Transcript Stryer An overview of the citric acid cycle

Lecture 6B – outline
Mitochondrial function (e.g. hepatocytes)
1) citric acid cycle as an energy source
a) pyruvate or a-ketoglutarate dehydrogenase
b) lipoic acid therapy
2) the respiratory chain as an energy source
3) oxidative phosphorylation and uncouplers
4) membrane transporters and shuttles
a) cytosolic NADH oxidation
b) acetyl CoA (NADPH export)
c) transport systems in the mitochondria
d) gluconeogenesis and glucose transport
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Compartmentalization
of the major pathways
of metabolism
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1.
CITRIC ACID CYCLE AS AN ENERGY SOURCE
An overview of the citric acid cycle
Stryer
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Acetyl CoA + 3 NAD+ + FAD + GDP + Pi + 2 H2O 
2 CO2 + 3 NADH + FADH2 + GTP + 2H+ + CoA
toxic!
120uM plasma citrate
complexes Fe
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The citric acid cycle is a source of biosynthetic precursors
Glucose
Pyruvate
ATP, CO2
Phosphoenolpyruvate
Acetyl CoA
ADP, Pi
Amino
acids
Oxaloacetate
Succinyl
CoA
Porphyrins
Citrate
Stryer Fig. 20-17.
Biosynthetic roles of the
citric acid cycle.
Intermediates drawn off
for biosyntheses are
replenished by the
formation of oxaloacetate
from pyruvate.
aketoglutarate
Amino
acids
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Control of the
citric acid cycle
Stryer Fig. 20-22.
Control of the
citric acid cycle and
the oxidative
decarboxylation of
pyruvate: * indicates
steps that require an
electron acceptor
(NAD+ or FAD) that
is regenerated by the
respiratory chain.
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2. THE MITOCHONRIAL
RESPIRATORY CHAIN AS AN
ENERGY SOURCE
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The mitochondrial respiratory chain
NADH
Diagram of a mitochondrion
FMNH2
complex I
NADH-Q
reductase
2Fe-2S
4Fe-4S
Q
FADH2
in flavoproteins
succinate:Q reductase
(complex II)
complex III Cytochrome
reductase
Chemiosmotic theory of oxidative phosphorylation
cyt c
complex IV Cytochrome
oxidase
O2
Sequence of electron
carriers in the
respiratory chain
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Origin of mitochondria: the endosymbiont hypothesis
The endosymbiont hypothesis suggests that mitochondria have evolved
from anaerobic bacteria which were phagocytosed by eukaryote cells
at the time oxygen appeared on earth,
Similarities between mitochondria and bacteria include the presence of:
• cardiolipin
•transporters
• ribosomes
• circular RNA and DNA
Therefore mitochondria protein synthesis should be inhibited by:
• TETRACYCLINE
• CHLORAMPHENICOL.
E.g. The extensive use of these drugs can inhibit
1. Bone marrow mitochondrial protein synthesis leading to a
decline in the production of white or red cells.
2. Intestinal epithelial cells causing them to cease dividing.
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NADH coenzyme Q reductase: complex I
M
N
N
A
D
H F
+
N
A
D
O
O
F
M
N
H
2
H
C
OC
3
C C
H
3
C
O
C
H
3
-
e
N
C (C
H
C C C
H
)10 H
2
2
H
Q
H
2x
O
C
H
C
OC
3
Q e
R
+
H
C
OC
3
H
C
OC
3
C
O
H
eC C
H
3
C R
C
O
H
+
+
A
H
C
OC
3
H
C
OC
3
C
C C
H
3
C R
C
O
H
e
C
o
eR
1
0S
I
n
(
U
B
The reduction of ubiquinone to ubiquinol proceeds through a semiquinone
anion intermediate.
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Q:Cytochrome c reductase (Complex III)
Q
cyt b (+2)
QH
Fe-S(+2)
cyt c1(+3)
cyt c(Fe+2)
QH
cyt b (+3)
QH2
Fe-S(+3)
cyt c1(+2)
cyt c(Fe+3)
Stryer p. 537
cytochrome c reductase
Stryer Fig. 21-11
Model of a portion of
Q: cytochrome c reductase
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Cytochrome oxidase (Complex IV)
Lodish Fig. 17-30
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Electron transport can be
blocked by specific inhibitor
poisons
NADH
NADH-Q
Reductase
QH2
Blocked by
rotenone and
amytal
Cytochrome b
Blocked by
antimycin
Cytochrome c1
Sites of action of some
inhibitors of electron
transport
Cytochrome c
Cytochrome Oxidase
Blocked by
CN- , N3 -, and CO
O2
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Cytochrome C - catalytic site
RC C
H
2
H
V
i
o
f
+
n
C
H
3
H
SC
H
C 2
y
o
t
R
y
l
f
h
The heme in cytochromes c and c1 is
covalently attached to 2 cysteine side chains
by thioether linkages
'
s
e
R C S C
H
t
e
iH
2
g Tr
o
h
t
h
e
h
e
R
'
n
e
u
i
opp
m
The iron atom of the heme group in
cytochrome c is bonded to a methionine
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sulfur atom and a histidine nitrogen atom
Cytochrome C - soluble NOT membrane bound
1. 26/104 amino acids residues have been invariant for > 1.5 x 109 years.
2. Met 80 and His 18 - coordinate Fe.
3. 11 residues from number 70 - 80 lining a hydrophobic crevice have
remained virtually unchanged throughout all cytochrome c regardless
of species or even kingdom.
4. A number of invariant arginine and lysine clusters can be found on
the surface of the molecule.
Cytochrome c has a dual function in the cell. Electron transport for ATP
production AND the major cause of most programmed cell death
(apoptosis) is initiated by the release of cytochrome c into the cytosol!
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3. OXIDATIVE
PHOSPHORYLATION AND
UNCOUPLERS
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Oxidative phosphorylation
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4. Mitochondrial MEMBRANE
TRANSPORTERS
A) Cytosolic NADH oxidation
B) Acetyl CoA (NADPH export)
C) Transport systems in the mitochondria
D) Gluconeogenesis and glucose transport
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a) Cytosolic NADH oxidation: membrane transporters glycerol
phosphate shuttle (Bucher shuttle)
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b) Acetyl CoA/NADPH export to cytosol for fatty acid synthesis/
drug metabolism
Gl uc o s e
Cyt osol
Py r uv a t e
Ac e t y l CoA
at hi gh c on c e n t r a t i o n
Ci t r a t e
Ci t r a t e
Ac e t y l CoA Sy nt h as e
Ci t r a t e
+ATP
+CoA
f a t t y a c i d s yn t he
or dr ug m e t a b ol i
( N- a c e t y l a t i on )
ATP c i t r at e l y as e
Ox a l o a c e t a t e NADH
Mi t ochondr i al Mat r i x
ma l at e d e hy dr o ge n as e
Ox a l o a c e t a t e
NAD+
ADP
M al a t e
NADP+
ma l i c e n z y me
CO2
Py r uv a t e
Py r uv a t e
ATP
NADPH
CO2
f a t t y a c i d s yn t he s i s
or P4 50 c a t a l y z e d dr ug
m e t a bol i s m
Th e r e f or e m a l i c e nz ym e s u ppl i e s NADPH
Ci t r a t e Lya s e s up pl i e s a c e t yl CoA.
Pe nt o s e Pho s ph a t e Pa t hwa y
NADPH
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Isocitrate as an NADPH shuttle for drug metabolism
Gl uc os e
Py r u va t e
Ac e t yl CoA
CYTO SO L
M I TO CH O NDRI AL M ATRI X Ci t r a t e
Ox a l oa c e t a t e
NADH
M al at e
Fu m a r a t e
NAD+
CI TRI C
ACI D
CYCLE
I s oc i t r a t e
NAD+
NADH
Su c c i n a t e
i s oc i t r a t e
de hy dr og e n as e
CO 2
a- k e t og l u t a r a t e
NADH
I s oc i t r a t e
NADP+
i s oc i t r a t e
de hy dr og e n as e
NADPH
a- k e t og l u t a r a t e
NAD+
P4 50 c at al yz e d
DRUG M ETABO LI S M
Su c c i n yl
CoA
CO2
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d) Gluconeogenesis
and glucose export
by the liver !
3 irreversible steps
Major antidiabetic drug
METFORMIN
Inhibits gluconeogenesis
Decr Hepatic Glucose Synth.
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Glucagon 51aa & Insulin 29aa
• Pancreas synthesises both peptide hormones
• Insulin required for cells (e.g.liver,muscle,fat) to take up
glucose and synthesise glycogen.
• Glucagon hepatocyte receptors signals glycogenolysis
(glycogen breakdown to glucose then increases
gluconeogenesis pyruvate -- glucose)
• Drugs. Dipeptidyl peptidase-4 inhibitor (Januvia, new anti
type 2 diabetes) increases incretin , a GI hormonal peptide
inhibitor of glucagon which lowers plasma glucose.
• Metformin, antidiabetic drug inhibits gluconeogenesis but
also can inhibit mitoch.complex I causing lactic acidosis.
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