Dr. Aga Syed SameerCSIR Lecturer

Department of Biochemistry,

Medical College,

Sher-I-Kashmir Institute of Medical Sciences,

Bemina, Srinagar, Kashmir, 190018. India.

Biological

Oxidation

Introduction

EntropyEnthalpyFree energyRedox Potential

Introduction

Oxidation:

Addition of Oxygen

Removal of Electrons

Removal of Hydrogen

Reduction:

Removal of Oxygen

Addition of Electrons

Addition of Hydrogen

Biological Oxidation

It refers to cellular oxidation of various metabolic fuels and/or metabolites by:

Addition of Oxygen

Removal of Electrons

Removal of Hydrogen

Occurs within the Cellular confines facilitated by mitochondria

Usually takes place with the help of enzymes

Biological OxidationMetabolites are aerobically oxidised ultimately to

CO2 and H2O via ETC

Free energy released during oxidation is captured and used in the synthesis of “high energy” phosphate bonds in ATP molecule

Oxidation of metabolites in cells leads to formation of two types of reducing equivalents/cofactors NADH and FADH2

In ETC of mitochondria these cofactors are oxidisedin presence of O2 rthereby regenerating NAD and FAD

Enthalpy

∆H: measure of the change in heat content of reactants and products

∆ H = Hproducts – Hreactants

Hproduct < Hreactants :::::: ∆H is negativeExothermicAlways spontaneous

Hproduct > Hreactants :::::: ∆H is positiveEndothermicSometimes spontaneous

Entropy

∆S: measure of the change in randomness or disorder of reactants and products

∆S = Sproduct – Sreactant

Sproduct > Sreactants :::::: ∆S is positive

Sproduct < Sreactants :::::: ∆S is negative

Solids have a very low entropy

Liquids have a slightly higher entropy

Gases have a very high entropy

Entropy

∆S: is a function of state which is defined for a reversible transformation by

∆S = dQ/T

In all thermodynamically reversible transformations the entropy change of the system plus its surroundings is Zero

All irreversible changes are accompanied by an increase in entropy

Free Energy

∆G: measures the change in free energy; and the direction in which reaction will proceed at any specified concentration of reactants and products

∆G = ∆H – T∆S

If ∆G is negative the reaction is spontaneous

If ∆G is positive the reaction is non-spontaneous

If ∆G is 0 the reaction is at equilibrium

Standard Free Energy∆GO: measure the change in free energy when

reactants and products are at concentration of 1mol/L

∆G = ∆GO + RT ln [B]/[A] for A → B

∆GO is standard free energy change R is gas constant (1.987 Cal/mol. degree) T is absolute temperature (K)

At equilibrium, ∆G = 0, Q = Keq so∆G = 0 = ∆Go + RT ln Keq∆Go = - RT ln Keq

∆GO for Redox Reactions

∆GO: measures the change in free energy when reactants and products are at concentration of 1mol/L

∆GO = -nF ∆ EO

n is number of electrons transferred

F is Faraday’s constant (23,062 Cal/Volt.Mol)

∆EO is difference between EO of electron accepting pair and EO of electron donating pair

∆ GO is change in standard free energy

ProcessesExergonic: Reaction is one in which ∆G is

negative and which can drive other processes and is therefore capable of doing useful work

Endergonic: Reaction is one in which ∆G is positive and which must be driven

Exothermic: Reaction is one in which ∆H is negative and which releases heat

Endothermic: Reaction is one in which ∆H is positive and which absorbs heat

Redox Potential

Also known as Reduction potential ; Oxidation / Reduction potential, is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced

Reduction potential is measured in volts (V), or millivolts (mV)

It is responsible for the transfer of electrons in the ETC for the oxidation of reducing equivalents

Mitochondria

Questions?