CHAPTER 04

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Chapter 4:

From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Chapter 4 Section 4.1: Thermodynamics Section 4.2: Free Energy Section 4.3: The Role of ATP Energy Overview

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Every event in the universe involves energy change Energy is the capacity to do work In living organisms, work is powered with the energy provided by ATP Thermodynamics is the study of energy transformations that accompany physical and chemical changes in matter Bioenergetics is the branch that deals with living organisms

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Bioenergetics is especially important in understanding biochemical reactions These reactions are affected by three factors: Enthalpy - total heat content Entropy- state of disorder Free Energy- energy available to do chemical work Figure 4.1 A Thermodynamic Universe

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Three laws of thermodynamics: First Law of Thermodynamics- Energy cannot be created nor destroyed, but can be transformed Second Law of Thermodynamics- Disorder always increases Third Law of Thermodynamics- As the temperature of a perfect crystalline solid approaches absolute zero, disorder approaches zero Figure 4.1 A Thermodynamic Universe

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press First two laws are powerful biochemical tools Thermodynamic transformations take place in a universe composed of a system and its surroundings Energy exchange between a system and its surroundings can happen in two ways: heat ( q ) or work ( w ) Work is the displacement or movement of an object by force Figure 4.1 A Thermodynamic Universe

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press First Law of Thermodynamics Expresses the relationship between internal energy in a closed system and heat ( q ) and work ( w ) Total energy of a closed system (e.g., engine) is constant D E = q + w Unlike a human body, which is an open system Enthalpy (H) is related to internal energy by the equation: H = E + PV D H is often equal to D E ( D H = D E )

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press First Law of Thermodynamics Continued If D H is negative ( D H <0) the reaction gives off heat: exothermic If is D H positive ( D H >0) the reaction takes in heat from its surroundings: endothermic In isothermic ( D H =0) no heat is exchanged Reaction enthalpy can also be calculated: D H reaction = SD H products + SD H reactants Standard enthalpy of formation per mole (25°C, 1 atm ) is symbolized by D H f °

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Second Law of Thermodynamics Physical or chemical changes resulting in a release of energy are spontaneous Nonspontaneous reactions require constant energy input All spontaneous processes occur in the direction that increases disorder Figure 4.2 A Living Cell As A Thermodynamic System

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press As a result of spontaneous processes, matter and energy become more disorganized Gasoline combustion The degree of disorder is measured by the state function entropy (S) Figure 4.3 Gasoline Combustion

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Section 4.1: Thermodynamics From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Second Law of Thermodynamics Continued Entropy change for the universe is positive for every spontaneous process D S univ = D S sys + D S surr Living systems do not increase internal disorder; they increase the entropy of their surroundings For example, food consumed by animals to provide energy and structural materials needed to maintain their complex bodies are converted to disordered waste products (i.e., CO 2 , H 2 O and heat) Organisms with a D S univ = 0 or equilibrium are dead

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Section 4.2: Free Energy From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Free energy is the most definitive way to predict spontaneity Gibbs free energy change or D G Negative D G indicates spontaneous and exergonic Positive D G indicates nonspontaneous and endergonic When D G is zero, it indicates a process at equilibrium Figure 4.4 The Gibbs Free Energy Equation

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Section 4.2: Free Energy From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Standard Free Energy Changes Standard free energy, D G°, is defined for reactions at 25°C,1 atm , and 1.0 M concentration of solutes Standard free energy change is related to the reactions equilibrium constant, K eq D G° = -RT ln K eq Allows calculation of D G° if K eq is known Because most biochemical reactions take place at or near pH 7.0 ([H + ] = 1.0 X 10 -7 M), this exception can be made in the 1.0 M solute rule in bioenergetics The free energy change is expressed as D G°′

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Coupled Reactions Many reactions have a positive D G°′ Free energy values are additive in a reaction sequence If a net D G°′ is sufficiently negative, forming the product(s) is an exergonic process Figure 4.5 A Coupled Reaction Section 4.2: Free Energy

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Conversion of glucose-6-phosphate to fructose-1,6-bisphosphate First step is endergonic ( D G°′ = +1.7 kJ/mol) so the reaction should not proceed The second reaction is exergonic ( D G°′ = -14.2 kJ/mol) due to cleavage of ATP’s phosphoanhydride bond The overall D G°′ for the coupled reactions is negative ( D G°′ = -12.5 kJ/mol), therefore the reaction does proceed in the direction written at standard conditions Figure 4.5 A Coupled Reaction Section 4.2: Free Energy

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Section 4.2: Free Energy From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The Hydrophobic Effect Revisited Understanding the spontaneous aggregation of nonpolar substances is enhanced by understanding thermodynamic principles The aggregation decreases the surface area of their contact with water increasing its entropy The free energy of the process is negative, therefore it proceeds spontaneously Spontaneous exclusion of water is important in membrane formation and protein folding

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Adenosine triphosphate is a nucleotide that plays an extraordinarily important role in living cells Hydrolysis of ATP  ADP + P i provides free energy Figure 4.6 Hydrolysis of ATP Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Drives reactions of several types: 1. Biosynthesis of biomolecules 2. Active transport across membranes 3. Mechanical work such as muscle contraction Figure 4.7 The Role of ATP Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Structure of ATP is ideally suited for its role as universal energy currency Its two terminal phosphoryl groups are linked by phosphoanhydride bonds Specific enzymes facilitate ATP hydrolysis Figure 4.8 Structure of ATP Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press The tendency of ATP to undergo hydrolysis is an example of its phosphoryl group transfer potential ATP acts as energy currency, because it can carry phosphoryl groups from high energy compounds to low energy Figure 4.9 Transfer of Phosphoryl Groups Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Several factors need to be considered to understand why ATP is so exergonic: 1. At physiological pH, ATP has multiple negative charges 2. Because of resonance hybridization , the products of ATP hydrolysis are more stable than ATP Figure 4.10 Contributing Structure of the Resonance Hybrid of Phosphate Section 4.3: The Role of ATP

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From McKee and McKee, Biochemistry , 4th Edition, © 2009 Oxford University Press Several factors need to be considered to understand why ATP is so exergonic (Continued): 3. Hydrolysis products of ATP are more easily solvated 4. Increase in disorder with more molecules Section 4.3: The Role of ATP

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