24.7 Urea Cycle The ammonium ion, the end product of amino acid degradation, is toxic if it is allowed to accumulate. The urea cycle converts ammonium ions to urea, which is transported to the kidneys to form urine. Learning Goal Describe the formation of urea from an ammonium ion.

Urea Cycle The urea cycle in the liver cells consists of reactions that occur in the mitochondria and cytosol. detoxifies ammonium ions from amino acid degradation. begins with the conversion of ammonium ions to carbamoyl phosphate using energy from two ATP.

Reaction 1: Transfer, Carbamoyl Group Reaction 1 of the urea cycle occurs in the mitochondrial matrix, where ornithine transcarbamylase catalyzes transfer of the carbamoyl group from carbamoyl phosphate to ornithine. the reaction yields citrulline, which is transported across the mitochondrial membrane by the citrulline transporter into the cytosol. hydrolysis of the high-energy phosphate bond in carbamoyl phosphate provides the energy to drive the reaction.

Reaction 1: Transfer, Carbamoyl Group

Reaction 2: Condensation with Aspartate Reaction 2 of the urea cycle occurs in the cytosol, where argininosuccinate synthetase catalyzes the condensation of citrulline with aspartate, producing argininosuccinate. the hydrolysis of two phosphate bonds in ATP to yield AMP and two inorganic phosphates provides energy for the reaction.

Reaction 3: Cleavage of Fumarate Reaction 3 of the urea cycle occurs in the cytosol, where argininosuccinate lyase splits argininosuccinate. the products are arginine and fumarate, a citric acid cycle intermediate.

Reaction 4: Hydrolysis of Arginine Reaction 4 of the urea cycle occurs in the cytosol, where arginase catalyzes the hydrolysis of arginine. the products are urea and ornithine. Ornithine returns to the mitochondrial matrix to repeat the urea cycle.

Urea Cycle, Overview

Study Check Identify the site for each as either mitochondrion or cytosol. A. formation of urea B. formation of carbamoyl phosphate C. combination of aspartate with citrulline D. cleavage of fumarate E. formation of citrulline

Solution Identify the site for each as either mitochondrion or cytosol. A. formation of urea cytosol B. formation of carbamoyl phosphate mitochondrion C. combination of aspartate with citrulline cytosol D. cleavage of fumarate cytosol E. formation of citrulline mitochondrion