Chapter 25, Problem 25.52CP

Interpretation Introduction

Interpretation:

Statement “the total energy cost is four ATP in urea cycle” has to be explained.

Concept introduction:

The cyclic biological pathway that produces urea for excretion is called urea cycle.

• Step 1: It include carbamoyl phosphate transfers its $H_{2}NC=O$ group to ornithine to give citrulline in a reaction catalyzed by ornithine transcarbamolyase.
• Step 2: Citrulline combines with aspartate to give argininosuccinate synthase
• Step 3: Argininosuccinate is split into arginine and fumarate. The enzyme is argininosuccinase.
• Step 4: The carbon-nitrogen bond of the arginine is hydrolyzed in a reaction catalyzed by arginase to give the cycle product, urea, plus ornithine, ready to repeat step 1