Amino Acid Catabolism Flashcards
why are amino acids degraded
when they are not being used as building blocks for anabolism
location of amino acid degradation
liver
absorption of amino acids
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
describe protein turnover of absorbed amino acids
tightly regulated
takes place at different rates - important for rapid changes
damaged proteins have to be removed
describe amino acids containing nitrogen
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
examples of molecules containing excess nitrogen molecules
urea
uric acid
creatinine
ammonium ion
where is urea formed
liver
3 steps of synthesising urea
transamination
de-animation
urea cycle (ornithine)
transamination
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
how is blood carried to the liver in transamination
alanine and glutamine are major carriers of nitrogen in the blood to the liver
where does de-amination and the urea cycle take place
liver
de-amination and urea cycle
amino group of glutamate is converted to free ammonium ion
urea synthesised
how is urea synthesised
urea cycle
one nitrogen from free ammonium, the other from aspartic acid
carbon from carbon dioxide
fumarate
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
reactants/products of urea cycle
CO2 + NH4+ + 3ATP + aspartate + 2H2O –>
urea + 2ADP + 2Pi + AMP + PPi + fumarate
degradations of carbon skeletons
after removal of the alpha amino group, the remaining carbon skeletons are converted into major metabolic intermediates
metabolic intermediates of carbon skeletons
glucose
oxidised in TCA cycle
degradation of ketogenic amino acids
degraded to acetyl-CoA or acetoacetyl-CoA
can give rise to ketone bodies or fatty acids
degradation of glucogenic amino acids
degraded to pyruvate or TCA cycle intermediates
can be converted into phosphoenolpyruvate and then into glucose
inherited disorders of amino acid degradation
alcaptonuria
maple syrup urine disease
phenylketonuria
alcaptonuria
degradation of phenylalanine and tyrosine is blocked
maple syrup disease
degradation of valine, isoleucine and leucine is blocked
urine smells like maple syrup
mental and physical retardation
prevented by appropriate diet
phenylketonuria
phenylalanine accumulates in all body fluids
leads to severe mental retardation if untreated
therapy - low phenylalanine diet
disorders of urea cycle
defects in an urea cycle enzyme
describe defects in an urea cycle enzyme
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
treatment for defects in urea cycle enzyme
low protein diet
drugs to remove nitrogen - form complexes with amino acids which are excreted, gene therapy in hepatocytes
