Chem Lecture 4 Enzymes Part 1

Transcription

1 Chem Lecture 4 Enzymes Part 1 Question of the Day: Enzymes are biological catalysts. Based on your general understanding of catalysts, what does this statement imply about enzymes? Introduction Enzymes are very good at what they do: As highly effective catalysts; They greatly enhance the rates of reactions. They can be incredibly selective. Important themes we will examine; of enzyme catalyzed reactions Catalysis by transition state stabilization The Michaelis-Menten Model for an enyzme catalyzed reaction The regulation of enzyme activity 2 Enzymes are Powerful and Highly Selective Example: Hydration of CO2 Carbonic anhydrase increases the rate 8 million-fold. Each Carbonic anhydrase molecule catalyzes 1 million reactions per second! 3 Enzymes are Powerful and Highly Selective Example: Hydration of CO2 Carbonic anhydrase increases the rate 8 million-fold. Each Carbonic anhydrase molecule catalyzes 1 million reactions per second! 3

2 Enzymes can be incredibly selective; In the types of reactions they catalyze In the reactants (substrates) they accept In the products they form 4 Enzymes can be very selective In the types of reactions they catalyze In the reactants (substrates) they accept In the products they form 5 Enzymes can be very selective In the types of reactions they catalyze In the reactants (substrates) they accept In the products they form 5 Enzymes can be very selective Substrate specificity Papain L-amino acids Trypsin L-Arg & L-Lys Thrombin L-Arg-Gly 6

3 Enzymes sometimes need help Cofactors Metal ions Coenzymes (Organic molecules) apoenzyme + cofactor = holoenzyme 7 Enzymes sometimes need help Cofactors Metal ions Coenzymes (Organic molecules) apoenzyme + cofactor = holoenzyme 7 Enzymes sometimes need help Cofactors Metal ions Coenzymes (Organic molecules) apoenzyme + cofactor = holoenzyme Do any of the names of the names of these coenzymes look familiar to you? 7 Enzymes sometimes need help Cofactors Metal ions Coenzymes (Organic molecules) apoenzyme + cofactor = holoenzyme 7

4 Enzymes sometimes need help Cofactors Metal ions Coenzymes (Organic molecules) apoenzyme + cofactor = holoenzyme 7 The cardinal rules for a chemical reaction, aa + bb = cd +dd: A reaction occurs spontaneously only if G < 0 (exergonic reactions) aa + bb cd +dd 8 The cardinal rules for a chemical reaction, aa + bb = cd +dd: A reaction is at equilibrium if G = 0 aa + bb cc+ dd 9 The cardinal rules for a chemical reaction, aa + bb = cd +dd: A reaction is non-spontaneous if G > 0 (endergonic reactions) aa + bb cc+ dd An input in free energy is required to make non-spontaneous reactions spontaneous. 10

5 The cardinal rules for a chemical reaction, aa + bb = cd +dd: G depends only on the initial (A + B) and final states (C + D) of a reaction, not on the pathway used to get from A + B to C + D. 11 The cardinal rules for a chemical reaction, aa + bb = cd +dd: G provides no information about the the rate of a reaction, it only indicates which way a reaction will proceed spontaneously. 12 The free energy change for a reaction, G, is related to the concentrations of the products and the reactants: C ΔG = ΔG o + RT ln A [ ] c [ D] d [ ] a [ B] b where ΔG o is the standard free energy change 13 The standard free energy change for a reaction, G, is the free energy change for a reaction carried out under standard conditions For solutions, [A] = [B] = [C] = [D] = 1 M For the solvent, [H2O] = 1 For gases, pa = 1 atm T = 25 C 14

6 Biochemists modify the definition of the standard state: For solutions, [H+ ] = 10-7 M (ph 7) instead of [H + ] = 1M. This is indicated by G and G. 15 G is related to the equilibrium ΔG' = 0 (at equlibrium) C 0 = ΔG '+ RT ln A C ΔG ' = RT ln A [ ] c [ D] d [ ] a [ B] b [ ] c [ D] d [ ] a [ B] b eq eq 16 G is related to the equilibrium ΔG' = 0 (at equlibrium) C 0 = ΔG '+ RT ln A C ΔG ' = RT ln A [ ] c [ D] d [ ] a [ B] b = RT ln K ' eq [ ] c [ D] d [ ] a [ B] b eq eq 16 G is related to the equilibrium ΔG ' = RT ln K ' eq K ' eq = 2.303RT log K ' eq ΔG '/2.303RT = 10 ΔG '/5.71 kj/mol = 10 at T = 298 K 17

7 G is related to the equilibrium K eq ΔG ' = RT ln K ' eq = 2.303RT log K ' eq K ' eq = 10 ( ΔG '/2.303RT ) ΔG '/5.71 kj/mol = 10 at T = 298 K 17 G is related to the equilibrium K eq ΔG ' = RT ln K ' eq = 2.303RT log K ' eq K ' eq = 10 ( ΔG '/2.303RT ) ΔG '/5.71 kj/mol = 10 at T = 298 K Excess reactants at equilibrium 17 G is related to the equilibrium K eq ΔG ' = RT ln K ' eq = 2.303RT log K ' eq K ' eq = 10 ( ΔG '/2.303RT ) ΔG '/5.71 kj/mol = 10 at T = 298 K Excess reactants at equilibrium Excess products at equilibrium 17 G is related to the equilibrium ΔG ' = RT ln K ' eq K ' eq = 2.303RT log K ' eq ΔG '/2.303RT = 10 ΔG '/5.71 kj/mol = 10 at T = 298 K 17

8 Problem Calculate the standard free energy change, G, for the following reaction from its equilibrium constant, K eq: glutamate + oxaloacetate aspartate aminotransferase K' eq = 6.8 aspartate+ α-ketoglutarte 18 Problem Calculate the equilibrium constant, K eq, for the following reaction from its standard free energy change, G : glucose 6-phosphatase glucose 6-phosphate + H 2 O glucose + P i ΔG ' = kj/mol 19 Problem Consider the following reaction: fructose 6-phosphate glucose 6-phosphate isomerase K' eq = 1.97 glucose 6-phosphate A) What is G for this reaction at 25 C? B) If the concentration of fructose 6-phosphate is set at 1.5 mm and that for glucose 6-phosphate is set at 0.5 mm, what is G? 20 Enzymes (as catalysts) affect only the rate of a reaction, not the G. 21

9 and Enzymes Enzymes, as catalysts, have no effect on the thermodynamics of a reaction. 22 Enzymes, as catalysts, affect only the rate of a reaction, not the G. Consequently, an affects they have on the kinetics of a reaction must must be equal in both the forward and back reactions: S P Rate at equilibrium forward = k f [ S] Rate Rate forward = Rate back back = k b [ P] k f [ S] = k b [ P] at equilibrium [ P] S [ ] eq = k f k b K eq = k f k b 23 Enzymes and Kinetics Enzymes accelerate the rates of state in a reaction The transition state lies between the substrate and products of a reaction. S X P 24 Enzymes and Kinetics Enzymes accelerate the rates of state in a reaction The transition state lies between the substrate and products of a reaction. S X P 24

10 Enzymes accelerate the rates of state S K X k P ΔG = G X G S V X V = k X = kt [ h S]e ΔG / RT Enzyme speed up reactions by lowering G, the activation free energy. 25 Enzymes accelerate the rates of state S K X k P ΔG = G X G S V X V = k X = kt [ h S]e ΔG / RT Enzyme speed up reactions by lowering G, the activation free energy. activation free energy 25 Enzymes accelerate Boltzmann the Distribution rates of [S]e reactions by stabilizing - G /RT the transition state S K X k P ΔG = G X G S V X V = k X = kt [ h S]e ΔG / RT Enzyme speed up reactions by lowering G, the activation free energy. activation free energy 25 Enzymes accelerate the rates of state S K X k P ΔG = G X G S V X V = k X = kt [ h S]e ΔG / RT Enzyme speed up reactions by lowering G, the activation free energy. activation free energy 25

11 Enzymes accelerate the rates of state S K X k P ΔG = G X G S V X V = k X = kt [ h S]e ΔG / RT Enzyme speed up reactions by lowering G, the activation free energy. 26 I think that enzymes are molecules that are complementary in structure to the activated complexes of the reactions that they catalyze, that is, to the molecular configuration that is intermediate between the reacting substance and the products of reaction for these catalyzed processes. The attraction of the enzyme molecule for the activated complex would thus lead to a decrease in its energy and hence to the decrease in the energy of activation of the reaction and to the increase in the rate of the reaction. - Linus Pauling (Nature 161 (1948): ) 27 Next up Enzymes (Chapter 8) Michaelis-Menten model for enzyme catalzyed reactions. 28