BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 14. OXIDATIVE PHOSPHORYLATION

14.2. Oxidation-Reduction Potentials and Free-Energy Changes

In Oxidative Phosphorylation, the electron-transfer potential inherent in NADH or FADH₂ is converted into the phosphoryl-group transfer potential characteristic of ATP. These forms of Free energy must be expressed quantitatively. We are already familiar with the measurement of phosphoryl-group transfer potential, which is derived from the ∆G⁰' for the Hydrolysis of a phosphate compound. The corresponding expression for electron-transfer potential is the oxidation-reduction potential (also referred to as the redox potential).

The oxidation-reduction potential is an electrochemical parameter. Let us consider, for example, a substance that can exist in an oxidized form X and a reduced form X^-. Such a pair is called a redox couple (Fig. 14.3). The oxidation-reduction potential of this couple can be determined by measuring the electromotive force developed by a test half-Cell relative to a standard reference half-cell. The test half-cell consists of an electrode immersed in a solution of 1 M oxidant (X) and 1 M reductant (X^-). The standard reference half-cell consists of an electrode immersed in a 1 M solution of H⁺ in equilibrium with H₂ gas at a pressure of 1 atm. The electrodes are connected to a voltmeter, and an Agar salt bridge provides electrical continuity between the half-Cells. A flow of electrons takes place from one half-cell to the other. If the reaction proceeds in the direction

X^- + Н⁺ → X + ¹/₂Н_2,

then the following reactions will occur in the half-cells:

X^- → X + e^-,

X + е- → ¹/₂Н_2.

Class="center">Fig. 14.3. Apparatus for measuring the standard oxidation-reduction potential of a redox couple

Thus, electrons move from the test half-cell to the reference half-cell; consequently, the electrode in the test half-cell is negatively charged relative to the electrode of the standard half-cell. The oxidation-reduction potential of the X : X^- couple corresponds to the voltage at THE START OF the experiment (when the concentrations of X, X^-, and H⁺ are equal to 1 M). The oxidation-reduction potential of the H⁺ : H₂ couple is defined as 0 V (volts).

The Significance of the oxidation-reduction potential is now apparent. A negative oxidation-reduction potential indicates that the substance has a lower affinity for electrons than H₂ does (as in the example above). A positive oxidation-reduction potential indicates a higher affinity for electrons than that of H₂. These relationships apply to standard conditions, where the concentrations of the oxidant, reductant, and H⁺ are 1 M and the H₂ pressure is 1 atm. Thus, a strong reducing agent (e.g., NADH) has a negative oxidation-reduction potential, whereas a strong oxidizing agent (O₂) has a positive oxidation-reduction potential.

Table 14.1. Standard oxidation-reduction potentials of selected reactions

The oxidation-reduction potentials of many biologically important redox couples are known (Table 14.1). The free-energy change of an oxidation-reduction reaction can be readily calculated from the difference in oxidation-reduction potentials of the reacting species. Consider, for example, the reduction of Pyruvate by NADH:

a) Pyruvate + NADH + Н⁺ ⇄ Lactate + NAD⁺.

The oxidation-reduction potential for the NAD⁺ : NADH couple is -0.32 V, and for the pyruvate : lactate couple it is -0.19 V. By convention, oxidation-reduction potentials pertaining to half-reactions are written as follows:

Oxidant + e^- → Reductant. Then

б) Пируват + 2Н⁺ + 2е^- → Лактат

E_о= - 0, 19 В,

в) NAD + Н⁺ + 2е^- → NADH

E_о = - 0, 32 В.

Subtracting reaction c) from reaction b), we obtain the desired reaction a) and ∆ E^'0 = + 0.13 V. Now we can calculate ∆G⁰' for the reduction of pyruvate by NADH. The change in Standard Free Energy ∆G⁰' is related to the change in redox potential ∆E'⁰ by the equation

∆G⁰' = -_nF ∆E_о,

where n is the number of electrons transferred, F is the Faraday constant (23.062 kcal • V^-1 mol^-1), ∆Е'⁰ is expressed in volts, and ∆G^0' in kilocalories per mole. For the reduction of

pyruvate n = 2, and therefore

∆С⁰' = - 2 • 23.062 • 0.13 = - 6 kcal/mol.

Note that a positive value of ∆Еo indicates the exergonic Nature of the reaction proceeding under standard conditions.

14.3. The redox potential span of the respiratory chain is 1.14 V, which corresponds to 53 kcal

The driving force of Oxidative phosphorylation is the electron transfer potential inherent in NADH or FАDН₂. Let us calculate ∆Eo and ∆С⁰' associated with The oxidation of NADH by O₂. The intermediate partial reactions are as follows:

а) ¹/₂O₂ + 2Н⁺ + 2е^- ⇄ Н₂O

E_o = + 0.82 V,

б) NAD⁺ + Н⁺ + 2е^-⇄ NADH

Е_o= - 0.32 V.

Subtracting reaction b) from reaction a), we obtain

в) ¹/₂O₂ + NADH + Н⁺⇄ Н₂O + NAD⁺

∆Е₀ = + 1.14 V.

The Free energy of oxidation for this reaction is

∆G^0, = _nF∆E_o = - 2 • 23.062 • 1.14 = - 52.6 kcal/mol.