Hans Adolf Krebs discovered the Krebs cycle in 1937. The Krebs cycle is a series of chemical reactions that produce carbon dioxide, ATP, and electron carriers to be used in cellular respiration. It occurs in the mitochondria of cells and consists of eight steps where molecules like citrate, isocitrate, and oxaloacetate undergo oxidation reactions, producing molecules like alpha-ketoglutarate, succinyl-CoA, fumarate, and malate. The cycle generates molecules like NADH, FADH2, and one ATP that supply energy to the electron transport chain.

1. KREB CYCLE
BY:Madeleine KATE SIY

2. WHO DISCOVER
KREB CYCLE?

3. Hans Adolf Krebs
Biochemist; born in Germany.
Worked in Britain. His
discovery in 1937 of the ‘Krebs
cycle’ of chemical reactions
was critical to the
understanding of cell
metabolism and earned him
the 1953 Nobel Prize for
Physiology or Medicine.

4. Kreb Cycle
•The Krebs cycle refers to a complex series of
chemical reactions that produce carbon dioxide
and Adenosine triphosphate (ATP), a compound
rich in energy. the created compound then goes
through a series of changes that produce energy.

6. Function of Kreb Cycle
•To supply the third stage of respirations with
electrons (gold arrow)
•Makes 3 NADH
•Makes 1 FADH2
•Makes 1 ATP

8. Phases of Kreb Cycle

9. Step One
•The acetic acid subunit of acetyl CoA is
combined with oxaloacetate to form a
molecule of citrate. The acetyl coenzyme
A acts only as a transporter of acetic acid
from one enzyme to another.

10. Step 2
•The citric acid molecule undergoes an
isomerization. A hydroxyl group and a
hydrogen molecule are removed from the
citrate structure in the form of water. The two
carbons form a double bond until the water
molecule is added back. Thus, isocitrate is
formed.

11. Step 3
•In this step, the isocitrate molecule is oxidized
by a NAD molecule. The NAD molecule is
reduced by the hydrogen atom and the
hydroxyl group. The NAD binds with a
hydrogen atom and carries off the other
hydrogen atom leaving a carbonyl group. This
structure is very unstable, so a molecule of
CO2 is released creating alpha-ketoglutarate.

12. Step 4
•In this step, our friend, coenzymeA, returns to oxidize
the alpha-ketoglutarate molecule. A molecule of NAD is
reduced again to form NADH and leaves with another
hydrogen. This instability causes a carbonyl group to be
released as carbon dioxide and a thioester bond is
formed in its place between the former alpha-
ketoglutarate and coenzymeA to create a molecule
of succinyl-coenzyme A complex.

13. Step 5
•A water molecule sheds its hydrogen atoms to
coenzyme A. Then, a free-floating phosphate
group displaces coenzyme A and forms a bond
with the succinyl complex. The phosphate is
then transferred to a molecule of GDP to
produce an energy molecule of GTP. It leaves
behind a molecule of succinate.

14. Step 6
•In this step, succinate is oxidized by a
molecule of FAD (Flavin adenine
dinucleotide). The FAD removes two hydrogen
atoms from the succinate and forces a double
bond to form between the two carbon atoms,
thus creating fumarate.

15. Step 7
•An enzyme adds water to the fumarate
molecule to form malate. The malate
is created by adding one hydrogen
atom to a carbon atom and then
adding a hydroxyl group to a carbon
next to a terminal carbonyl group.

16. Step 8
•In this final step, the malate molecule is
oxidized by a NAD molecule. The carbon that
carried the hydroxyl group is now converted
into a carbonyl group. The end product
is oxaloacetate which can then combine with
acetyl-coenzyme A and begin the Krebs cycle
all over again.

17. Total Energy/ Glucose
•3molecules of ATP from each NADH and H+ produced
(10NADH + H+ x 3 ATP molecules=30ATP molecules)
•2 moleculesATP from each FADH
(2 FADH x 2ATP molecule= 4 ATP molecules)
•1/2 oxygen + 2H+=water
•total:34ATP molecules

18. http://highered.mcgraw-
hill.com/sites/0072507470/stu
dent_view0/chapter25/anima
tion__how_the_krebs_cycle_
works__quiz_1_.html

19. Sources
• http://www.incolor.com/mcanaday/Krebs%20Phases.htm
• http://highered.mcgraw-
hill.com/sites/0072507470/student_view0/chapter25/animation__how_the_
krebs_cycle_works__quiz_1_.html
• http://www.wisegeek.org/what-is-the-krebs-cycle.htm
• http://freevideolectures.com/Course/2548/Biology/24#
• http://www.brightstorm.com/science/biology/cell-functions-and-
processes/krebs-cycle/