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BIOC/DENT/PHCY 230
LECTURE 4
Nitrogen Metabolism
Many nitrogen containing compounds
eg. Amino acids, nucleotides, porphyrins,
neurotransmitters
There is no dedicated store for nitrogen or
nitrogen compounds in humans
Nitrogen Balance
An individual’s nitrogen balance is
dependent on a combination of:
1) Dietary nitrogen intake
2) Physiological state
Nitrogen balance status can be:
1) In balance
2) Positive
3) Negative
1) In balance
Nitrogen intake = nitrogen excretion
Dietary amino acids,
nucleotides etc.
Urine, faeces, hair and
skin loss, perspiration
2) Positive
Nitrogen intake > nitrogen excretion
Possible causes:
Childhood and adolescent growth
Pregnancy
Body building
3) Negative
Nitrogen intake < nitrogen excretion
Possible causes:
Illness
Starvation
Post-surgery
Amino acids are the major source of dietary N
Excess or insufficient dietary amino acid
intake leads to the catabolism of amino acids
 Excess amino acids can be used for energy
 Insufficient dietary amino acids lead to the
catabolism of proteins
 Insufficient dietary energy leads to the
catabolism of proteins
 For amino acids to be utilised for energy, they
must have their a-amino groups removed
Deamination of amino acids
Deamination generates:
a carbon skeleton
can be used for anabolic
or catabolic reactions
a free amino group
generally excreted
Some amino acids can be directly
deaminated
Serine, threonine and glutamate can be directly
deaminated
Glutamate deamination is catalysed by
glutamate dehydrogenase (GDH)
GDH
glutamate + NAD(P)+ + H2O
a-ketoglutarate + NH4+ + NAD(P)H
Glutamine can be deaminated in a
two step process
glutaminase
glutamine + H2O
glutamate + NH3
Glutamate is then deaminated by GDH
Glutamine can also be synthesised
from glutamate
Glutamine synthesis is an energy requiring reaction
The reaction is catalysed by glutamine synthetase
(GS)
GS
glutamate + NH4+ + ATP
glutamine + ADP + Pi
Transamination
Those amino acids that can not be directly
deaminated have their amino groups transferred
to specific substrates
These substrates are keto acids found in
intermediary metabolism
a - ketoglutarate
oxaloaceatate
pyruvate
CAC
Addition of amino groups to these keto acids generates
amino acids
a - ketoglutarate
glutamate
oxaloacetate
aspartate
pyruvate
alanine
Most amino acids are deaminated by donating their
a-amino acids to one of these keto acids
Thus the deamination of most amino acids leads to the
production of either glu, asp, ala or gln.
An example transamination
glutamate a-keto acid
a-KG
glutamate aminotransferase
a-amino acid
Pyridoxal
phosphate
Derived from vitamin B6
Takes part in all amino
transferase reactions
Forms a Schiff base
intermediate with
substrates
Role of transamination in metabolism
Transamination allows for:
1) the generation of amino acids in short supply
2) the provision of carbon skeletons for energy
generation
3) the safe removal of excess amino groups
Free ammonia is a by-product of brain metabolism
The neurotransmitter GABA is inactivated by deamination
glutamate + NH4+ + ATP
GS
glutamine + ADP + Pi
However when ammonia concentrations are high:
GDH
a-ketoglutarate + NH4+ + NADPH
glutamate + NADP+ + H2O
 Brain requires large amounts of ATP
 This must be generated via oxidative phosphorylation
 Therefore the CAC must function efficiently
Free ammonia is also produced in muscle
Amino groups can be liberated:
 during normal muscle turnover
 during starvation
 during severe muscle activity
ATP
ADP + Pi
2ADP
ATP + AMP
AMP
IMP + NH4+
AMP
deaminase
 Pyruvate is usually abundant in active muscle
 Muscle uses pyruvate as an acceptor keto acid
alanine
aminotransferase
glutamate + pyruvate
a-ketoglutarate + alanine
 Thus in muscle most amino groups are shuttled to
alanine (via glutamate)
 Alanine is then exported to the liver where the amino
groups can be liberated
AMP
The take home message
 Nitrogen balance status depends on the intake and
use of N containing compounds
 Excess N from amino acids must be excreted
 A series of aminotransferase and deamination
reactions shuttle nitrogen to appropriate molecules and
tissues
 Brain and muscle can generate large amounts of
excess nitrogen as part of their metabolism
 The liver is an important tissue for processing
excess nitrogen