The Urea Cycle
Formation of Urea
– protecting the body from toxic ammonia
urea is only made in the liver – uses
four ATP
(as much as a peptide bond) – two ATP from ATP
Þ
AMP + PP
I
urea is very stable to protect the nitrogen from forming dangerous amide salts
deficiency of any of the four enzymes produces disease in humans – often manifests as too much systemic glutamine
ornithine
is made from the breakdown of arginine
can regenerate aspartate from fumerate:
fumerate
Þ
OAA (through the TCA cycle), then
OAA
Þ
aspartate (aspartate transaminase)
nitogen atoms in urea come from free ammonia (made into carbamoyl phosphate) and from aspartate
The Urea Cycle
Regulation of the Urea Cycle
Rate-limiting enzyme is
carbamoyl phosphate synthase
– upregulated by
N-acetyl glutamate
(glutamate + acetyl CoA)
high glutamate levels indicate presence of nitrogen
high acetate levels indicate fasting state
