Amino Acid Catabolism

Question 1

why are amino acids degraded

Answer 1

when they are not being used as building blocks for anabolism

Question 2

location of amino acid degradation

Answer 2

liver

Question 3

absorption of amino acids

Answer 3

proteolytic enzyme sin stomach and intestine produce single amino acids and di- and tri- peptides
they are absorbed into intestinal cells and released into blood for absorption by other tissues

Question 4

describe protein turnover of absorbed amino acids

Answer 4

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

Question 5

describe amino acids containing nitrogen

Answer 5

nitrogen present on side chain - breakdown produces ammonia and ammonium ions
ammonium ions are toxic and so the excess nitrogen needs to be excreted safely

Question 6

examples of molecules containing excess nitrogen molecules

Answer 6

urea
uric acid
creatinine
ammonium ion

Question 7

where is urea formed

Answer 7

liver

Question 8

3 steps of synthesising urea

Answer 8

transamination
de-animation
urea cycle (ornithine)

Question 9

transamination

Answer 9

aminotransferase moves the amino group from alpha-amino acids to alpha-ket acid (alpha-ketoglutarate);
gives glutamate

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

Question 10

how is blood carried to the liver in transamination

Answer 10

alanine and glutamine are major carriers of nitrogen in the blood to the liver

Question 11

where does de-amination and the urea cycle take place

Answer 11

liver

Question 12

de-amination and urea cycle

Answer 12

amino group of glutamate is converted to free ammonium ion

urea synthesised

Question 13

how is urea synthesised

Answer 13

urea cycle
one nitrogen from free ammonium, the other from aspartic acid
carbon from carbon dioxide

Question 14

fumarate

Answer 14

end product of urea cycle in the cytosol

its conversion to malate enable its carbon to be transported back to mitochondrial matrix via malate-asprate shuttle

Question 15

reactants/products of urea cycle

Answer 15

CO2 + NH4+ + 3ATP + aspartate + 2H2O –>

urea + 2ADP + 2Pi + AMP + PPi + fumarate

Question 16

degradations of carbon skeletons

Answer 16

after removal of the alpha amino group, the remaining carbon skeletons are converted into major metabolic intermediates

Question 17

metabolic intermediates of carbon skeletons

Answer 17

glucose

oxidised in TCA cycle

Question 18

degradation of ketogenic amino acids

Answer 18

degraded to acetyl-CoA or acetoacetyl-CoA

can give rise to ketone bodies or fatty acids

Question 19

degradation of glucogenic amino acids

Answer 19

degraded to pyruvate or TCA cycle intermediates

can be converted into phosphoenolpyruvate and then into glucose

Question 20

inherited disorders of amino acid degradation

Answer 20

alcaptonuria
maple syrup urine disease
phenylketonuria

Question 21

alcaptonuria

Answer 21

degradation of phenylalanine and tyrosine is blocked

Question 22

maple syrup disease

Answer 22

degradation of valine, isoleucine and leucine is blocked
urine smells like maple syrup
mental and physical retardation
prevented by appropriate diet

Question 23

phenylketonuria

Answer 23

phenylalanine accumulates in all body fluids
leads to severe mental retardation if untreated
therapy - low phenylalanine diet

Question 24

disorders of urea cycle

Answer 24

defects in an urea cycle enzyme

Question 25

describe defects in an urea cycle enzyme

Answer 25

accumulation of urea cycle intermediates
glutamine levels increase in the circulation
alpha-ketoglurarate is no longer regenerated
alpha-ketoglurarate levels become too low to fix more free ammonium ion - elevated levels of ammonia in the blood are toxic for the nervous system

Question 26

treatment for defects in urea cycle enzyme

Answer 26

low protein diet

drugs to remove nitrogen - form complexes with amino acids which are excreted, gene therapy in hepatocytes