Section 8. Amino Acid Metabolism

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Section 8. Amino Acid Metabolism

• Urea cycle

11/18/05
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Substrates for the Urea Cycle

• Above, amino groups are transferred to glutamate,
  from which ammonium is produced, and then used to
  make carbamoyl phosphate.
• Below, amino groups are transferred to produce
  aspartate.

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Urea Cycle

• Aspartate and carbamoyl phosphate each deliver an
  amino group to the cycle.
• Notice that the carbamoyl phosphate production
  and condensation occur in the mitochondrial
  matrix.

Fig. 23.16
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NH4 from Oxidative Deamination of Glutamate

• Hexameric glutamate dehydrogenase is is
  controlled allosterically.
• High energy levels inhibit (ATP and GTP).
• Low energy levels activate (ADP and GDP).
• NADP can replace NAD.
• NH4 , which is toxic to humans, is produced in
  the mitochondria and used to make carbamoyl
  phosphate.

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Carbamoyl Phosphate Synthesis
(p. 645)

• Carbamoyl phosphate synthetase is in
  mitochondrial matrix.
• NH4 is source of NH3.
• The hydrolysis of two ATP make this reaction
  essentially irreversible.
• N-acetyl glutamate is an allosteric activator.
  (see S08L05)

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2 P used

• 1. ARGININOSUCCINATE SYNTHASE 2.
  ARGININOSUCCINASE
• 3. ARGINASE
  4. ORNITHINE TRANSCARBAMOYLASE

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Connection to Krebs Cycle

• Fumarate is oxidized to oxaloacetate by Krebs
  cycle enzymes, producing NADH.
• Oxaloacetate accepts an amino group instead of
  being condensed with acetyl CoA.

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Amino Acids to Urea

• Glutamate Dehydrogenase is the control site
  ADP (), GDP (), ATP (-), GTP (-) and
  NADH (-).
• Control at other sites by glucagon (), cortisol
  (), insulin (-), growth hormone (-).

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Summary of Reactions and Energetics - 1

• H20 aa NAD ? ?-keto acid NH4 NADH H
• and
• H20 fumarate aa NAD ? aspartate ?-keto
  acid NADH H
• then
• aspartate NH4 HCO3- 3 ATP ?
• urea fumarate 2 H20 2 ADP AMP 4 Pi
  H
• Four high energy phosphate bond equivalents are
  used for these reactions (- 4 P).
• Two NADH are produced.

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Summary of Reactions and Energetics - 2

• Now consider NADH oxidation
• 2 H 2 NADH O2 ??2 NAD 2 H20 (5
  P)
• The net reaction is then
• 2 aa HCO3- O2 ?
• 2 ?-keto acid urea H 2 H20
  (1P)

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Hyperammonemia

• Normal blood NH4 is 10-40 mM.
• Deficiencies of carbamoyl phosphate synthetase or
  of any enzyme in the urea cycle cause high
  NH4.
• Affects CNS and can lead to irreversible brain
  damage.
• Treatment strategies depend on which enzyme is
  deficient.

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Argininosuccinase Deficiency

• Low dietary protein reduces need for urea cycle.

• High dietary arginine provides a path for
  carbamoyl phosphate and aspartate nitrogens to
  produce argininosuccinate, which is excreted.

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Carbamoyl Phosphate Synthetase Deficiency
Fig. 23.20

• Hippurate and phenylacetylglutamine are excreted.
• Amino groups to glycine and glutamine by
  transamination.

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Ketogenic and Glucogenic Amino Acids
Fig. 23.21

• After removal of the amino group, the keto acids
  are used to make Krebs cycle intermediates,
  pyruvate, acetyl CoA and acetoacetyl CoA.

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Nitrogen for Oral Bacteria

• Urea is a major source of nitrogen for oral
  bacteria.
• It diffuses through most membranes and is in
  saliva.
• Bacterial urease produces NH4.
• Glutamate dehydrogenase incorporates NH4 into
  ?-keto acids to obtain?amino acids for bacterial
  growth.

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Nitrogen for Bacterial Amino Acid Synthesis

• When NH4 is limiting, it does not bind
  glutamate dehydrogenase, and the lower two
  reactions are used.

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Engineered Oral Bacteria to Fight Caries?

• Streptococcus Salivarius urease activity affects
  oral microbial ecology.
• It produces NH3, which in addition to promoting
  growth, neutralizes acids produces by other
  bacteria.
• S. Salivarius urease gene was introduced into
  Streptococcus mutans GS5. It was expressed and
  during glucose metabolism reduced pH decrease and
  duration.
• (Clancy Burne,1997 FEMS Microbiol Lett
  151205)

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• Web links
• Nitrogen Fixation. A summary of the topic.
• Nitrogen Cycle. The biological big picture.
• Amino Acid Metabolism. Reviews reactions.
• Next topic Porphyrins, heme, bile pigments