Download Nitrogen Metabolism

TUMS
Dr. Azin Nowrouzi
Tehran University of Medical Sciences
Many essential biomolecules contain N
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Major Functions of Nitrogen
Derived from Dietary Protein
Heme
Blood cell
Choline
PL
Glycosamine
Sugar
Nucleotides
DNA
Protein
synthesis
Protein
Biogenic amines
Neurotransmitters
Carnitine
Heart
Creatine
phosphate
« Energy »
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The
nitrogen
cycle
• No biomolecules
are dedicated to
the storage of N
• excess N is
excreted
• N must be
replenished by
dietary protein
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A. Ammonia Is Incorporated
into Glutamate
• Reductive amination of a-ketoglutarate by
glutamate dehydrogenase occurs in plants,
animals and microorganisms
• In mammals & plants, located in mitochondria.
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B. Glutamine Is a Nitrogen Carrier
in Many Biosynthetic Reactions
• A second important route in assimilation of ammonia is via glutamine
synthetase
• It is present in all organisms.
• In humans it is most active in the liver.
• Glutamine is transported from the liver to other tissues via the blood.
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Assimilation of Ammonia
Major Ammonium ion carrier
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Glutamate synthetase
• Glutamate synthatase is not present in
humans.
• It is found in Bacteria
• It is used by blue-green algae and by Rhisobia.
Transamination
Reversible transfer of Ammonia between amino acids and a-ketoacids
by aminotransferase
•
•
Glutamate provides the amino group for
the synthesis of many
other amino acids through trasamination
reactions
Prosthetic group involved
in amino transfer
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Aspartate Aminotransferase
Overall mechanism of transamination
Alanine Aminotransferase
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Central role of the aminotrasferases and
glutamate dehydrogenase in nitrogen metabolism
1.
Proteins are degraded to amino acids
2.
Removal of nitrogen is first step in
degradation
3.
Ammonium is converted into urea
4.
C-skeleton enters known pathways
5.
Amino acids are made from intermediates
of other pathways
Amino Acid Synthesis
• The ability of an organism to live and grow is dependent on
protein synthesis
• Therefore, a supply of all 20 aa is necessary.
• Higher plants are able to synthesize all 20 aas.
• Many microorganisms and higher animals make fewer
• Humans make 10 of the 20 aas (these are called nonessential
amino acids.
• The remainder must be supplied in the diet, usually in the
form of plant or animal proteins (these are called essential
amino acids).
Metabolic Classification of the
Amino Acids
• All 20 amino acids are essential for life,
• They are necessary for protein
synthesis
– Essential or indispensable: 9
– Nonessential or dispensable: 11
• Review complete vs. incomplete
• All natural, unprocessed animal and
plant foods contain all twenty amino
acids
• A lack of any one of them leads to
severe metabolic disruption and
ultimate death.
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What are the nonessential amino acids
synthesized from?
• Their synthesis depends on the availability of the appropriate
carbon skeletons and a source of ammonia.
• Glucose is ultimately the source of carbon skeletons for most
nonessential aa.
• Two essential aa, phenylalanine and methionine, are used to
make tyrosine and cysteine, respectively.
• Since ammonia is available in the fed state, amino acids
become essential to our diet when we are not able to
synthesize their carbon skeletons.
α-keto acids required for synthesis of
nonessential amino acids
α-keto acid
Pyruvate
Oxaloacetate
α-keto glutarate
Pyruvate, 3-phosphoglycerate
Amino acid
Alanine
Aspartate, Asparagine
Glutamate, glutamine, Proline, Arginine
Serine
Biosynthesis of all amino acids in plants and
microorganisms
Biosynthesis of Amino
Acids
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Synthesis of Tyrosine
Amino acids are precursors of
some other biomolecules
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Obtained in large amounts
in diet as proteins
Not like carbohydrate and
fat
most N is in proteins
• Because no large stores of
protein in the body
AA’s are the source of N in the
diet
Amino Acids
If not enough consumption
Tissue breakdown occurs
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Protein Quality
Animal vs. Plant protein
• Important in maintaining N balance
• Proteins have different biological value (BV)
• Major reasons why animal protein is called BV protein,
whereas plant protein is of lower BV:
– Animal protein is “complete” - contains all essential amino acids
– Contains essential amino acids in larger amounts and in proper
proportion for optimal utilization
• Note: Soybean protein even though from a plant, is
comparable to animal protein
o In children leads to kwashiorker
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28 grams
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Note
• The 56-g protein requirement for adult male
can be met by 45 g of animal protein
• Same requirement would necessitate 65 g
plant protein
• Combining plant products (legumes + grains)
provides all essential amino acids
• Mixture of 30% animal protein and
70 % plant protein similar to use of animal
protein alone
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Protein RDA varies in
different stages of life cycle
•
•
•
•
•
•
•
0-0.5 years: 1 g/lb
0.5-1 years: .71 g/lb
1-6 years: .56 g/lb
7-14 years: .45 g/lb
15-18 years: .41 g/lb
19+ years: .36 g/lb
1Ib = 0.4536 kg
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Protein Turnover
• Body proteins turn over; t1/2= min - wks
• 400g of protein are synthesized per day and
400g are broken down
– Secretory proteins such as digestive enzymes,
polypeptide hormones, and antibodies, turn over
rapidly
– Structural proteins are much more metabolically
stable.
Chemical Signals for Turnover
• ubiquitinatin
– A small, heat stable protein (ubiquitin) reacts with
other proteins to mark them for destruction
• Oxidation of amino acid resides- Pro, Arg, Lys
• Pest sequences- one or more regions rich in
proline (P), gltamate (E),serine (S), and
threonine (T)
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Nitrogen Balance
• No biomolecules are dedicated to the storage of N;
• Excess N is excreted.
• Definition:
• Balances
– Zero for healthy adults
– Positive
–
–
–
–
childhood growth
pregnancy
muscle building
Healing
– Negative for
• Protein malnutrition
• Essential aa malnutrition
• “stress response” (fever, recovery from surgery, etc)
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Protein Digestion
• Gastric Digestion
– Function of pH
• Kills bacteria
• Denatures
proteins…
– Activation and
Action of Pepsin
• Intestinal Digestion
– Pancreatic
enzymes
– Intestinal enzymes
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Denaturation
of
Proteins
Protein Digestion in the Stomach
at low pH
Low pH
N
C
Protein in Native
Conformation
Protein Denatured
Pepsin
N
N
N
C
C
C
N
Oligopeptides
C
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Activation of the Gastric and
Pancreatic Zymogens
Trypsinogen
Enteropeptidase
Val-Asp-Asp-Asp-Asp-Lys
Trypsin
Chymotrypsinogen
Proelastase
Elastase
Procarboxypeptidase
Chymotrypsin
Carboxypeptidase
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Zymogens Activation
slow, nonenzymatic
+
pepsinogen
(zymogen)
pepsin
fast, enzymatic
denatured
proteins
hydrolysis
autocatalytic
conversion of
pepsinogen to pepsin
large peptide
fragments
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Other details of
Intestinal Enzymes
Enzyme
Occurrence
pH optimum
Major site of action
Trypsin
Intestine
7.5 to 8.5
Arginyl, lysyl bonds
Chymotrypsin
Intestine
7.5 to 8.5
Pepsin
Stomach
1.5 to 2.5
Aromatic amino acyl
bonds (Phe, Trp, Tyr)
Wide range of specificity
Carboxypeptidas e Intestine
7.5 to 8.5
C-terminal amino acid
Aminopeptidase
Intestinal
mucosa
N-terminal amino acid
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Final conversion of peptides to
free amino acids
di- and tripeptides
aa's
aminopeptidase
Brush Border
di- and tripeptides
peptidases
amino acids
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Amino Acid Absorption
• Secondary active
transport driven by
Na+ gradient
• Facilitated diffusion
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Intestinal
lumen
Di- and
Tripeptides
Amino
acid Na+
peptides
peptidases
Amino
acid
Na+
ATP
ADP
+Pi
K+
Facilitated
transporter
Serosal
side
Brush
border
Na+
Active
transporter
Na+
K+
Transepithelial
amino acid
transport.
Portal vein
Amino
acid
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Gamma-Glutamyl Cycle
• A metabolic cycle for transporting amino acids
into cells.
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Disorders of Amino Acid
transportation or absorption
Amino acid specificity
Amino acids transported
1. Small neutral amino
acids
Alanine, serine, throenine
2. Large neutral and
aromatic amino acids
Isoleucine, leucine, valine,
tyrosine, tryptophan,
phenylalanine
Arginine, lysine, omithine,
cystine
3. Basic amino acids
4. Proline, glycine
5. Acidic amino acids
Human disease
Hartnup disease
Cystinuria
Glycinuria
Glutamic and aspartic acids
Uptake (transport) systems exist especially in intestine & kidney.
Lack of specific transporter results in a disease state.
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This can be partially overcome through uptake of peptides.