THE UREA CYCLE lecture outline sem.pptx
- 1. THE UREA CYCLE G.H. MSONI
- 2. • the breakdown of macromolecules in the body such as proteins, purines, pyrimidines and other amino group containing molecules generates ammonia(NH3) • Ammonia can further be converted to ammonium(NH4) which is a CNS irritant • Accumulation of ammonia and ammonium in the body is detrimental as these products are toxic and death usually results.
- 3. • Organisms have developed different mechanisms to get rid of ammonia and ammonium by converting it to less toxic substances that the body can tolerate • Birds and other reptiles get rid of ammonia by converting it to uric acid which is passed in stool and are thus called URICOTERIC animals
- 4. • Fish and other reptiles directly get rid of ammonia through their gills and are thus called AMMONOTERIC animals • Humans and other mammals get rid of ammonia by converting it to urea which is less toxic and is excreted via the kidneys and are thus called UREOTERIC
- 5. • The urea cycle in humans and other mammals occurs in the liver and to a less extent in the kidneys • It occurs in two sites of the cell that is the mitochondrion matrix and the cytoplasm • The urea cycle involves the use of rare amino acids ornithine, citruline and Arginosuccinate • The overall reaction involves the input of 3ATPs two amino groups to synthesise urea
- 6. • The urea cycle is an amphibolic pathway
- 7. PATHWAY
- 8. Rxns of the urea cycle • Step 1 ammonia from metabolism reacts with bicarbonate(HCO3) under the influence of carbomyl phosphate synthase to form carbomyl phosphate • Step 2 Carbomyl phosphate then reacts with ornithine to form citruline • Step 3 citruline then leaves the mitochondrion into the cytoplasm where it reacts with aspartic acid which donates the second amino group to form Arginosuccinate
- 9. • This is the second rxn at which ATP is utilised this rxn is catalysed by Arginosuccinate synthase • Step 4 is a cleavage rxn catalysed by Arginosuccinate lyase to form arginine and fumarate • Step 5 is the urea production step catalysed by arginase to form urea and ornithine which is taken back to the mitochondrion to continue the cycle