Bioenergetics and Biological Oxidation Dr Usman Shah Nawaz
Bioenergetics and Biological Oxidation Dr Usman Shah. Nawaz
. • Bioenergetics is the quantitative study of the energy transduction that occur in living cells and of the nature and function of the chemical processes underlying these transductions
Bioenergetics • Bioenergetics describes the transfer and utilization of energy in biologic systems. • It makes use of a few basic ideas from the field of thermodynamics, particularly the concept of free energy. • Changes in free energy (ΔG) provide a measure of the energetic feasibility of a chemical reaction and can, therefore, allow prediction of whether a reaction or process can take place. • Bioenergetics concerns only the initial and final energy states of reaction components, not the mechanism or how much time is needed for the chemical change to take place. • In short, bioenergetics predicts if a process is possible, whereas kinetics measure how fast the reaction occurs
Free energy • Free energy, G, expresses the amount of energy capable of doing work during a reaction at constant temperature and pressure. • Exergonic reactions: When a reaction proceeds with the release of free energy (that is, when the system changes so as to possess less free energy), the free-energy change ΔG, has a negative value and the reaction is said to be exergonic. • In endergonic reactions, the system gains free energy and ΔG is positive
FREE ENERGY • The direction and extent to which a chemical reaction proceeds is determined by the degree to which two factors change during the reaction. – Enthalpy – Entropy
Enthalpy • Enthalpy (ΔH) is a measure of the change in heat content of the reactants and products. • It reflects the number and kinds of chemical bonds in the reactants and products. • Negative ΔH: When a chemical reaction releases heat, it is said to be exothermic; the heat content of the products is less than that of the reactants and ΔH has, by convention, a negative value. • Positive ΔH: Reacting systems that take up heat from their surroundings are endothermic and have positive values of ΔH.
Entropy • Entropy (ΔS) is a measure of the change in randomness or disorder of reactants and products. • When the products of a reaction are less complex and more disordered than the reactants, the reaction is said to proceed with a gain in entropy.
• Neither of these thermodynamic quantities by itself is sufficient to determine whether a chemical reaction will proceed spontaneously in the direction it is written. • However, when combined mathematically, enthalpy and entropy can be used to define a third quantity, free energy (G), which predicts the direction in which a reaction will spontaneously proceed.
FREE ENERGY CHANGE • The change in free energy comes in two forms, – ΔG° • ΔG: ΔG (without the superscript "o"), is the more general because it predicts the change in free energy and, thus, the direction of a reaction at any specified concentration of products and reactants. • ΔG°: This contrasts with the change in standard free energy, ΔG° (with the superscript "o"), which is the energy change when reactants and products are at a concentration of 1 mol/L.
• The concentration of protons is assumed to be 107 mol/L – that is, p. H=7. • Although ΔG° represents energy changes at these nonphysiologic concentrations of reactants and products, it is nonetheless useful in comparing the energy changes of different reactions. • Further, ΔG° can readily be determined from measurement of the equilibrium constant
Sign of ΔG predicts the direction of a reaction • The change in free energy, ΔG, can be used to predict the direction of a reaction at constant temperature and pressure. • Consider the reaction: A B
Negative ΔG • If ΔG is a negative number, there is a net loss of energy, and the reaction goes spontaneously – that is, A is converted into B. • The reaction is said to be exergonic.
Positive ΔG • If ΔG is a positive number, there is a net gain of energy, and the reaction does not go spontaneously from B to A. • The reaction is said to be endergonic, and energy must be added to the system to make the reaction go from B to A.
ΔG is zero • If ΔG = 0, the reactants are in equilibrium. • When a reaction is proceeding spontaneously – that is, free energy is being lost – then the reaction continues until ΔG reaches zero and equilibrium is established.
ΔG of the forward and back reactions • The free energy of the forward reaction (A → B) is equal in magnitude but opposite in sign to that of the back reaction (B → A). • For example, if ΔG of the forward reaction is 5000 cal/mol, then that of the backward reaction is +5000 cal/mol.
ΔG depends on the concentration of reactants and products • ΔG of the reaction A → B depends on the concentration of the reactant and product. • At constant temperature and pressure, the following relationship can be derived:
• ΔG° is the standard free energy change • R is the gas constant cal/mol • degree • T is the absolute temperature (°K) • [A] and [B] are the actual concentrations of the reactant and product • ln represents the natural logarithm.
ΔG depends on the concentration of reactants and products • A reaction with a positive ΔG° can proceed in the forward direction (have a negative overall ΔG) if the ratio of products to reactants ([B]/[A]) is sufficiently small (that is, the ratio of reactants to products is large). For example, consider the reaction:
Example • Figure shows reaction conditions in which the concentration of reactant, glucose 6 -phosphate, is high compared with the concentration of product, fructose 6 -phosphate. • This means that the ratio of the product to reactant is small, and RT ln([fructose 6 -phosphate] [glucose 6 -phosphate]) is large and negative, causing ΔG to be negative despite ΔG° being positive. • Thus, the reaction can proceed in the forward direction.
Standard free energy change, ΔG° • ΔG° is called the standard free energy change because it is equal to the free energy change, ΔG, under standard conditions – that is, when reactants and products are kept at 1 mol/L concentrations.
Standard free energy change, ΔG° • Under these conditions, the natural logarithm (In) of the ratio of products to reactants is zero (In 1 = 0) and, therefore, the equation becomes:
At equilibrium • At equilibrium ΔG = 0 • So the reaction does not move in any direction.
ΔG° is predictive only under standard conditions • Under standard conditions, ΔG° can be used to predict the direction a reaction proceeds because, under these conditions, ΔG° is equal to ΔG. • However, under physiologic conditions, ΔG° cannot predict the direction of a reaction, because it is composed solely of constants (R, T, and Keq) and is, therefore, not altered by changes in product or substrate concentrations.
Relationship between ΔG° and Keq • In a reaction A → B, a point of equilibrium is reached at which no further net chemical change takes place – that is, when A is being converted to B as fast as B is being converted to A. • In this state, the ratio of [B] to [A] is constant, regardless of the actual concentrations of the two compounds: • where Keq is the equilibrium constant • [A]eq and [B]eq are the concentrations of A and B at equilibrium.
• If the reaction A ↔ B is allowed to go to equilibrium at constant temperature and pressure, then at equilibrium the overall free energy change (ΔG) is zero. • Therefore, • where the actual concentrations of A and B are equal to the equilibrium concentrations of reactant and product [A]eq and [B]eq • their ratio as shown above is equal to the Keq.
Thus, • The standard free-energy change of a chemical reaction is simply an alternative mathematical way of expressing its equilibrium constant • OR • ΔG°` is the difference between the free energy content of the products and the free energy content of the reactants, under standard conditions • .
ΔG° of two consecutive reactions are additive • The standard free energy changes (ΔG°) are additive in any sequence of consecutive reactions, as are the free energy changes (ΔG). For example: • Glucose + ATP → glucose 6 -P + ADP ΔG° = -4000 cal/mol • Glucose 6 -P → fructose 6 -P ΔG° = +400 cal/mol • Glucose + ATP → fructose 6 -P + ADP ΔG° = -3600 cal/mol
ΔGs of a pathway are additive • This additive property of free energy changes is very important in biochemical pathways through which substrates must pass in a particular direction for example, A → B → C → D. • As long as the sum of the ΔGs of the individual reactions is negative, the pathway can potentially proceed, even if some of the individual component reactions of the pathway have a positive ΔG. • The actual rate of the reactions does, of course, depend on the activity of the enzymes that catalyze the reactions.
Applications of Bioenergetics Knowledge of bioenergetics helps in the understanding of • – – obesity hyperthyroidism myocardial infarction. It also explains how the human beings able to maintain body temperature. • (i) Bioenergetics is the basis for calculating the person’s daily requirement of energy, which is dependent on height, weight and the work one does, and (ii) recommending the amount and type of food intake.
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