The introduction of Kreb Cycle for basic biovhemistry

1. CITRIC ACID CYCLE /
TRICARBOXYLIC ACID CYCLE
(TCA CYCLE) / KREBS CYCLE

2. Lesson Learning Outcomes
Upon completion of this chapter, students
should be able to:
explain the steps of the citric acid cycle
differentiate between citric acid cycle and
glyoxylate cycle
relate citric acid cycle as an energy source

3. INTRODUCTION
⚫Also known as Kreb cycle or tricarboxylic acid cycle
(TCA cycle)
⚫Three processes play central roles in aerobic
metabolism
◦ the citric acid cycle
◦ electron transport
◦ oxidative phosphorylation
⚫Metabolism consists of
◦ catabolism: the oxidative breakdown of nutrients
◦ anabolism: the reductive synthesis of biomolecules
⚫The citric acid cycle is amphibolic; that is, it plays a
role in both catabolism and anabolism

4. Overall cellular respiration

6. oxidation and the
release ofenergy
Fats Proteins
Fatty acids
and glycerol
Amino
Acids
Small
molecules
Anabolism
of proteins
beakdown
of larger
molecules
to smaller
ones
Catabolism Excretion
Products ofanabolism,
including proteins and
nucleic acids
energy and
reducing
agents
Some nutrients and
products ofcatabolism
Monosac-
charides
Polysac-
charides
Excretion Anabolism
Catabolism Anabolism

7. The role of ATP as an energy source

8. ATP
•An ATP molecule consists of one adenosine
and three (tri) phosphate groups.
•ATP is essentially the energy currency of the
body. It is the breakdown of ATP that
releases energy which the body’s tissues such
as muscle can use.

9. ATP Structure

10. The Citric Acid Cycle

12. Where citric acid cycle happens?
• Takes place in the matrix of mitochondria except for
one in which the intermediate electron acceptor is
FAD (inner mitochondrail membrane)

13. • In the citric acid cycle and the pyruvate
dehydrogenase reactions, one molecule of pyruvate
is oxidized to 3 molecules of CO2 as a result of
oxidative phosphorylation.
• The oxidations are accompanied by reductions.
• 4 NAD+ are reduced to NADH (pyruvate to acetly
Co-A – 1 NAD+, citric acid cycle – 3 NAD+)
• 1 FAD is reduced to FADH2
• 1 GDP is phosphorylated to GTP

14. Pyruvate to Acetyl-CoA
⚫Step 1: pyruvate loses CO2 and HETPP is
formed
⚫Step 2: requires lipoic acid
⚫the active form of lipoic acid is bound to the
enzyme by an amide bond to the amino group
of a lysine
O
CH CCOO- +
3
Pyruvate
pyruvate
dehydrogenase
TPP CO2 + 3
CH CH-TPP
OH
Hydroxyethyl-TPP
S S
Lipoic acid
COOH COOH
HS SH
Dihydrolipoic acid
reduction
oxidation

15. Pyruvate to Acetyl-CoA
• Step 3: the acetyl group is transferred to the
sulfhydryl group of coenzyme A
SH
Dihydrolipoamide
O
C-NH- Enz
+
CoA-SH
Coenzyme A
O
CoA-S-CCH3
Acetyl-CoA
+ HS SH
Dihydrolipoamide
dihydrolipoyl transacylase
O
C-NH- Enz
O
CH3 C-S

16. Pyruvate to Acetyl-CoA
S S
NAD+
NADH
O
C-NH- Enz
Lipoamide
SH Dihydrolipoamide
HS
• Step 4: Oxidation of dihydrolipoamide
O
C-NH- Enz

18. The Citric Acid Cycle
• Step 1: condensation of acetyl-CoA
oxaloacetate;
with
+
CH2 -COO-
HO C-COO-
CH2 -COO-
Citrate
CoA-SH
Coenzyme A
citrate
synthase
O
CH3C-SCoA
Acetyl-CoA
+
O C-COO-
CH2 -COO-
Oxaloacetate

19. The Citric Acid Cycle
• Step 2: dehydration and rehydration gives isocitrate;
catalyzed by aconitase
only one of the 4 stereoisomers of isocitrate is
formed in the cycle
HO C-COO-
CH2-COO-
Citrate
CH2-COO- CH2-COO-
C-COO-
CH-COO-
Aconitate
CH2-COO-
H C-COO-
HO CH-COO-
Isocitrate

20. The Citric Acid Cycle
• Step 3: oxidation of isocitrate followed by
decarboxylation
 isocitrate dehydrogenase is an allosteric enzyme; it is
inhibited by ATP and NADH, activated by ADP and NAD+
CH2 -COO-
H C-COO-
HO CH- COO-
Isocitrate
CH2 -COO-
NAD+ NADH
isocitrate
dehydrogenase
CO2 CH2 -COO-
H C-H
O C-COO-
-Ketoglutarate
H C-COO-
O C-COO-
Oxalosuccinate

21. The Citric Acid Cycle
– like pyruvate dehydrogenase, this enzyme is a
multienzyme complex and requires coenzyme A,
thiamine pyrophosphate, lipoic acid, FAD, and
NAD+
• Step 4: oxidative decarboxylation of α-ketoglutarate
to succinyl-CoA
CoA-SH
CH2 -COO-
CH2
C-COO-
-Ketoglutarate
O
NAD+ NADH
-ketoglutarate
dehydrogenase
complex
CH2 -COO-
CH2
O C SCoA
Succinyl-CoA
+ CO2

22. The Citric Acid Cycle
• Step 5: formation of succinate
 this is the first energy-yielding step of the cycle
 the overall reaction is slightly exergonic
CH2-COO-
CH
2
O C SCoA
Succinyl-CoA
i
+ GDP+ P + GTP + CoA-SH
Succinate
succinyl-CoA
synthetase
CH2-COO-
CH2-COO
-

23. The Citric Acid Cycle
• Step 6: oxidation of succinate to fumarate
• Step 7: hydration of fumarate
FAD FADH 2
CH2 - COO-
CH2 - COO-
Succinate
succinate
dehydrogenase
C
- O O C
C
H
Fumarate
H COO-
C
C
H COO-
- OOC H
Fumarate
H2 O
-
HO CH- COO
CH2 -COO-
L-Malate
fumarase

24. The Citric Acid Cycle
• Step 8: oxidation of malate
O C-COO-
CH2-COO-
Oxaloacetate
NAD+
HO CH-COO- NADH
CH2-COO-
malate
dehydrogenase
L-Malate

26. From Pyruvate to CO2
Pyruvate dehydrogenase complex
+ CoA-SH + NAD+
Acetyl-CoA
Citric acid cycle
Acetyl-CoA +3NAD+
+ FAD + GDP + P + 2H O
i 2
2CO2 + CoA-SH+ 3NADH+ 3H+
+ FADH2 + GTP
Pyruvate + 4NAD
+
+ FAD + GDP + Pi + 2H2O
Pyruvate
+ NADH+ CO2 + H+
3CO2 + 4NADH + FADH2 + GTP + 4H+

27. Control of the CA Cycle
⚫Three control points within the cycle
◦ citrate synthase: inhibited by ATP, NADH, and succinyl CoA; also
product inhibition by citrate
◦ isocitrate dehydrogenase: activated by ADP and NAD+, inhibited
by ATP and NADH
◦ -ketoglutarate dehydrogenase complex: inhibited
NADH, and succinyl CoA; activated by ADP and NAD+
⚫One control point outside the cycle
by ATP,
◦ pyruvate dehydrogenase: inhibited by ATP and NADH; also
product inhibition by acetyl-CoA

28. Control of the CA Cycle

29. The Glyoxylate Cycle
• Plants and some bacteria, but not animals, use a
modification of the citric acid cycle to produce four-
carbon dicarboxylic acids and eventually glucose
the glyoxylate cycle bypasses the two oxidative
decarboxylations of the citric acid cycle
instead, it routes isocitrate via glyoxylate to
malate
key enzymes in this cycle are isocitrate lyase and
malate synthase

30. Glyoxylate Cycle

31. The Glyoxylate Cycle
• The glyoxylate cycle takes place
in plants: in glyoxysomes, specialized organelles devoted
to this cycle
in yeast and algae: in the cytoplasm
• Helps plants grow in the dark
seeds are rich in lipids, which contain fatty acids
during germination, plants use the acetyl-CoA produced
in fatty acid oxidation to produce oxaloacetate and
other intermediates for carbohydrate synthesis
once plants begin photosynthesis and can fix CO2,
glyoxysomes disappear

32. CA Cycle in Catabolism
• The catabolism of proteins, carbohydrates, and
fatty acids all feed into the citric acid cycle at one
or more points
Pro t eins
A m i n o Acids
A c e t y l - C o A
C a r b o h y d r a t e s F a t t y A cids
P y r u v a te
 - K e t o g l u t a r a t e
Succiny l - C o A
F u m a r a t e
M a l a t e
O x a l o a c e t a t e
i n t e r m e d ia t e s
of the citric
a c i d c y c l e

33. CA Cycle in Anabolism
• The citric acid cycle is the source of starting materials
for the biosynthesis of other compounds. Examples:
- -
O
OOCCH2CH2CCOO
-Ketoglutarate
transamination
-OOCCH CCOO- -OOCCH CHCOO-
NH3
+
-OOCCH2CH2CHCOO-
Glutamate
NH3
+
2
Aspartate
O
2
Oxaloacetate
transamination

34. End of lecture