TRANSAMINATION

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TRANSAMINATION
In addition to being the basis of protein
synthesis, amino acids form an important source
of energy for the body.
Animals do not have storage proteins in which to
lock away excess amino acids. Therefore

• Excess dietary amino acids must be metabolised.

• Amino acids resulting from protein turnover must
  be metabolised.

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TRANSAMINATION
Amino acids are metabolised to organic acids by
removal of the amino group by TRANSAMINATION and
DEAMINATION
The liver plays a major role in these processes.
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TRANSAMINATION
Transamination involves transfer of an amino
group to an organic acid by enzymes called
transaminases.
Glutamate-oxaloacetate aminotransferase
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TRANSAMINATION
Removal of the amino group from many amino acids
produces organic acids that can be used as an
energy source for the citric acid (TCA) cycle.
Alanine, cysteine, glycine and serine can be
converted to pyruvate, which can then be
converted to glucose via gluconeogenesis.
Most frequently ketoglutarate is used as the
acceptor for an amino group, resulting in
synthesis of glutamate.
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AMINO ACID CATABOLISM FEEDS CARBOHYDRATE
METABOLISM
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TRANSFER OF NITROGEN FROM MUSCLES TO THE LIVER
In skeletal muscle cells, alanine
aminotransferase transfers amino group to
pyruvate to form alanine
pyruvate a-amino acid keto organic acid
alanine
Alanine is placed into the blood by muscle cells.
It is removed from the blood by liver cells.
In liver cells alanine aminotransferase then
transaminates alanine to regenerate pyruvate,
which can be used in gluconeogenesis to make
glucose.
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DEAMINATION

In the liver, amino acids can be directly or
indirectly be transaminated to form glutamate.
Glutamate undergoes reversible OXIDATIVE
DEAMINATION in liver cell mitochondria, producing
ammonia.
NAD
NADH H
COO
H2N CH CH2
CH2 COO
COO
C O CH2
CH2
COO

Glutamate H2O a-ketoglutarate NH3
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DEAMINATION
Ammonia cannot be allowed to accumulate in
mitochondria of liver cells. Ammonia combines
with a-ketoglutarate to form glutamate, depriving
the TCA cycle of a-ketoglutarate.
Glutamate H2O a-ketoglutarate NH3
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THE UREA CYCLE (ORNITHINE CYCLE)
A series of reactions that is catalysed by
enzymes partially in the cytosol and partially in
the mitochondria of liver cells.
The urea cycle converts the alkaline, toxic,
reactive nitrogenous waste ammonia into the
neutral, much less toxic and unreactive urea.
The liver excretes urea into the bloodstream from
where it is removed from the body by the kidneys.
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THE UREA CYCLE
-NH3 from deamination
NH2 (CH2)3 CH2 COO
NH3
ornithine
O COO
H2N C NH
(CH2)3 CH NH3
NH2 C O NH2
urea
citrulline
mitochondrion
NH2
H2N CH NH
(CH2)3
CH COO NH3
aspartate
COO CH
CH2 COO NH
NH3
H2N C NH
(CH2)3 CH2 CH COO
arginine
-NH2 from transamination
arginosuccinate
fumarate
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THE UREA CYCLE combining ammonia and carbon
dioxide.
Oxidative deamination of glutamate in hepatocyte
mitochondria produces ammonia.
Glutamate H2O a-ketoglutarate NH3
This ammonia is combined with carbon dioxide to
form carbamoyl phosphate.
NH2 C O O
PO Carbamoyl phosphate
NH3 CO2
2ATP
2ADP Pi
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THE UREA CYCLE synthesis of citrulline.
NH2 (CH2)3
CH2 COO NH3 Ornithine
ornithine transcarbamoylase
NH2 C O O
PO Carbamoyl phosphate

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THE UREA CYCLE conversion of citrulline to
arginosuccinate.
NH2 COO C NH
CH CH2 COO NH
(CH2)3 CH COO
NH3 Arginosuccinate
arginosuccinate synthetase
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THE UREA CYCLE conversion of arginosuccinate to
arginine.
CO O CH CH
COO Fumarate
arginosuccinate lyase
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THE UREA CYCLE production of urea and
regeneration of ornithine.
arginase