, ΔG >0 b) Yes, it is in the forward direction, ΔG <0 c) ΔG = kj/mol d) Yes, ΔG <0 e) glucose +AT glucose-6- + AD f) AT hydrolysis is a highly favorable process (negative ΔG) and can be used to drive other, non-favorable processes. The hydrolysis of AT to AD releases a very large amount of energy. If this reaction is coupled to a reaction that requires energy (positive ΔG) the overall process will still release energy and can now occur. 4. For the reagents to react they have to be close enough to actually be able to interact. The higher the concentration of reactants the more likely it is for the two (or more) reacting partners to be close to each other. An enzyme creates an artificially high local concentration of substrates by bringing the reactants much closer together than they would be if free in solution. 5. a) At first the product ratio is determined by the difference in the activation energies of the reactions. Since the activation energy for A+B+C AB+C is lower than the activation energy for A+B+C A+BC, more AB will be formed initailly. b) Add heat (energy) Add more B and C Add a catalyst specific for the A+B+C A+BC reaction c) When the reaction is at equilibrium the ratio of products from the two reactions is determined by the free energies of the products. Since the free energy of A+BC is lower that the free energy of AB+C, more BC will be formed.

7 6. a) The unprotonated histidine acts as a catalytic base to deprotonate the 2 - group, making it a stronger nucleophile. The protonated histidine acts as an acid catalyst by protonating the oxygen attached to the 5 carbon, this allows the resulting group to break away. [ote: this is only describing the reaction up to the above intermediate]. b) E + TS E-TS Uncatalyzed reaction E + S E-S Enzyme-catalyzed reaction E + E- The same picture as the figure from the lecture. [ote: the above mechanism is really showing only half of the reaction] c) o, DA does not have a 2 - group, therefore its hydrolysis cannot be catalysed by Rase A.