Definitions of oxidation and reduction (redox), Quinones, Flavoproteins, Iron–sulfur proteins, and Cytochromes

1. Bacteriology
Presented by
D. Mona Othman Albureikan
Lectures (15,16)

2. • The oxidation and
reduction (redox) are the
transfer of oxygen,
hydrogen and electrons.
• It also explains the terms
oxidising agent and
reducing agent.
Definitions of oxidation and reduction (redox)

3. • Oxidation is the loss of
electrons, hydrogen, or
both, or the gain of
oxygen.
• Reduction is the gain of
electrons, hydrogen, or
both, or the loss of oxygen.
Definitions of oxidation and reduction (redox)

4. • Because both reduction
and oxidation are going on
side-by-side, this is known
as a redox reaction.
• Each time a substance is
oxidized, another is reduced.
• NAD+ is the oxidized form;
NADH is the reduced form.
Definitions of oxidation and reduction (redox)

5. • Glucose is a reduced molecule; energy is released during a cell’s oxidation of glucose.
Definitions of oxidation and reduction (redox)

6. • Enzymes called oxidoreductases facilitate these reactions, which occur in both
catabolic and anabolic pathways.
Definitions of oxidation and reduction (redox)

7. Definitions of oxidation and reduction (redox)

8. • In some key energy-release and storage reactions, hydrogen atoms are removed
from organic molecules through a process called dehydrogenation.
Definitions of oxidation and reduction (redox)

9. • Electrons flow spontaneously
down a potential energy gradient
toward acceptors that have a
more positive electrode potential.
• Since electrons can do great
damage to the molecular structure
of the cell, they must be retained
and held by special receptor
substances called electron-
transport molecules.
Definitions of oxidation and reduction (redox)

10. • These molecules are
easily reduced and
oxidized.
• The electrons flow from
primary electron donors
to terminal electron
acceptors through a
series of ((electron
carrier proteins)) and a
class of lipids called
quinones.
Definitions of oxidation and reduction (redox)

11. • The electrons flow through a series of
electron carriers.
• Some of these carry hydrogen as well as
electrons, and some carry only electrons.
• The electron carriers are as follows:
1. Quinones (hydrogen and electron carriers)
2. Flavoproteins (hydrogen and electron carriers)
3. Iron–sulfur proteins (electron carriers)
4. Cytochromes (electron carriers)
The Electron Carriers

12. • Some are derived from vitamins and are key components that complete the
structure of enzymes called coenzymes,
• such as nicotinamide adenine dinucleotide (NAD+),
• nicotinamide adenine dinucleotide phosphate (NADP+)
• and flavine adenine dinucleotide (FAD+).
• Others, such as flavoproteins and cytochromes are embedded in the cell
membrane of bacteria.
Definitions of oxidation and reduction (redox)

13. • The quinones are lipids, whereas the other electron carriers are proteins, which exist in
multiprotein enzyme complexes called oxidoreductases.
• The electrons are not carried in the protein molecule, but in a nonprotein molecule
bound to the protein.
• The nonprotein portion that carries the electron is called a prosthetic group.
• The quinones are also hydrogen carriers.
1. Quinones (hydrogen and electron carriers)

14. 1. Quinones (hydrogen and electron carriers)
- Some are believed to be highly mobile in the lipid phase of the membrane, carrying hydrogen
and electrons to and from the complexes of protein electron carriers that are not mobile.
- Bacteria make two types of quinone that function during respiration: ubiquinone (UQ), a
quinone also found in mitochondria, and menaquinone (MQ, or sometimes MK).

15. • A flavoprotein (Fp) is an electron carrier that has as its prosthetic group an organic
molecule called a flavin.
• The term is derived from the Latin word flavius, which means yellow, in reference to
the color of flavins.
• The flavins FAD and FMN are synthesized by cells from the vitamin ribofl avin
(vitamin B2).
2. Flavoproteins (hydrogen and electron carriers)

16. • when flavins are reduced they carry 2H• (equivalent to two electrons and two
protons), one on each of two ring nitrogens.
• There are many different flavoproteins, and they catalyze diverse oxidation–
reduction reactions in the cytoplasm, not merely those of the electron
transport chain in the membranes.
2. Flavoproteins (hydrogen and electron carriers)

17. • Although all the flavoproteins have FMN or FAD as their prosthetic group,
they catalyze different oxidations and have different redox potentials.
• These differences are due to differences in the protein component of the
enzyme, not in the flavin itself.
2. Flavoproteins (hydrogen and electron carriers)

18. • Iron–sulfur proteins contain nonheme iron and usually acid-labile sulfur.
• The term “acid-labile sulfur” means that when the pH is lowered to
approximately 1, hydrogen sulfi de is
released from the protein.
• This is because there is sulfi de attached
to iron by bonds that are ruptured in acid. Generally, the proteins contain clusters
in which iron and acid-labile sulfur are present in a ratio of 1:1.
3. Iron–sulfur proteins (electron carriers)

19. • There may be more than one
iron–sulfur cluster per
protein.
• The FeS clusters have
different Eh values, and the
electron travels from one FeS
cluster to the next toward the
higher Eh.
3. Iron–sulfur proteins (electron carriers)

20. • Cytochromes are electron carriers
that have heme as the prosthetic
group.
• In the center of each heme there is
an iron atom that is bound to the
nitrogen of the pyrrole rings. The
iron is the electron carrier and is
oxidized to ferric or reduced to
ferrous ion during electron
transport.
4. Cytochromes (electron carriers)

21. • The Eh values of the
different cytochromes
vary depending on
the protein and the
molecular
interactions with
surrounding
molecules.
4. Cytochromes (electron carriers)

22. 1- Oxidation and reduction in terms of oxygen transfer
• Definitions
• Oxidation is gain of oxygen.
• Reduction is loss of oxygen.
• For example, in the extraction of iron :

23. 1-Oxidation and reduction in terms of oxygen transfer
• Oxidising and reducing agents;
• An oxidising agent is substance which oxidises something else. In the example, the iron(III)
oxide is the oxidising agent.
• A reducing agent reduces something else. In the equation, the carbon monoxide is the
reducing agent.
• Oxidising agents give oxygen to another substance.
• Reducing agents remove oxygen from another substance.

24. 2- Oxidation and reduction in terms of hydrogen
transfer
• Definitions;
• Oxidation is loss of hydrogen.
• Reduction is gain of hydrogen.
• Notice that these are exactly the opposite of the oxygen definitions.
• For example, ethanol can be oxidised to ethanal

25. 2- Oxidation and reduction in terms of hydrogen
transfer
• Definitions;
• You would need to use an oxidising agent to remove the hydrogen from the ethanol. A
commonly used oxidising agent is potassium dichromate(VI) solution acidified with
dilute sulphuric acid.
• Ethanal can also be reduced back to ethanol again by adding hydrogen to it. A possible
reducing agent is sodium tetrahydridoborate, NaBH4.

26. 2- Oxidation and reduction in terms of hydrogen
transfer
• Oxidising and reducing agents;
• Oxidising agents give oxygen to another substance or remove hydrogen from it.
• Reducing agents remove oxygen from another substance or give hydrogen to it.

27. 3- Oxidation and reduction in terms of electron
transfer
• Definitions
• Oxidation is loss of electrons.
• Reduction is gain of electrons.

28. 3- Oxidation and reduction in terms of electron
transfer
• A simple example;
• The equation shows a simple redox reaction which can obviously be described in
terms of oxygen transfer.
• Copper (II) oxide and magnesium oxide are both ionic. The metals obviously aren't. If
you rewrite this as an ionic equation, it turns out that the oxide ions are spectator ions
and you are left with:

29. 3- Oxidation and reduction in terms of electron
transfer
• Oxidising and reducing agents
• If you look at the equation above, the magnesium is reducing the copper(II) ions by
giving them electrons to neutralise the charge. Magnesium is a reducing agent.
• Looking at it the other way round, the copper (II) ions are removing electrons from
the magnesium to create the magnesium ions. The copper (II) ions are acting as an
oxidising agent.

30. Summary
• An oxidising agent oxidises something else.
• Oxidation is loss of electrons (OIL RIG).
• That means that an oxidising agent takes
electrons from that other substance.
• So an oxidising agent must gain electrons.