How the Body Handles Nitrogen

Question 1

State the main nitrogen-containing molecules of the body.

Answer 1

1. Amino acids/proteins
2. Nucleic acids
3. Nucleotides
4. Amines
5. Haem-containing molecules.

Question 2

Describe the fate of dietary protein?

Answer 2

The plants/animals we eat get their nitrogen from nitrogen-fixing bacteria called diazotrophs. Nitrogen fixation is the process of converting nitrogen into a biologically usable form such as ammonia. This occurs anaerobically. After this, N undergoes nitrification to convert it to nitrate a usuable form.

Dietary proteins are enzymatically hydrolyzed. In the stomach pepsin cuts the protein into peptides. In the small intestine trypsin cuts it into smaller peptides. Chymotrypsin does the same. Aminopeptidases and carboxypeptidases degrade peptides into amino acids.

Question 3

Describe the role of glutamate in the transfer of nitrogen to, from and between amino-acids.

Answer 3

The flow of nitrogen from ammonia to other biomolecules occurs through glutamate in a process called transamination. Most organs can’t fix nitrogen so they conserve it by transferring amino groups. Glutamate acts as a temporary storage molecule for nitrogen and can donate the amino group as required for amino acid synthesis.

Question 4

Describe the catabolism of body protein.

Answer 4

Proteins are broken down during exercise, starvation or diabetes if required. During anaerobic glycolysis pyruvate is produced which can’t be metabolised aerobically so lactic acid builds up. Glutamate and the enzyme alanine aminotransferase can help convert this pyruvate to alanine for transport to the liver where glucose can be re-synthesised.

Question 5

Describe how nitrogen is transported through plasma to the liver.

Answer 5

Nitrogen is transported through the blood to the liver in the form of glutamine or alanine. This is because they have no charge unlike glutamate which is negatively charged.

Question 6

Decsribe the formation of urea.

Answer 6

The urea cycle converts excess ammonia into urea in the mitochodria of liver cells. The urea then enters the blood stream, is filtered by the kidneys and excreted in urine. See notes for diagram and equations.

Question 7

Describe the fate of the carbon of catabolised body protein.

Answer 7

1. Glugogenic amino acids. These amino acids can be converted to glucose through gluconeogenesis in which the carbon skeleton is used.
2. Ketogenic amino acids. These can be degraded directly into acetyl CoA releasing ketone bodies formed from the carbon skeleton.

Question 8

Describe examples of how metabolic defects in the urea cycle give rise to clinical problems.

Answer 8

Wherever there is an enzyme there can be a disorder. 6 inherited disorders of the urea cycle.
E.g. ornithine transcarbamoylase (OTC) deficiency. Most common, X-linked inheritance (rest are autosomal recessive), characterised by hyperammonaemia which is highly toxic. A medical emergency as if blood passes the blood brain barrier it can cause permanent brain damage.
Or if the concentration of an amino acid is too low the urea cycle can’t turn leading to a build up of ammonia.

Amino acid disorders: inherited gene defects cause decreased enzyme activity so there is a decrease in the product and increase in the precursors which can lead to other metabolic products. E.g. phenylketonuria (PKU) an absence/deficiency in phenylalanine hydroxylase which can cause a build up of phenylalanine levels which untreated can cause impaired brain development. It’s autosomal recessive and treated with a low protein diet supplememted with Tyrosine.