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Lecture 5

BIOC 3300 Lecture Notes - Lecture 5: Maleic Acid, Transamination, Riboflavin


Department
Biochem & Molecular Biology
Course Code
BIOC 3300
Professor
Mc Leod Roger
Lecture
5

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1. Role of the TCA
Cycle
(a) Central oxidative pathway.
(b) Krebs cycle / tricarboxylic acid cycle.
(c) carbohydrate, fat and protein all can
enter the cycle.
(d) oxidize organic metabolites, coupled
to the reduction of cofactors.
(e) amphibolic
2. TCA tricarboxylic acid cycle.
Both citric acid and isocitric acid are
tricarboxylic acids.
3. Why TCA cycle is
central oxidative
pathway?
carbohydrate, lipid and protein are
catabolized in aerobic systems, occuring
in mitochondrial matrix.
4. How three
macromolecules
enter the TCA
cycle
carbohydrate enters as pyruvate,
fat as acetylCoA from α-oxidation,
protein enters as citric acid cycle
intermediates of amino acid metabolism
5. Function of TCA
cycle?
to oxidize organic metabolites, coupled to
the reduction of cofactors.
6. why is TCA cycle
amphibolic?
contain both catabolic and anabolic two
distinct phases:
(1) oxidation to produce CO2,
(2) generation of oxaloacetate
7.
Pyruvate
Dehydrogenase
complex
8.
PDC complex
A complex of three enzymes and five
coenzymes that convert pyruvate into
acetyl-CoA by a process called pyruvate
decarboxylation.
9. Three enzymes in
Pyruvate
Dehydrogenase
complex
E1 = pyruvate dehydrogenase
(decarboxylase).
E2 = dihydrolipoamide transacetylase.
E3 = dihydrolipoamide dehydrogenase.
E2 and E3 are present in complex as
dimers.
10. Coenzymes in TCA
cycle and
Pyruvate
Dehydrogenase
complex
(a) thiamine pyrophosphate (TPP).
(b) lipoic acid.
(c) Coenzyme A.
(d) Flavin coenzymes- FAD, FMN.
(e) NAD+.
11. Coenzyme (a):
thiamine
pyrophosphate
cofactor for decarboxylations of α-ketoacids.
Forms a nucleophilic carbanion that attacks
an α-keto carbon.
In Pyruvate Dehydrogenase complex:
Bound to E1.
Function: Decarboxylates pyruvate,
yielding a hydroxyethyl-TPP carbanion
12. Coenzyme (b):
lipoic acid
As lipoamide, acyl group and electron
carrier.
Reduced by 2H+ and 2e- to 2 -SH groups
with ring opening.
In Pyruvate Dehydrogenase complex:
Covalently linked to a Lys on E2 (lipoamide).
Accepts the hydroxyethyl carbanion from
TPP as an acetyl group
13. lipoamide
14. Coenzyme (c):
Coenzyme A
- components are ATP, pantothenic acid
and β-mercaptoethylamine.
- free energy of thioester hydrolysis is high
relative to oxyester.
- nucleophilic properties of the thioalkoxide
anion.
- activation and transfer of acetyl and acyl
groups.
In Pyruvate Dehydrogenase complex:
Substrate for E2.
Accepts the acetyl group from
acetyldihydrolipoamide.
15. Coenzyme (d):
flavin
coenzymes
FAD, FMN; electron transfer reactions via
reduction of ring system.
Each contains a conjugated double bond
system in the isoalloxazine ring that can be
reduced or oxidized to transfer electrons.
In Pyruvate Dehydrogenase complex:
Bound to E3.
Reduced by dihydrolipoamide
Lecture 5 Citric Acid Cycle
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16. riboflavin
17. FMN
18. FAD
19. Coenzyme (e):
NAD+ Nicotinamide adenine dinucleotide
(NAD)
In Pyruvate Dehydrogenase complex:
Substrate for E3.
Reduced by FADH2.
20. Acteyl CoA A reactive thioester with CoA as leaving
group.
Free energy of hydrolysis of thioester is
large and can be used to drive acyl group
transfer reactions.
Free energy of hydrolysis is -35 kJ/mole for
acetyl-CoA.
Acetyl group of acetyl-CoA is linked by a
"high energy" thioester to the sulfhydryl
portion of the
β-mercaptoethylamine.
21. Main function
Acteyl CoA
To convey the carbon atoms within the
acetyl group to the citric acid cycle to be
oxidized for energy production.
22. Why is acetyl-
CoA "high
energy"
compounds?
Thioester bond.
Hydrolysis of the thioester bond is highly
exergonic.
23. Where is Acteyl
CoA produced?
Acetyl-CoA is produced during breakdown
of carbohydrates through glycolysis as well
as fatty acid oxidation and enters the citric
acid cycle.
24. Structure of
coenzyme A
(CoASH or CoA)
It consists of a β-mercaptoethylamine group
linked to the vitamin pantothenic acid
through an amide linkage.
25. Pyruvate
Dehydrogenase
Complex
26. Coenzyme A
O
CH2O P
HO C HO
HO C H
HO C H
O
O P O
O
N
CH2
N N
N
O
H
H3C
H2C
N
N
CH2
E
O
His
75a
9a
7a
8a
8
910
4a
10a 1
4
3
2
5
6
H
O
CH2
CH2O P
HO C HO
HO C H
HO C H
O
O P O
O
N
CH2
N N
N
O
H
H3C
H2C
N
N
CH2
E
O
His
75a
9a
7a
8a
8
910
4a
10a 1
4
3
2
5
6
O
HH
HH
OH OH
N
NH2
NN
N
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