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25.6 Digestion of Proteins

25.7 Degradation of Amino Acids

25.8 Urea Cycle

Chapter 25 Metabolic Pathways for Lipids and Amino Acids

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Proteins in the Body

Proteins provide: Amino acids for

protein synthesis. Nitrogen atoms for

nitrogen-containing compounds.

Energy when carbohydrate and lipid resources are not available.

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Digestion of Proteins

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Transamination

In transamination: Amino acids are degraded in the liver. An amino group is transferred from an amino

acid to an -keto acid, usually -ketoglutarate. The reaction is catalyzed by a transaminase or

aminotransferase. A new amino acid, usually glutamate, and a new

-keto acid are formed.

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A Transamination Reaction

NH3+ O Alanine

| || aminotransferaseCH3—CH—COO- + -OOC—C—CH2—CH2—COO-

Alanine -Ketoglutarate

O NH3+

|| |CH3—C—COO- + -OOC—CH—CH2—CH2—COO-

Pyruvate Glutamate

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Oxidative DeaminationOxidative deamination: Removes the amino group as an ammonium ion from

glutamate. Provides -ketoglutarate for transamination. NH3

+ Glutamate | dehydrogenase-OOC—CH—CH2—CH2—COO- + NAD+ + H2O Glutamate

O || -OOC—C—CH2—CH2—COO- + NH4

+ + NADH -Ketoglutarate

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Urea Cycle

The urea cycle: Detoxifies ammonium ion from amino acid

degradation. Converts ammonium ion to urea in the liver. O ||

H2N—C—NH2 urea

Provides 25-30 g urea daily for urine formation in the kidneys.

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Carbamoyl Phosphate In the mitochondria, an ammonium ion reacts

with CO2 from the citric acid cycle, 2 ATP, and water.

NH4+ + CO2 + 2ATP + H2O

O O || ||H2N—C—O—P—O- + 2ADP + Pi

| O-

Carbamoyl phosphate

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Reaction 1 Transfer of Carbamoyl Group

The carbamoyl group is transferred to ornithine to form citrulline.

Citrulline moves across the mitochondrial membrane into the cytosol.

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Reaction 2 Condensation with Aspartate

In the cytosol, citrulline combines with aspartate.

Hydrolysis of ATP to AMP provides energy.

The N in aspartate is part of urea.

Cytosol

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Reaction 3 Cleavage of Fumarate

Fumarate: Is cleaved from argininosuccinate. Enters the citric acid cycle.

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Reaction 4 Hydrolysis Forms Urea

Hydrolysis of arginine: Forms urea. Forms ornithine,

which returns to the mitochondrion to pick up another carbamoyl group to repeat the urea cycle.

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Urea Cycle

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Summary of Urea Cycle

The urea cycle converts: Ammonium ion to urea Aspartate to Fumarate 3ATP to 2ADP, AMP, 4Pi

NH4+ + CO2 + 3ATP + Aspartate + 2H2O

Urea + 2ADP + AMP + 4Pi + Fumarate