Section 8. Amino Acid Metabolism

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Section 8. Amino Acid Metabolism

• Overview

11/15/05
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Nitrogen Fixation
Fig. 24.2

• Source of nitrogen for life processes. 2 x 1011
  kg/year.
• Rhizobium, other root nodule bacteria, and blue
  green algae.
• Nitrogenase complex
• reductase is a dimer with two 4Fe 4S clusters
• nitrogenase is an a2b2 tetramer with two P
  clusters (Fe, S) and two FeMo cofactors.

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Nitrogenase Complex Reactions

• FeMo cofactor reaction
• N2 ? 2 NH3
• N2 triple bond is 225 kcal/mole.
• Notice that H2 is made also.
• Reductase reaction
• provides 8 e-
• 16 ATP ? 16 ADP 16 Pi.
• Ferredoxin supplies e- to reductase.
• 8 ferredoxin(red) ? 8 ferredoxin(ox) 8 e-.
• Ferredoxin(ox) is reduced by photosynthetic
  centers.

Stryer 4th
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Nitrogen Incorporation into Amino Acids
(p. 669)

• Glutamate dehydrogenase catalyzes NH4 addition
  to a-ketoglutarate to form a protonated Schiffs
  base, which is reduced to glutamate.
• Coupled to oxidation of NADPH (or NADH).
  Requires cofactor pyridoxal phosphate.
• Glutamine synthetase incorporates a second NH4
  to make glutamine (see below).
• Incorporated nitrogens transferred to make other
  amino acids.

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(p. 669)
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Amino Acid Metabolism in Humans
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Nitrogen Excretion in Humans

• Urea 90
• Ammonium 4
• Creatinine 3
• Amino acids 1.4
• Uric acid 1
• Other 0.6

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Examples of Non-protein Nitrogenous Compounds

• heme
• pyrimidines
• purines
• choline (serine)
• creatine
• bile salts (glycine)
• Melanin (tyrosine)
• porphyrins
• epinephrine (phenylalanine)
• nicotinic acid (tryptophan)

• Almost all nitrogen in human metabolism comes
  from dietary amino acids.
• To the left are some examples of
  nitrogen-containing compounds that are made from
  amino acids.

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Essential and Nonessential Amino Acids

• Essential Nonessential
• histidine alanine proline
• isoleucine arginine serine
• leucine asparagine tyrosine
• lysine aspartate
• methionine cysteine
• phenylalanine glutamate
• threonine glutamine
• tryptophan glycine
• valine

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Biosynthetic Family Schemes

• These schemes, for bacteria and plants, show
  biosynthetic families of amino acids made from
  major metabolic precursors (blue) and from other
  amino acids (yellow).
• The amino acids in bold are essential for humans.
  

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Amino Acids and ?-Keto Acids

• Every amino acid has a corresponding ?-keto acid
  (carbon skeleton).
• Each amino acid and its keto acid are
  interconvertible by transaminiation reactions.
• It is our inability to synthesize the carbon
  skeletons that makes certain amino acids
  essential to the human diet.

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Vitamin B6 and Pyridoxal Phosphate

• The vitamin form, an alcohol, is oxidized to the
  aldehyde level.
• The coenzyme is pyridoxal phosphate.
• During transaminiation reactions, the pyridoxal
  group is transiently converted to pyridoxamine.

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Catalytic Intermediate

• A Schiff base is formed by pyridoxal phosphate
  and an amino acid.
• All the bonds to the ?-carbon on the amino acid
  are weaker and more labile.
• a aminotransferase
• b decarboxylase
• c aldolase
• N of lower structure contributes to weakening
  the a, b and c bonds.

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Reactions in an Aminotransferase Active Site

• Dehydration attaches an amino acid to
  enzyme-bound PLP, forming a Schiff base.
• The aldimine is converted to ketimine.
• Hydrolysis frees the ?-keto acid.

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Transamination Mechanism

• Upper Left side Amino acid 1 binds and is
  converted to a keto acid.
• Lower right side Keto acid 2 binds and is
  converted to an amino acid.

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Catabolism of Amino Acids

• Amino acids provide about 15 of human metabolic
  energy.
• Peripheral tissues are the primary sites of amino
  acid catabolism, converting the carbon skeletons
  into pyruvate, acetyl CoA, acetoacetyl CoA,
  ?-ketoglutarate, succinyl CoA, fumarate or
  oxaloacetate.
• Amino groups are removed, converted to urea and
  excreted.
• When necessary, peripheral tissue amino groups
  can be transported to the liver as part of
  alanine (or glutamine).
• In the liver, alanine is converted back into
  pyruvate (suitable for gluconeogenesis), and the
  amino group is transferred to glutamate and then
  to the urea cycle.

Peripheral tissue
Liver
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Common Amino Group Carriers

• Alanine aminotransferase and glutamate
  aminotransferase are the catalysts.

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Clinical Measures of Tissue Damage

• Alanine aminotransferase (ALT), also known as
  serum glutamic pyruvic transaminase (SGPT) is
  normally more concentrated in the liver, compared
  to other locatons. It is released into the
  bloodstream as the result of liver injury and
  serves as a fairly specific indicator of liver
  status.
• Aspartate aminotransferase (AST), also known as
  serum glutamic oxaloacetic transaminase (SGOT)
  is, by contrast, normally found in a diversity of
  tissues including liver, heart, muscle, kidney,
  and brain. It is released into serum when any one
  of these tissues is damaged. For example, its
  level in serum rises with heart attacks and with
  muscle disorders.

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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 Urea cycle