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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.

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slide2

Nitrogen Metabolism

Many nitrogen containing compounds

eg. Amino acids, nucleotides, porphyrins, neurotransmitters

There is no dedicated store for nitrogen or nitrogen compounds in humans

slide3

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

slide4

Nitrogen intake = nitrogen excretion

Dietary amino acids, nucleotides etc.

Urine, faeces, hair and skin loss, perspiration

1) In balance

slide5

2) Positive

Nitrogen intake > nitrogen excretion

Possible causes:

Childhood and adolescent growth

Pregnancy

Body building

slide6

3) Negative

Nitrogen intake < nitrogen excretion

Possible causes:

Illness

Starvation

Post-surgery

slide8

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
slide9

Deamination of amino acids

Deamination generates:

a carbon skeleton a free amino group

can be used for anabolic or catabolic reactions

generally excreted

slide10

GDH

glutamate + NAD(P)+ + H2O a-ketoglutarate + NH4+ + NAD(P)H

Some amino acids can be directly deaminated

Serine, threonine and glutamate can be directly deaminated

Glutamate deamination is catalysed by glutamate dehydrogenase (GDH)

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Glutamine can be deaminated in a two step process

glutaminase

glutamine + H2O glutamate + NH3

Glutamate is then deaminated by GDH

slide12

GS

glutamate + NH4+ + ATP glutamine + ADP + Pi

Glutamine can also be synthesised from glutamate

Glutamine synthesis is an energy requiring reaction

The reaction is catalysed by glutamine synthetase (GS)

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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

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Addition of amino groups to these keto acids generates amino acids

glutamate

aspartate

alanine

a - ketoglutarate

oxaloacetate

pyruvate

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.

slide15

glutamate

a-keto acid

a-KG

a-amino acid

glutamate aminotransferase

An example transamination

slide16

Pyridoxal

phosphate

Derived from vitamin B6

Takes part in all amino transferase reactions

Forms a Schiff base intermediate with substrates

slide18

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

slide19

Free ammonia is a by-product of brain metabolism

The neurotransmitter GABA is inactivated by deamination

GS

glutamate + NH4+ + ATP 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
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ATP ADP + Pi

2ADP ATP + AMP

AMP IMP + NH4+

AMP deaminase

Free ammonia is also produced in muscle

  • Amino groups can be liberated:
  • during normal muscle turnover
  • during starvation
  • during severe muscle activity
slide22

alanine aminotransferase

glutamate + pyruvate a-ketoglutarate + alanine

  • Pyruvate is usually abundant in active muscle
  • Muscle uses pyruvate as an acceptor keto acid
  • 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
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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