Bioenergetics and Oxidative Phosphorylation PDF

21:02 16.8 KB/s lippincott's Book-1...

21:02 16.8 KB/s lippincott's Book-1 UINII II. Intermediary Metabolism Bioenergetics and Oxidative Phosphorylation 6 I. OVERVIEW AG: CHANGE IN FREE ENERGY Bioenergeticsdescribes the transfer and utilization of energy in biologic Energy available to do work. systems. It makes use of a few basic ideas from the field of thermo Approaches zero as reaction dynamics, particularly the concept of free energy. Changes in free proceeds to equilibrium. energy (AG) provide a measure of the energetic feasibility of a chemical Predicts whether a reaction is favorable. reaction and can, therefore, allow prediction of whether a reaction or process can take place. Bioenergeticsconcerns only the initial and final AH: CHANGE IN ENTHALPY energy states of reaction components, not the mechanism or how much Heatreleased or absorbed time is needed for the chemical change to take place. In short, bio during a reaction. energetics predicts a process is possible, whereas kinetics measures Does not predict whether a if reaction is favorable. how fast the reaction occurs (see p. 54). FREE ENERGY II. AG =AH-TAS The direction and extent to which a chemical reaction proceeds is AS: CHANGE IN ENTROPY determined by the degree to which two factors change during the Measure of randomness. reaction. These are enthalpy (AH, a measure of the change in heat con Does not predict whether a reaction is favorable. tent of the reactants and products) and entropy (AS, a measure of the change in randomness or disorder of reactants and products, Figure 6.1). Neitherof these thermodynamic quantities by itself is sufficient to Figure 6.1 determine whether a chemical reaction will proceed spontaneously in Relationship between changes in the direction it is written. However, when combined mathematically (see free energy (G),enthalpy (H), and Figure 6.1), enthalpy and entropy can be used to define a third quantity, entropy (Š). Tis the absolute free energy (G), which predicts the direction in which a reaction will temperature in degrees Kelvin spontaneously proceed. (°K): °K =°C+ 273. 69 70 6.Bioenergetics and Oxidative Phosphorylation A B III. FREE ENERGY CHANGE A (Reactant) (Product) ansition Traisit The change in free energy is represented two ways,AG and AG°. The in first, AG (without the superscript "o"), the change in free represents energy and, thus, the direction of a reaction at any specified concentra tion of products and reactants. AG,then, is a variable. This contrasts with (G) the standard free energy change,AG° (with the superscript "o"), which is A the energy change when reactants and products are at a concentration Initial state of 1 mol/L. [Note: The concentration of protons is assumed to be 10 energy mol/L, that is, pH =7.] Although AG° represents energychangesat these Free AG is negative nonphysiologic concentrations of reactants and products, it is nonethe Change in neray of reaction B less useful in comparing the energy changes of different reactions. Furthermore, AG° can readily be determined from measurement of the Final state equilibrium constant (see p. 72). This section outlines the uses of AG; Progress of reaction AG° is described on p. 71. B B Transition A A. Sign of AG predicts the direction of a reaction state The change in free energy, AG, can be used to predict the direction of a reaction at constant temperature and pressure. Consider the reaction: (G) A B Final state energy Free 1. Negative AG: If AG is a negative number, there is a net loss of AGis positive energy, and the reaction goes spontaneously as written-that is, B A is converted into B (Figure 6.2A). The reaction is said to be Initial state exergonic. Progress of reaction 2. Positive AG: If AG is a number, there is a net gain of positive energy, and the reactiondoes not go spontaneouslyfrom B to A (see Figure 6.2B). Energy must be added to the system to make the reaction go from B to A, and the reaction is said to be ender Figure 6.2 gonic. Change lOwer free eneray in a reaction. A. The product (G)than reactant. B. The product has free energy(4G) during h 3. AG is a reaction zero: If is AG =0, the proceeding reactants are in spontaneously--that equilibrium. [Note: is, energy is free When a higher free energythan the being lostthen the reaction continues until AG reacheszeroand reactant. equilibrium is established.] B.AG of the forwardand back reactions The free energy of the forward reaction (A B) is equal in magni tude but opposite in sign to that of the back reaction (B A). For example, AG if of the forward reaction is -5 kcal/mol, then that of the back reaction is +5 kcal/mol. [Note: AG can also be expressed in kilo joules per mole or kJ/mol (1 kcal =4.2 kJ).] C.AG depends on the concentration of reactants and products The AG A of the reaction B depends on the concentration of the reactant and At constant temperature and pressure, product. the fol lowing relationship can be derived: II.Free Energy Change 71 [B) A Nonequilibrium conditions AG = AG° + RT In A= 0.9 mol/L =0.09 molL where AG° is energy change (see below) the standard Ris the gas constant (1.987 cal/mol Tis the absolute temperature (°K) free · degree) AA A AG = -0.96 kcal/mol [A]and [B]are the actual concentrations of the A BA A reactant and product A In represents the natural logarithm A A A reaction with a positive AG° can proceed in the forward direction A (have a negative overall AG) if the ratio of products to reactants Glucose 6-P Fructose 6-P (BA]) is small (that is, the ratio suficiently of reactants to products is large). For example, consider the reaction: B Standard conditions Glucose 6-phosphate fructose 6-phosphate =1mo/L -1mo/L Fiqure 6.3A shows reaction conditions in which the concentration of reactant, glucose 6-phosphate, is high compared with the concen tration of product, fructose 6-phosphate. This means that the ratio of B AG = AG° =+0.4kcal/mol the product to reactant is small, and RT In([fructose 6-phosphate]/ [glucose 6-phosphate)) is large and negative, causing AG to be A negative despite AG° being positive. Thus, the reaction can proceed A A in the forward direction. A) D. Standard free energy change, AG° The standard free energy change, AG°, is so called because it is C Equilibrium conditions equal to the free energy change, AG, under standard conditions that is, when reactants and products are at 1 mol/L concentrations =0.66 molL =0.33 mo/L (see Figure 6.3B). Under these conditions, the natural logarithm of the ratio of products to reactants is zero (In1 = 0) and, the therefore, AABA A A equation shown at the top of this page becomes: AG = AG°+0 AG = 0kcal/mol A A A 1. AG° dard is predictive only under standard conditions: Under stan conditions, AG° can be used to predict the direction a reac AA tion proceeds because, under these conditions, AG° is equal B AG. However, AG° cannot predict the direction of a reaction under [Fructose 6-phosphatel physiologic conditions, because is composed solely of constants Keg o.504 it [Glucose 6-phosphate] (R, T,and Keg) and is, therefore, not altered by changes in product or substrate concentrations. 2. Relationship between AG° and Keg: In a reaction AB, a point of Figure 6.3 equilibrium reached at which no further net chemical change is AG of a reaction depends on the takes place-that is, when A is being converted to as fast as B B concentration of reactant (B) and of product For the conversion is being converted to A. In this state, the ratio of [B] to [AJ is con glucose 6-P to fructose 6-P, stant, regardless of the actual concentrations of the two com AG is negative when the ratio of pounds: reactant (A) to product (B) is large (top, panel A); is positive under standard conditions (middle, B); and is zero at equilibrium (Alea bottom, panel C). 72 6. Bioenergetics and Oxidative Phosphorylation where Keg is the eqilibrium constant, and [AJeg and [B]eg are the concentrations ofA and B at equilibrium. If thereaction A B is A Favorable process (1G is negative) allowed to go to equilibrium at constant sure, then at equilibrium the overall free energy temperature and pres change (AG)is zero. Therefore, AG =0 = AG°+ RT In (B) Ala where the actual concentrations of A and B are equal to the equi librium concentrations of reactant and product [AJeg and [Bleq: and their ratio as shown above is equal to the Keg- Thus, AGO =-RTIn Keg BUnfavorable process (AG is positive) This equation allows some simple predictions: AG°=0 A B If Keg =1, then If Keg >1, then AG°