Download Amino Acid Oxidation and the Urea Cycle

Amino Acid Oxidation
and the Urea Cycle
Amino Acids:
• Final class of biomolecules whose oxidation
contributes significantly to the generation of
energy
• Undergo oxidation in three metabolic
circumstances in animals:
During normal synthesis and degradation of proteins
When a diet is rich in protein; amino acids are not
stored
During starvation or in diabetes mellitus
Overview of Amino Acid Catabolism:
Dietary
protein
Intracellular
protein
Glucose
TCA
Cycle
Respiration
α-keto acid
Amino
Acids
NH44++
Biosynthesis
of amino acids
nucleotides,
biological
amines
Excretion
Metabolic Fates of Amino Groups
Most amino acids are metabolized in the
liver
• Some of the ammonia generated is used in
biosynthesis; the excess is excreted
• Excess ammonia generated in extrahepatic
tissues is transported to liver for conversion
to the appropriate excreted form
Removal
Removal of
of the
the α-amino
α-amino groups
groups occurs
occurs in
in
the
the cytosol
cytosol by
by transamination
transamination reactions
reactions
catalyzed
catalyzed by
by aminotransferases
aminotransferases
(transaminases):
(transaminases):
α-keto
amino acid
acid11
α-keto acid
acid22 ++ amino
α-keto
α-keto acid
acid11 ++ amino
amino acid
acid22
In
In liver
liver α-keto
α-keto acid
acid11 is
is usually
usually α-Kg;
α-Kg; in
in muscle
muscle
it
it is
is usually
usually pyruvate
pyruvate
COO--
COO-COO--
C=O
+
CH22
CH22
COO-α-Kg
--
COO
C=O
CH22
CH22
COO-α-Kg
+
++
33
NH
H33N++ C H
R
amino acid
COO-H33N++ C H
CH33
alanine
C-H
CH22
+
CH22
COO-glutamate
GPT
alanine
alanine
aminoamino-
COO-NH33++ C-H
CH22
+
CH22
transferase
transferase
COO-glutamate
COO-C=O
R
α-keto
acid
COO-C=O
CH33
pyruvate
COO-C=O
CH22
CH22
COO-α-Kg
--
+
COO
H33N++ C H
CH22
COO-aspartate
COO-NH33++ C-H
CH22
+
CH22
COO--
COO-C=O
CH22
COO-OAA
glutamate
Aspartate aminotransferase or Glutamate-OAA
transaminase (GOT)
Serum
Serum GPT
GPT (SGPT)
(SGPT) and
and GOT
GOT (SGOT)
(SGOT) are
are
sensitive
sensitive indicators
indicators for
for aa number
number of
of disease
disease
conditions.
conditions.
• During heart attacks, damaged heart cells
leak aminotransferases.
• Damaged liver cells also leak
aminotransferases. SGPT and SGOT levels
are monitored in people exposed to
industrial chemicals.
The
The effect
effect of
of transamination
transamination is
is to
to collect
collect
amino
amino groups
groups from
from many
many amino
amino acids
acids and
and
convert
glutamate
convert them
them into
into one,
one, glutamate
• Glutamate channels amino groups into
biosynthetic pathways or into reactions
where nitrogenous waste products are
formed
How
How are
are amino
amino groups
groups removed
removed from
from
glutamate
glutamate and
and prepared
prepared for
for excretion?
excretion?
Glutamate is transported into the mitochondrial matrix
where it undergoes oxidative deamination catalyzed by
glutamate d’hase:
Glutamate
glutamate d’hase
NAD(P)+
NAD(P)H
α-Kg + NH44
Ammonia
Ammonia is
is extremely
extremely toxic
toxic to
to animal
animal
tissues;
tissues; it
it is
is converted
converted to
to glutamine
glutamine for
for
transport
transport from
from extrahepatic
extrahepatic tissues
tissues to
to the
the
liver
liver or
or kidneys.
kidneys.
COO--
COO-NH33++ C-H
CH22
CH22
COO-glutamate
glutamine synthetase
NH44
NH33++ C-H
CH22
CH22
H22N C=O
glutamine
In
In the
the liver
liver glutamine
glutamine is
is converted
converted back
back to
to
glutamate
glutaminase:
glutamate by
by glutaminase:
COO-NH33++ C-H
CH22
CH22
H22N C=O
COO-glutaminase
H22O
NH44++
glutamine
UREA
NH33++ C-H
CH22
CH22
COO-glutamate
• Glutamine is the major transport form of ammonia; it
is present in blood in much higher concentrations
than other amino acids.
• Alanine also plays a role in transport of amino groups
to the liver by the glucose-alanine cycle:
Muscle
AA
glutamate
Liver
glucose
pyr + glutamate
glutamate
pyr
alanine
+ α−Kg
α−Kg
alanine
Habitat
Habitat determines
determines the
the Molecular
Molecular Pathway
Pathway for
for
Nitrogen
Nitrogen Excretion
Excretion
• Aquatic organisms (bacteria, protozoa, fish) release
ammonia to their aqueous enviroment (ammonotelic)
• Birds and reptiles convert amino nitrogen into uric acid;
they cannot carry enough water for the excretion of
nitrogen as urea (uricotelic)
• Terrestrial animals excrete amino nitrogen in the form
of urea (ureotelic)
• Plants recycle virtually all amino groups; there is no general
pathway for nitrogen excretion
The Urea Cycle
• Discovered by Hans Krebs and Kurt
Hanseleit
• Occurs in the liver
• Takes place in two intracellular
compartments; the cytosol and the
mitochondrial matrix
++
44
NH
+
carbamoyl phosphate
HCO33-synthetase I
2 ATP
2 ADP,
Pi
Pi
NH33++
O
H22N-C-NH-(CH22)33-CH-COO--
O
O
H22N-C-O-P-O-O-carbamoyl phosphate
ornithine
ornithine
transcarbamoylase
transcarbamoylase
NH33++
NH33-(CH22)33-CH-COO-Ornithine
Citrulline
Transported to cytosol
NH33++
O
H22N-C-NH-(CH22)33-CH-COO-Citrulline
Aspartate
arginosuccinate
synthetase
ATP
ATP
PPi
PPi
NH33++
COO-- NH22++
-OOC-CH22-CH-NH-C-NH-(CH22)33-CH-COO-Arginosuccinate
NH33++
COO-- NH22++
-OOC-CH22-CH-NH-C-NH-(CH22)33-CH-COO-Arginosuccinate
arginosuccinate lyase
OOC-CH=CH-COO--
--
Fumarate
NH33++
NH22++
-)
-CH-COO
NH2-C-NH-(CH
22 33
2
Arginine
NH33++
NH22++
-)
-CH-COO
NH2-C-NH-(CH
22 33
2
Arginine
arginase
NH33++
NH33-(CH22)33-CH-COO-Ornithine
O
H22N-C-NH22
UREA
Back to mitochondrion
Overall equation for Urea synthesis:
NH33 + HCO33-- + Aspartate + 3 ATP
urea + fumarate + 2ADP + 2Pi + AMP + PPi
Regulation of the Urea Cycle:
• Carbamoyl Phosphate Synthetase I is
allosterically activated by
N-acetylglutamate.
• High levels of transamination during amino
acid breakdown lead to elevated glutamate
with concommitant increases in the
concentration of N-acetylglutamate .
Breakdown of Individual Amino Acids
Degradation of the carbon skeletons of the
20 common amino acids yields one of 7
intermediates: a-Kg, succinyl CoA, pyruvate,
fumarate, OAA, acetoacetate, acetyl CoA
α-Kg, succinyl CoA, pyruvate, fumarate,
OAA can all serve as precursors for glucose
synthesis; hence amino acids giving rise to
these intermediates are Glucogenic.
Acetoacetate and acetyl CoA can serve as
precursors for fatty acid or ketone
synthesis; hence amino acids giving rise to
these compounds are termed Ketogenic
Alanine, Serine, Glycine,
Cysteine, Threonine,
Tryptophan
Pyruvate
Isoleucine, Leucine,
Threonine, Tryptophan
Acetyl CoA
Leucine, Lysine,
Phenylalanine,
Tyrosine
Acetoacetate
Arginine, Glutamine,
Glutamate, Proline, Histidine
Isoleucine, Methionine,
Valine
Aspartate, Tyrosine,
Phenylalanine
Aspartate,
Asparagine
α−Kg
Succinyl
CoA
Fumarate
OAA
Threonine
H
CH22 C-COO-NH22
NH
Tryptophan
H
HS-CH22C-COO-NH22
Alanine
H
Cysteine
CH33 C-COO-NH22
O
CH33-C-COO-Pyruvate
HH
H33C-C-C-COO-HO NH22
H
Glycine H C-COO-NH22
H
HOCH22 C-COO-Serine
NH22
• Alanine is converted to pyruvate by transamination.
• Asparagine is converted to Aspartate by Asparaginase:
H
C-CH22 C-COO-NH22
NH22
O
H
C-CH22 C-COO--O
NH22
O
• Aspartate can be converted to OAA by transamination:
Aspartate + α-KG
• Aspartate degradation via the
Glutamate + OAA
urea cycle yields fumarate.
Methionine, Valine, Isoleucine
O
CH33-CH22-C-S-CoA
Propionyl CoA
O
-OOC-CH22-CH22-C-S-CoA
Succinyl CoA
Arginine, Glutamine, Histidine, Proline
Glutamate
Glutamate D’hase
α−Kg
Leucine
Leucine and
and Lysine
Lysine are
are the
the only
only two
two purely
purely
ketogenic
ketogenic amino
amino acids.
acids. HMG-CoA
HMG-CoA is
is an
an
intermediate
intermediate in
in leucine
leucine degradation.
degradation.
• Initial steps in valine, leucine, isoleucine
degradation are identical:
transamination to the corresponding a-keto acid
decarboxylation to the corresponding CoA derivative
dehydrogenation to form a double bond
Defect in the decarboxylation reaction results
in Maple Syrup Urine Disease, which is
fatal unless treated early in life.
Phenylalanine is converted to Tyrosine by
Phenylalanine Hydroxylase; then the pathway
proceeds with the breakdown of tyrosine to fumarate
plus acetoacetate. Homogentisate is an intermediate
in this pathway.
Alkaptonuria results in the urinary excretion of
excess homogentisate; air oxidation causes this
compound to turn dark. This disease is not fatal,
individuals tend to suffer arthritis in later life.
OH
-OOC-H22C
homogentisate
OH
Phenylketonuria results from absence of
phenylalanine hydroxylase. Phe is converted to
phenylpyruvate and excreted. Severe mental
retardation occurs unless infants are immediately
placed on a diet low in Phe.
CH22
C=O
COO-
phenylpyruvate
Human
Human Genetic
Genetic Disorders
Disorders of
of Amino
Amino Acid
Acid
Catabolism
Catabolism
Incidence
Condition (per
(per 100,000
100,000 births)
births) Defective Enzyme Symptoms
Dark
Dark pigment
pigment in
in urine;
urine;
arthritis
arthritis in
in late
late life
life
Alkaptonuria
Alkaptonuria
0.4
0.4
Homogentisate
Homogentisate
Dioxygenase
Dioxygenase
Maple
Maple Syrup
Syrup Urine
Urine
Disease
Disease
0.4
0.4
Branched
Mental retardation;
retardation;
Branched chain
chain α-keto
α-keto Mental
acid
convulsions; early
early
acid dedydrogenase
dedydrogenase convulsions;
death
death
Phenylketonuria
Phenylketonuria
Methylmalonic
Methylmalonic
Acidemia
Acidemia (MMA)
(MMA)
88
(<0.5)
(<0.5)
phenylalanine
phenylalanine
hydroxylase
hydroxylase
Neonatal
Neonatal vomiting;
vomiting;
mental
mental retardation
retardation
Methylmalonyl
Methylmalonyl CoA
CoA
Mutase
Mutase
Mental
Mental retardation;
retardation;
convulsions;
convulsions; early
early
death
death