Download 24.7 Urea Cycle

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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Reaction 1: Transfer, Carbamoyl Group
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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.
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
Urea Cycle, Overview
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.
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
General, Organic, and Biological Chemistry: Structures of Life, 5/e
Karen C. Timberlake
© 2016 Pearson Education, Inc.