Respiratory Chain

Content

• Biological oxidation

• Redox-potential

• Co-enzymes of oxidation reduction

Objective

• At the end of this lecture, student will be able to

• Explain Redox-potential

• Describe the enzymes and co-enzymes involved in oxidation
reduction

Biological
oxidation

• Oxidation is defined as the loss of electrons and
reduction as the gain of electrons. E.g. Interconversion of ferrous ion (Fe²⁺)
to ferric ion (Fe³⁺)

• The electron lost in the oxidation is accepted by an
acceptor which is said to be reduced. Thus, the oxidation-reduction is a
tightly coupled

• The general principle of oxidation-reduction is applicable
to biological systems also

• E.g. Oxidation of NADH to NAD+ coupled with the reduction
of FMN to FMNH2

• There are two redox pairs NADH/NAD+ and FMN/FMNH2 the
redox pairs differ in their tendency to lose or gain electrons

Redox
Potential (E_o)

• The oxidation-reduction potential or simply, redox
potential is a quantitative measure of the tendency of a redox pair to lose or
gain electrons

• Redox pairs are assigned specific standard redox potential
(Eo volts) at pH 7.0 and 25oC

• The more negative redox potential represents a greater
tendency to lose electrons

• The electrons flow from a redox pair with more negative Eo
to another redox pair with more positive Eo

• The redox potential (Eo) is directly related to the change
in the free energy

Enzymes and
co-enzymes involved in oxidation reduction

• Biological oxidation is catalyzed by enzymes, which
function in combination with coenzymes or electron carrier proteins

1. Oxido-reductases:
these enzymes catalysed the removal of hydrogen from substrate and add the
other substrate e.g. glyceraldehyde-3-phosphate dehydrogenase

2. Oxidases:
these enzymes catlyse the removal of hydrogen from the substrate and add
directly to the molecular oxygen e.g. Cytochrome oxidases, tyrosinases,
uricases etc

3. Oxygenases:
these enzymes incorporate oxygen in to the subatrate

a. Mono-oxygenases:
add one atom of O2 to the substrate

b. Di-oxygenases:
add both the atom of O2 to the substrate

4. Aerobic
dehydrogenases: these enzymes remove hydrogen from the substrate and add it
either directly to oxygen or any other artificial acceptor like methylene blue
and the product formed is H2O2

5. Anaerobic
dehydrogenases: these enzymes use other substrate to donate the hydrogen.
They transfer hydrogen to other hydrogen acceptor, but not directly to oxygen.
Thus hydrogen acceptor are NAD, FMN, FAD, cytochrome also receive hydrogen (b,
c1, a, a3)

6. Hydrogen
peroxidases: these enzymes have either hydrogen peroxide or organic
peroxide as their substrate.

There are two types of hydrogen peroxidases

1. Peroxidases

2. Catalases

Their prime function is to destory H2O2

Summary

• Oxidation is defined as the loss of electrons and
reduction as the gain of electrons

• The oxidation-reduction potential or simply, redox
potential is a quantitative measure of the tendency of a redox pair to lose or
gain electrons

• Biological oxidation is catalyzed by enzymes, which
function in combination with coenzymes or electron carrier proteins