Energy in Chemical and Biochemical Reactions

Transcription

1 Energy in Chemical and Biochemical Reactions

2 Reaction Progress Diagram for Exothermic Reaction Reactants activated complex Products ENERGY A + B Reactants E a C + D Products Δ rxn Reaction coordinate The energy of a colliding system can be represented using an activation-energy diagram. When the energy content of the product bonds is less than that of the reactant bonds, the difference in energy appears as heat energy. Reactions of this type are called exothermic reactions.

3 Reaction Progress Diagram for Endothermic Reaction Reactants activated complex Products ENERGY E a A + B Reactants C + D Products Δ rxn Reaction coordinate When the energy content of the product bonds is less than that of the reactant bonds, the difference in energy appears as heat energy absorbed. Reactions of this type are called endothermic reactions.

4 Energy in Chemical and Biochemical Reactions ENERGY A + B E a E a C + D Reactants Δrxn C + D A + B Products Δrxn Products Reactants Reaction coordinate Reaction coordinate The energy difference between reactants and products is called the heat of reaction, Δrxn. For exothermic reactions, Δrxn is negative. For endothermic reactions, Δrxn is positive.

5 Energy in Chemical and Biochemical Reactions The terms exothermic and endothermic refer to the energy contained in the chemical bonds of reactants and products, and not to the total useful energy which can be obtained from the given reaction.

6 Equilibrium Constants and Their Use Any chemical system at equilibrium can be described by a number called the equilibrium constant, Keq, an associated mathematical expression involving the reactants and the products of the reaction. a A + b B c C + d D K eq = [C]c [D] d [A] a [B] b (The upper-case letters represent the concentrations of the chemical substances involved and the lower-case letters represent the balancing coefficients for the reaction.)

7 Equilibrium Constants and Their Use The magnitude of the equilibrium constant indicates whether a particular reaction is product-dominated or reactantdominated at equilibrium. K eq = [C]c [D] d [A] a [B] b When Keq >> 1 (i.e. 1 x 10 3 ), products dominate the reaction mixture. The equilibrium is called favorable. When Keq << 1 (i.e. 1 x 10-3 ), reactants dominate the reaction mixture. The equilibrium is called unfavorable.

8 Equilibrium Constants and Their Use Note: A reaction can have a very large equilibrium constant, yet be very slow. Conversely, a reaction can have an equilibrium constant close to one and be very fast. This is because the equilibrium constant is related to the energy difference between the reactant molecules and the product molecules. The equilibrium constant tells us nothing about the reaction rate which is related to the magnitude of the energy of activation for the reaction. Nevertheless, equilibrium constants can be useful in calculating the final reactant and product concentrations, given that there is sufficient energy of activation for the reaction to occur.

9 Equilibrium Constants and Their Use Equilibrium Constants for a Sequences of Reactions A metabolic conversion, such as the conversion of glucose to lactic acid in muscle during glycolysis, is accomplished by a series of connected reactions. A B C D The equilibrium constant for a set of connected reactions can be obtained by multiplying together the equilibrium constants for the individual reactions: A B, K1 = 1 x 10-3 (unfavorable) B C, K2 = 1 x 10 2 (favorable) C D, K3 = 1 x 10 3 (favorable) A D, K4 = K1 x K2 x K3 = 1 x 10 2 (favorable)

10 Equilibrium Constants and Their Use 1) P ) ATP + 2 ADP + P4 3- Keq = 3.8 x 10-3 Keq = 2.3 x 106 Equilibrium Constants for Coupled Reaction ATP + 3) + ADP Keq = 8.6 x 103

11 Another Way of Writing a Coupled Reaction C 2 ATP ADP C 2 P 3 2- D-Glucose D-Glucose-6-phosphate K3 = K1 x K2 Keq = 8.6 x 103

12 Free Energy Changes (energy available for work) and Chemical Reactions (ΔG = Free Energy Change ΔG0 = Standard Free Energy Change)

13 Free Energy Changes and Equilibrium Constants Δ ΔG = RTlnK

14 Relationship Between G and Keq Gº (kj/mol) Keq % product at equilibrium % % % % % % %

15 Free Energy Changes The previous reaction can be described in terms of free energy changes. Reaction 1: Glucose + Pi <=> glucose-6-phosphate + 2 Reaction 2: ATP + 2 <=> ADP + Pi (ΔGo' = kj/mol, endergonic) (ΔGo' = kj/mol, exergonic) To couple the two reactions, add reactants on left, add products on right, and add ΔGo' values to get ΔGo' for coupled reaction: Glucose + ATP <=> glucose-6-phosphate + ADP (ΔGo' = kj/mol)

16 Downward motion of an object releases potential energy (right side, exergonic) that can be used to do mechanical work, moving another object upward (left side, endergonic). A coupling mechanism (rope) is required to enable exergonic process to drive endergonic one.

17 The coupled reaction is exergonic; it will go spontaneously (forward, left to right) in the cell, but will it proceed at a rate consistent with cellular needs? There's N information about rates in the value of a ΔG -- we can't answer this question from bioenergetics. Most biological reactions would proceed at a very slow rate indeed if they're not catalyzed. The biological catalyst enabling the coupled reaction above to proceed on a biological timescale (as opposed to a geological timescale!) is an enzyme, hexokinase. Free energy coupling, with enzymes as catalysts, is the strategy used in metabolic pathways.

18 igh Energy Compounds Energy released during oxidation of nutrients is trapped in the form of a few energy-rich or "high energy" compounds. A "high-energy" compound is a compound with a functional group (in many cases, phosphoryl group) whose free energy of transfer to another compound proceeds with a large negative ΔG. We can compare how "high" the energy is in compounds by comparing their free energy of transfer to a common compound. X-P X- + P3 2- free energy of transfer to water = ΔG for hydrolysis

19 ydrolysis of igh Energy Compounds

20 ydrolysis of igh Energy Compounds N 2 N N N N P P P C 2 2 N N 2 N N N P P P C 2

21 Standard Free Energies of ydrolysis of Some Phosphorylated Compounds X-P X- + P3 2- G º(kJ/mol) G º(kcal/mol) Phosphoenolpyruvate + 2 Pyruvate + Pi ,3-Bisphosphoglycerate Phosphoglycerate + Pi Phosphocreatine + 2 Creatine + Pi ATP + 2 AMP + PPi ADP + 2 AMP + Pi ATP + 2 ADP + Pi PPi + 2 Pi + Pi AMP + 2 Adenosine + Pi Glucose-1-phosphate + 2 Glucose + Pi Fructose-6-phosphate + 2 Fructose + Pi Glucose-6-phosphate + 2 Glucose + Pi Glycerol-1-phosphate + 2 Glycerol + Pi

22 ATP s energy of hydrolysis : 1 ATP > ADP + Pi G º = kj/mol 2 Energy needed to synthesize ATP from ADP: ADP + Pi > ATP G º = kj/mol Glu-6-P energy of hydrolysis : 3 Glu-6-P > Glu + Pi G º = kj/mol 4 Energy needed to synthesize Glu-6-P from Glu: Glu + Pi > Glu-6-P G º = kj/mol ATP + Glu > Glu-6-P +ADP G º = kj/mol +

23 G º= kj/mol (i.e ) C 2 ATP ADP C 2 P 3 2- D-Glucose exokinase D-Glucose-6-phosphate

24 1 2 Phosphoenol (PEP) Pyruvate energy of hydrolysis : PEP > Pyruvate + Pi G º = kj/mol Energy needed to synthesize PEP from Pyruvate: Pyruvate + Pi > PEP G º = kj/mol ATP s energy of hydrolysis : 3 ATP > ADP + Pi G º = kj/mol 4 Energy needed to synthesize ATP from ADP: ADP + Pi > ATP G º = kj/mol ADP + PEP > ATP + Pyruvate G º = kj/mol +

25 G º= kj/mol (i.e ) P C C C ADP Pyruvate kinase ATP C C C Phosphoenolpyruvate Pyruvate

26 What is the difference between these two reactions? P C C C ADP Pyruvate kinase ATP C C C Phosphoenolpyruvate Pyruvate C 2 ATP ADP C 2 P 3 2- D-Glucose exokinase D-Glucose-6-phosphate

27 Standard Free Energies of ydrolysis of Some Phosphorylated Compounds X-P X- + P3 2- G º(kJ/mol) G º(kcal/mol) Phosphoenolpyruvate Pyruvate + Pi ,3-Bisphosphoglycerate 3-Phosphoglycerate + Pi Phosphocreatine Creatine + Pi ATP AMP + PPi ADP AMP + Pi ATP ADP + Pi PPi Pi + Pi AMP Adenosine + Pi Glucose-1-phosphate Glucose + Pi Fructose-6-phosphate Fructose + Pi Glucose-6-phosphate Glucose + Pi Glycerol-1-phosphate Glycerol + Pi