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 -amino groups removed

Deamination of amino acids

Deamination generates:

a carbon skeleton a free amino group

can be used for anabolic or catabolic reactions

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)

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

NAD(P)H

GDH

Glutamine can be deaminated in a two step process

glutamine + H2O glutamate + NH3

glutaminase

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)

glutamate + NH4+ + ATP glutamine +

ADP + Pi

GS

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

- ketoglutarate

oxaloaceatate

pyruvate

CAC

Addition of amino groups to these keto acids generates amino acids

Most amino acids are deaminated by donating their -amino acids to one of these keto acids

- ketoglutarate

oxaloacetate

pyruvate

glutamate

aspartate

alanine

Thus the deamination of most amino acids leads to the production of either glu, asp, ala or gln.

An example transamination

glutamate -KG -amino acid-keto acid

glutamate aminotransferase

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

Brain requires large amounts of ATP

This must be generated via oxidative phosphorylation

Therefore the CAC must function efficiently

glutamate + NH4+ + ATP glutamine +

ADP + Pi

The neurotransmitter GABA is inactivated by deamination

GS

GDH-ketoglutarate + NH4

+ + NADPH glutamate + NADP+ + H2O

However when ammonia concentrations are high:

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 deamina

se

Pyruvate is usually abundant in active muscle

Muscle uses pyruvate as an acceptor keto acid

glutamate + pyruvate -ketoglutarate + alanine

alanine aminotransferase

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