Nitrogen

Question 1

State the main nitrogen-containing molecules of the body

Answer 1

proteins, nucleic acids, nucleotides, biologically active amines, haem-containing molecules

Question 2

Describe the fate of dietary proteins

Answer 2

dietary protein is digested to produce amino acids for use in anabolic pathways

dietary proteins are enzymatically hydrolysed

Question 3

Describe the flow of nitrogen through the body

Answer 3

digestion of dietary protein

‘turn-over’ of body protein

excretion of nitrogen as urea

maintenance of a pool of intra- and extra-cellular free amino acids

Question 4

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

Answer 4

Nitrogen is transferred to, from and between amino-acids by oxidative deamination and transamination (aminotransfer) reactions involving glutamate

Glutamate is the only amino- acid that can obtain its nitrogen directly from ammonium ions

Question 5

Describe the catabolism of body protein

Answer 5

gastric glands in stomach lining - pepsin cuts protein into peptides in the stomach

Trypsin and chymotrypsin cut proteins and larger peptides into smaller peptides in the small intestines

amino peptidase and carboxypeptidase A and B degrade peptide into amino acids in the small intestine

villus, intestinal mucosa absorb amino acids

Question 6

Describe how nitrogen is transported through plasma to the liver

Answer 6

Nitrogen from turned-over body protein is transported through plasma to liver as glutamine, or, from skeletal muscle, as alanine

Transport of potentially toxic ammonia in large amounts is avoided

Question 7

Describe the formation of urea

Answer 7

is a metabolite (breakdown product) of amino acids

when the liver breaks down protein or amino acids, and ammonia

Excess ammonium ions are converted to urea in the urea cycle

Question 8

Describe the fate of the carbon of catabolised body protein

Answer 8

Carbon of turned- over amino-acids may be used to form intermediates of glycolysis, gluconeogenesis, citric acid cycle or lipid metabolism pathways

Carbon of amino-acids metabolised to pyruvate or citric acid cycle intermediates may become carbons of glucose: such amino-acids are called ‘glucogenic’

Amino-acids providing acetyl Co A or acetoacetate carbon atoms are called ‘ketogenic’

Question 9

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

Answer 9

Rare group of inherited metabolic disorders - 6 inherited disorders of the urea cycle
- most common = omithine transcarbamcylase
(OTC) deficiency -1 in 40000 births in UK

OTC has x-linked inheritance, rest are autosomal recessive

characterised by hyperammonaemia (elevated blood ammonia levels) - highly toxic - medical emergency

typically present in newborn babies

Question 10

Describe the Urea Cycle

Answer 10

cycle is split between mitochondria and cytosol, 5 enzymes catalyse cycle in the liver and their concentrations increase or decrease in response to high or low protein diets

CP51 is regulatory enzyme -(carbamoyl phosphate synthase) - Acetyl-glutamate is allosteric activator of CP51

Question 11

Describe Phenylketonuria

Answer 11

absence/ deficiency of phenylalanine hydroxylase (PAH) - classical PKU

autosomal recessive disorder

associated with increrased phenylalanine (PHe) levels -toxic - due to no breakdown by PAH

untreated individuals exhibit signs of impaired brain development

treatable condition - reduced protein diet supplemented with tyrosine