12 - Biochemistry amino acid metabolism

Question 1

what happens to amino acids that are not used as building blocks?

Answer 1

degraded

Question 2

what is the main site of amino acid degradation?

Answer 2

liver

Question 3

what are the 3 steps in synthesis of urea?

Answer 3

1. transamination
2. deamination/urea cycle
   3.

Question 4

where is urea formed?

Answer 4

liver

Question 5

what are major nitrogen containing excretory cycle?

Answer 5

• urea
  -uric acid
• creatinine
• ammonium ion

Question 6

what happens in transamination?

Answer 6

• Aminotransferases move the amino group from alpha-amino acids to alpha-keto acids
• usually alpha-ketoglutarate, a TCA intermediate
• gives glutamate
• occurs in all tissues

Question 7

what happens in transamination for transfer to liver?

Answer 7

• For transport to the liver:
• amino group of glutamate is transferred to pyruvate, giving alanine
• or glutamine synthase adds NH4+ to glutamate giving glutamine

Question 8

what are major carriers of nitrogen in the blood to liver?

Answer 8

alanine & glutamine

Question 9

what happens in deamination/urea cycle?

Answer 9

• In the liver
• Amino group of glutamate is converted to free ammonium ion
• Urea is synthesised in a complex series of reactions
  
  • urea/ornithine cycle
  • one nitrogen from free ammonium, the other from aspartic acid
  • carbon from CO2

Question 10

what is end product of urea cycle?

Answer 10

• Fumarate is generated as an end-product in the cytosol
• Its conversion to malate enables its carbon to be transported back to the mitochondrial matrix via the malate-aspartate shuttle.

Question 11

what happens to ketagenic amino acids?

Answer 11

• degraded to acetyl-CoA or acetoacetyl-CoA
• can give rise to ketone bodies or fatty acids

Question 12

what happens to glucogenic amino acids?

Answer 12

• degraded to pyruvate or TCA cycle intermediates
• can be converted into phosphoenolpyruvate and then into glucose

Question 13

what happens to remaining carbon skeletons after removal of alpha amino group?

Answer 13

• converted into major metabolic intermediates
• Can be converted to glucose or oxidised in the TCA cycle

Question 14

what is protein turnover way of amino acid degradation?

Answer 14

• tightly regulated
• takes place at different rates
• important for rapid changes
• damaged proteins have to be removed

Question 15

what is absorption way of amino acid degradation?

Answer 15

• proteolytic enzymes in stomach and intestine produce single amino acids and di- and tri-peptides
• absorbed into intestinal cells and released into blood for absorption by other tissues

Question 16

what is alcaptonuria?

Answer 16

degradation of phenylalanine and tyrosine is blocked

Question 17

what is maple syrup urine disease?

Answer 17

degradation of valine, isoleucine, and leucine is blocked

urine smells like maple syrup

mental and physical retardation

prevented by appropriate diet

Question 18

what is phenylketonuria?

Answer 18

phenylalanine accumulates in all body fluids

leads to severe mental retardation if untreated

therapy: low phenylalanine diet

Question 19

what are ways there can be defects in urea cycle enzyme?

Answer 19

• accumulation of urea cycle intermediates
• glutamine levels increase in the circulation
• α-ketoglurarate is no longer regenerated
• α-ketoglurarate levels become too low to fix more free ammonium ion
  
  • elevated levels of ammonia in the blood are toxic for the nervous system
• treatment with low-protein diet
• drugs which remove nitrogen
  
  • e.g. forming complexes with amino acids which are excreted
  • gene therapy in hepatocytes