16' Amino acid catabolism: the urea cycle

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16. Amino acid catabolism the urea cycle
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The urea cycle
2 ATP 2 ADP Pi
HCO3-
mitochondria
carbamoyl phosphate
NH4
citrulline
Pi
ornithine
H2N-CH2CH2CH2CH-CO2-
Asp
cytosol
ATP
AMP PPi
urea
H2O
argininosuccinate
arginine
fumarate
-O2C-CHCH-CO2-
3
Incorporation of ammonia into urea begins with
formation of carbamoyl phosphate
4
Carbamoyl phosphate reacts with ornithine to form
citrulline
ornithine
carbamoyl phosphate
Pi
H
citrulline
This step also occurs in the mitochondrial matrix.
5
Citrulline combines with aspartate to form
argininosuccinate
ATP
AMP PPi H2O
This reaction occurs only in the cytosol, so
citrulline first must leave the mitochondria. A
transporter exchanges ornithine for citrulline
plus a proton across the mitochondrial inner
membrane.
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Argininosuccinate splits into arginine and
fumarate
argininosuccinate
-O2C-CHCH-CO2-
fumarate
arginine
This reaction occurs in the cytosol.
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Hydrolysis of arginine releases urea and
regenerates ornithine
arginine
H2O
ornithine
H
urea
This reaction occurs in the cytosol. To continue
the cycle, ornithine must enter a mitochondrion.
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Formation of urea consumes 4 phosphate anhydride
bonds
HCO3-
2 ATP 2 ADP Pi
carbamoyl phosphate
NH4
citrulline
Pi
ornithine
ATP
2 Pi
PPi AMP
Asp
H2O
urea
argininosuccinate
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Aspartate used in the urea cycle can be
regenerated from fumarate that is produced
2 ATP 2 ADP Pi
CO2 NH4
carbamoyl phosphate
?-keto acids amino acids
Pi
aspartate-oxaloacetate aminotransferase
ornithine
citrulline
oxaloacetate
Urea cycle
ATP
urea
aspartate
AMP PPi
malate dehydrogenase
arginine
argininosuccinate
NADH
malate
NAD
fumarate
H2O
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Oxidation of malate in mitochondria generates ATP
2 e- to O2 via NADH dehydrogenase generates 2.5
ATP
2 ATP 2 ADP Pi
CO2 NH4
carbamoyl phosphate
oxaloacetate
mitochondrion
Pi
glutamate
aspartate
malate
ornithine
citrulline
?-ketoglutarate
?-ketoglutarate
citrulline
glutamate
ornithine
ATP
aspartate
urea
AMP PPi
amino acids a-ketoacids
arginine
argininosuccinate
malate
cytosol
H2O
fumarate
NADH, NAD and oxaloacetate cant cross the
mitochondrial inner membrane, but there are
transporters for malate, aspartate, glutamate and
a-ketoglutarate.
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Transport systems in the mitochondrial inner
membrane exchange aspartate for glutamate and
a-ketoglutarate for malate
mitochondrion
malate
glutamate H
aspartate
?-ketoglutarate
cytosol
Because the glutamate/aspartate transporter also
moves a proton across the membrane, it can be
driven by the electrochemical potential gradient.
Mutations in this transporter have been linked to
autism.
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????a-ketoglutarate/malate and aspartate/glutamate
transporters also participate in oxidation of
cytosolic NADH
2 e- to electron-transport chain
mitochondrion
NAD
NADH
oxaloacetate
malate
aspartate
?-ketoglutarate
glutamate
oxaloacetate
NAD
cytosol
NADH
glycolysis
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The urea cycle is regulated in two ways
1. Allosteric activation of carbamoylphosphate
synthetase by N-acetylglutamate
acetyl-CoA
N-acetylglutamate
CoA-SH
Glu
In mammals, N-acetylGlu appears to play only a
regulatory role. Carbamoyl-phosphate synthetase
is completely inactive in its absence. A genetic
deficiency in the enzyme that forms N-acetylGlu
can cause a lethal defect in the urea cycle.
2. A high-protein diet or starvation leads to
increased synthesis of all five enzymes used in
the urea cycle, including carbamoylphosphate
synthetase. Expression of the enzyme that
synthesizes N-acetylglutamate also increases.
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Inherited disorders of the urea cycle can have
severe effects
Approximate incidence per 100,000 births
Patients with these conditions cant be treated
by simply removing proteins from their diet
because humans are not able to synthesize all the
necessary amino acids.
Lehninger, Table 18-2
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Genetic deficiencies in some of the urea-cycle
enzymes can be treated pharmacologically













































phenylacetate
benzoate
benzoyl-CoA
phenylacetyl-CoA
phenylacetyl-glutamine
hippurate (benzoylglycine)
O
The amide products of these reactions (hippurate
and phenylacetylglutamine) are excreted in the
urine. Replenishing the Gly or Gln removes
ammonia.





16
Arginine also serves as a precursor of nitric
oxide (NO)
arginine
NADPH, O2
NADP, H2O
The enzyme nitric oxide synthase, which catalyzes
both steps, has four bound cofactors FMN, FAD,
heme and tetrahydrobiopterin
Hydroxyarginine (bound to enzyme)
1/2 NADPH, O2
NO
1/2 NADP, H2O
citrulline
NO acts as a short-lived messenger in control of
blood pressure, blood clotting, and
neurotransmission. It binds to a guanylyl
cyclase and activates production of c-GMP
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The carbon chains of the common amino acids
provide materials that feed into the citric acid
cycle
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Phenylalanine and tyrosine break down to fumarate
and acetoacetate
O2 NADH H2O NAD
phenylalanine hydroxylase
CO2 O2
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I wont expect you to know the details of
reactions 3-6.