L18 - Amino acid metabolism II

Question 1

Transport of nitrogen to the liver: why is it necessary?

Answer 1

Nitrogen in amino groups must be converted to urea - the liver is the main site of urea synthesis, other tissues can oxidise amino acids but cannot synthesise urea

There must be a mechanism to transport amino groups (ammonia) to the liver where they can be converted to urea and excreted

Question 2

Ammonia transport to the liver: what mechanisms are there for it?

Answer 2

• Glutamate synthase can combine glutamate with ammonia in an ATP-dependent reaction - first line of defence within the brain to prevent toxic ammonia buildup - this can then be transferred to the liver for urea production
• Transamination of pyruvate to alanine, transferring ammonia to the less toxic form of alanine - mainly undergone by muscle tissue - alanine then moves to the liver

Both methods end up with urea production through deamination by glutamate dehydrogenase as well as glucose production from the carbon skeleton

Question 3

Amino acid metabolism regulation

Answer 3

GDH - stimulated by ADP/GDP, inhibited by ATP/GTP

Question 4

Amino acid biosynthesis: where do each of the groups come from?

Answer 4

Carbon skeleton - intermediates of glycolysis, pentose phosphate pathway, and TCA cycle

Amino group - Gln/Glu which obtain their nitrogen from ammonia

Question 5

The 20 essential amino acids: how many are produced within humans, which organisms produce all of them, and why is this the case?

Answer 5

9-11 (two are complicated essential amino acids) - essential amino acids must be supplied in the diet

Plants and bacteria synthesize all twenty amino acids

In general, the more complex amino acids are essential amino acids - require enzymes that have been lost from the human genome over evolution - possible speculatory answer: we could gather it from the diet so we lost the taxing, complicated, high resource-requiring pathways to produce these amino acids when they can instead be simply supplied from the diet?

Question 6

Essential amino acids: what are they and what are the complicated ones?

Answer 6

• Isoleucine
• Leucine
• Valine
• Tryptophan
• Threonine
• Methionine
• Phenylalanine
• Histidine
• Lysine

Arginine - produced by the urea cycle but also needed in high amounts during pregnancy and childhood (periods of rapid growth) so must be obtained from the diet

Tyrosine - made from essential amino acid (phenylalanine) so is techniqually an essential amino acid

Question 7

Non-essential amino acids: what are they and what are the complicated ones?

Answer 7

• Alanine
• Proline
• Glycine
• Glutamate
• Glutamine
• Asparagine
• Aspartate
• Cysteine
• Serine
• Tyrosine

Arginine - produced by the urea cycle but also needed in high amounts during pregnancy and childhood (periods of rapid growth) so must be obtained from the diet

Tyrosine - made from essential amino acid (phenylalanine) so is techniqually an essential amino acid

Question 8

How are amino acids grouped: what?

Answer 8

Families according to the intermediates that they are made from

• alpha-ketoglutarate
• 3-phosphoglycerate
• Oxaloacetate
• Pyruvate
• Phosphoenolpyruvate and erythrose-4-phosphate
• Ribose-5-phosphate

For example, alanine, valine, isoleucine, and leucine are produced from pyruvate

Question 9

Amino acid biosynthesis: how is it regulated?

Answer 9

Feedback regulation from the amino acid to the first enzyme in the pathway

Question 10

Amino acids as metabolic precursors: what can they also be synthesised to?

Answer 10

• Hormones
• Heme groups (hemoglobin and cytochromes)
• Coenzymes
• Nucleotides
• Alkaloids
• Cell wall polymers
• Porphyrins
• Antibiotics
• Pigments
• Neurotransmitters