25'9 Fates of the Carbon Atoms from

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Chapter 25 Metabolic Pathways for Lipids and
Amino Acids

• 25.9 Fates of the Carbon Atoms from
• Amino Acids
• 25.10 Synthesis of Amino Acids

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Carbon Atoms from Amino Acids

• Carbon skeletons of amino acids are
  intermediates or converted to intermediates for
  use in the citric acid cycle.
• Three-carbon skeletons
• Alanine, serine, and cysteine Pyruvate
• Four-carbon skeletons
• Aspartate, asparagine Oxaloacetate
• Five-carbon skeletons
• Glutamine, glutamate, proline,
• arginine, histidine Glutamate

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Three carbon skeletons
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Four carbon skeletons
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Five-carbon skeletons
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Glucogenic and Ketogenic Amino Acids

• Amino acids are classified as
• Glucogenic if they generate pyruvate or
  oxaloacete, which can be used to synthesize
  glucose.
• Ketogenic if they generate acetoacetyl CoA or
  acetyl CoA, which can form ketone bodies or fatty
  acids.

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Ketogenic Glucogenic
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Sources of Amino Acids

• Essential amino acids must be obtained in the
  diet.
• Nonessential amino acids are synthesized in the
  body.

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Synthesis of Amino Acids

• In humans, transamination of compounds from
  glycolysis or the citric acid cycle produces
  nonessential amino acids.

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Synthesis of Glutamine

• Glutamine is synthesized by adding another amino
  group to glutamate.
• NH3 Glutamine
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  synthetase
• -OOCCHCH2CH2COO- NH3 ATP
• Glutamate
• NH3 O
• -OOCCHCH2CH2CNH2 ADP Pi
• Glutamine

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Phenylketonurea (PKU)

• In phenylketonurea (PKU)
• The gene that converts phenylalanine to tyrosine
  is defective.
• Phenylalanine forms phenylpyruvate
  (transamination), which goes to phenylacetate
  (decarboxylation).
• High levels of phenylacetate cause severe mental
  retardation.
• A diet low in phenylalanine and high in tyrosine
  is recommended.

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Phenylketonurea (PKU)
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Overview of Metabolism

• In metabolism
• Catabolic pathways degrade large molecules.
• Anabolic pathway synthesize molecules.
• Branch points determine which compounds are
  degraded to acetyl CoA to meet energy needs or
  converted to glycogen for storage.

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