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Q.
What is true of urea biosynthesis?
Excretory Products and their Elimination
Solution:
Urea cycle was discovered by Krebs and Hanseleit
• Urea cycle takes place in liver cells
• In urea cycle, $NH_{3}$ is converted into urea
• In urea cycle, 2 molecules of $NH_{3}$ and 1 molecule of $CO_{2}$ is involved
• Urea cycle removes two waste products from the blood in liver, i.e. $NH_{3}$ and $CO_{2}$
• Urea $(NH_{2}-CO-NH_{2})$ has 2 amino $(-NH_{2})$ groups, one derived from $NH_{3}$ and other from aspartate
• C-atom of urea is supplied by $CO_{2}$
• One molecule of $NH_{3}$ and one molecule of $CO_{2}$ react to form carbamoyl phosphate. Carbamoyl phosphate reacts with ornithine to form citrulline
• Citrulline reacts with aspartate to produce ar-ginosuccinate Arginosuccinate cleaves to give fumarate and arginine. Arginase cleaves to arginine to form urea and ornithine
• Ornithine is reused in urea cycle. Urea is transported to kidney for its removal
• 3 amino acids participating in urea cycle are ornith - ine, citrulline and arginine
