section 8. amino acid metabolism overviewoverview 11/15/05
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Section 8. Section 8. Amino Acid Amino Acid MetabolismMetabolism
Section 8. Section 8. Amino Acid Amino Acid MetabolismMetabolism
OverviewOverviewOverviewOverview
11/15/05
Nitrogen FixationNitrogen Fixation
• Source of nitrogen for life processes. ~2 x 10Source of nitrogen for life processes. ~2 x 101111 kg/year. kg/year.• Rhizobium, other root nodule bacteria, and blue green algae.Rhizobium, other root nodule bacteria, and blue green algae.• Nitrogenase complex:Nitrogenase complex:
– reductase is a dimer with two 4Fe 4S clustersreductase is a dimer with two 4Fe 4S clusters– nitrogenase is an nitrogenase is an 2222 tetramer with two P clusters (Fe, S) and tetramer with two P clusters (Fe, S) and
two FeMo cofactors.two FeMo cofactors.
• Source of nitrogen for life processes. ~2 x 10Source of nitrogen for life processes. ~2 x 101111 kg/year. kg/year.• Rhizobium, other root nodule bacteria, and blue green algae.Rhizobium, other root nodule bacteria, and blue green algae.• Nitrogenase complex:Nitrogenase complex:
– reductase is a dimer with two 4Fe 4S clustersreductase is a dimer with two 4Fe 4S clusters– nitrogenase is an nitrogenase is an 2222 tetramer with two P clusters (Fe, S) and tetramer with two P clusters (Fe, S) and
two FeMo cofactors.two FeMo cofactors.
Fig. 24.2
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Nitrogenase Complex Nitrogenase Complex ReactionsReactions
• FeMo cofactor reaction: FeMo cofactor reaction:
– NN22 2 NH 2 NH33
– NN22 triple bond is 225 kcal/mole. triple bond is 225 kcal/mole.
– Notice that HNotice that H22 is made also. is made also.
• Reductase reaction: Reductase reaction: – provides 8 eprovides 8 e--
– 16 ATP 16 ATP 16 ADP + 16 Pi. 16 ADP + 16 Pi.• Ferredoxin supplies eFerredoxin supplies e-- to reductase. to reductase.
– 8 ferredoxin(red) 8 ferredoxin(red) 8 ferredoxin(ox) + 8 e 8 ferredoxin(ox) + 8 e--..– Ferredoxin(ox) is reduced by photosynthetic centers.Ferredoxin(ox) is reduced by photosynthetic centers.
N2 + 8 e- + 16 ATP + 16 H2O
2 NH3 + H2 + 16 ADP + 16 Pi + 24 H+
Stryer 4th
2
Nitrogen Incorporation into Amino AcidsNitrogen Incorporation into Amino Acids
• Glutamate dehydrogenase catalyzes NHGlutamate dehydrogenase catalyzes NH44++ addition to addition to -ketoglutarate to -ketoglutarate to
form a protonated Schiff’s base, which is reduced to glutamate. form a protonated Schiff’s base, which is reduced to glutamate. • Coupled to oxidation of NADPH (or NADH). Requires cofactor pyridoxal Coupled to oxidation of NADPH (or NADH). Requires cofactor pyridoxal
phosphate.phosphate.
• Glutamine synthetase incorporates a second NHGlutamine synthetase incorporates a second NH44++ to make glutamine (see to make glutamine (see
below). below). • Incorporated nitrogens transferred to make other amino acids.Incorporated nitrogens transferred to make other amino acids.
(p. 669)
(p. 669)3
Amino Acid Metabolism in HumansAmino Acid Metabolism in Humans
amino acidpool
synthesis tissueproteins
foodproteins
excessamino acids
-keto acidsurea
CO2
NH4+
pyruvate,Krebs cycleintermediates,acetyl CoA
Non-protein nitrogenous compounds
(70 gm/day)synthesis
degradation
(140 gm/day)
renalexcretion catabolism
conversion
15-30 mM in cells2-4 mM in blood
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Nitrogen Excretion in HumansNitrogen Excretion in Humans
• Urea 90%Urea 90%• Ammonium 4%Ammonium 4%• Creatinine 3%Creatinine 3%• Amino acids 1.4%Amino acids 1.4%• Uric acid 1%Uric acid 1%• Other 0.6%Other 0.6%
creatinephosphate
creatinine
+ Pi
This reaction occurs nonezymatically. It is independent of metabolism.
NN
NH2
+
O
H
CH3
NH
N
NH2
+P
CH3
O OO
O
O
NH2
O
NH2
UREA
5
Examples of Non-protein Examples of Non-protein Nitrogenous CompoundsNitrogenous Compounds
• hemeheme• pyrimidinespyrimidines• purinespurines• choline (serine)choline (serine)• creatinecreatine• bile salts (glycine)bile salts (glycine)• Melanin (tyrosine)Melanin (tyrosine)• porphyrinsporphyrins• epinephrine (phenylalanine)epinephrine (phenylalanine)• nicotinic acid (tryptophan)nicotinic acid (tryptophan)
• Almost all nitrogen Almost all nitrogen in human in human metabolism comes metabolism comes from dietary amino from dietary amino acids.acids.
• To the left are some To the left are some examples of examples of nitrogen-containing nitrogen-containing compounds that are compounds that are made from amino made from amino acids.acids.
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Essential and Nonessential Amino AcidsEssential and Nonessential Amino AcidsEssential and Nonessential Amino AcidsEssential and Nonessential Amino Acids
EssentialEssential NonessentialNonessential histidine histidine alanine alanine prolineproline
isoleucine isoleucine argininearginine serineserine
leucine leucine asparagineasparagine tyrosinetyrosine
lysine lysine aspartateaspartate
methionine methionine cysteinecysteine
phenylalaninephenylalanine glutamateglutamate
threoninethreonine glutamineglutamine
tryptophantryptophan glycineglycine
valinevaline
EssentialEssential NonessentialNonessential histidine histidine alanine alanine prolineproline
isoleucine isoleucine argininearginine serineserine
leucine leucine asparagineasparagine tyrosinetyrosine
lysine lysine aspartateaspartate
methionine methionine cysteinecysteine
phenylalaninephenylalanine glutamateglutamate
threoninethreonine glutamineglutamine
tryptophantryptophan glycineglycine
valinevaline 7
Biosynthetic Family SchemesBiosynthetic Family SchemesBiosynthetic Family SchemesBiosynthetic Family Schemes
• These schemes, for bacteria and plants, show biosynthetic families These schemes, for bacteria and plants, show biosynthetic families of amino acids made from major metabolic precursors (blue) and of amino acids made from major metabolic precursors (blue) and from other amino acids (yellow).from other amino acids (yellow).
• The amino acids in The amino acids in boldbold are essential for humans. are essential for humans.
• These schemes, for bacteria and plants, show biosynthetic families These schemes, for bacteria and plants, show biosynthetic families of amino acids made from major metabolic precursors (blue) and of amino acids made from major metabolic precursors (blue) and from other amino acids (yellow).from other amino acids (yellow).
• The amino acids in The amino acids in boldbold are essential for humans. are essential for humans. 8
Amino Acids and Amino Acids and -Keto Acids-Keto Acids• Every amino acid has Every amino acid has
a corresponding a corresponding --keto acid (carbon keto acid (carbon skeleton).skeleton).
• Each amino acid and Each amino acid and its keto acid are its keto acid are interconvertible by interconvertible by transaminiation transaminiation reactions.reactions.
• It is our inability to It is our inability to synthesize the carbon synthesize the carbon skeletons that makes skeletons that makes certain amino acids certain amino acids essential to the essential to the human diet.human diet.
-ketoglutarate glutamate
oxaloacetate aspartate
alaninepyruvate
AMINO ACID-KETO ACID
OO
CH2
NH3
+
OO
OO
CH2
O
CH2
OO
OO
CH2
O
OO
OO
CH3
O
OO
CH2
NH3
+
CH2
OO
OO
CH3
NH3
+
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Vitamin BVitamin B6 6
and Pyridoxal and Pyridoxal PhosphatePhosphate
• The vitamin The vitamin form, an alcohol, form, an alcohol, is oxidized to the is oxidized to the aldehyde level.aldehyde level.
• The coenzyme The coenzyme is pyridoxal is pyridoxal phosphate.phosphate.
• During During transaminiation transaminiation reactions, the reactions, the pyridoxal group pyridoxal group is transiently is transiently converted to converted to pyridoxamine.pyridoxamine.pyridoxal phosphate
Reactivesite
N
CHO
CH3
OOH
P
O
O
O
(PLP)
N
CH2
OH
CH3
OHOH
N
CHO
CH3
OHOH
N
CH2
CH3
OHOH
NH2
pyridoxine (vitamin B6)
pyridoxal
pyridoxamine
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Catalytic IntermediateCatalytic Intermediate
• A Schiff base is formed by pyridoxal A Schiff base is formed by pyridoxal phosphate and an amino acid.phosphate and an amino acid.
• All the bonds to the All the bonds to the -carbon on the -carbon on the amino acid are weaker and more labile.amino acid are weaker and more labile.– a: aminotransferasea: aminotransferase– b: decarboxylaseb: decarboxylase– c: aldolasec: aldolase
• NN++ of lower structure contributes to of lower structure contributes to weakening the a, b and c bonds.weakening the a, b and c bonds.
Resonance structure
Ra
bc
+
N
CHN
CH3
OOH
P
O
O
O
O
O
H
H
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Schiff base (aldimine)
Ra
bc
+N
CHN
CH3
OOH
P
O
O
O
O
O
H
H Fig. 25-5
Reactions in an Reactions in an Aminotransferase Active SiteAminotransferase Active Site
Reactions in an Reactions in an Aminotransferase Active SiteAminotransferase Active Site
• Dehydration attaches an Dehydration attaches an amino acid to enzyme-amino acid to enzyme-bound PLP, forming a bound PLP, forming a Schiff base.Schiff base.
• The aldimine is The aldimine is converted to ketimine.converted to ketimine.
• Hydrolysis frees the Hydrolysis frees the --keto acid.keto acid.
aldimineN
CH
N
CH3
OOH
P
O
O
O
O
O
H
R
N
CH
N
CH3
OOH
P
O
O
O
O
OH
H
R
+
H+
N
CH2
N
CH3
OOH
P
O
O
O
O
O
H
R
+ ketimine
H+
-keto acid + PMP
+
R
OO
O
N
CH2
CH3
OOH
P
O
O
O
H
NH2
+
H20
amino acid + PLP
+
R
NH2
O
OH
N
CH
CH3
OOH
P
O
O
O
H
O
+
H20
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Transamination MechanismTransamination Mechanism• Upper Left Upper Left
side: Amino side: Amino acid 1 binds acid 1 binds and is and is converted to a converted to a keto acid.keto acid.
• Lower right Lower right side: Keto acid side: Keto acid 2 binds and is 2 binds and is converted to converted to an amino acid.an amino acid.
2
2
2
Aldimine
+
PMPenzyme
E
R
2
CH
CO
N =
Ketimine
+
H N-CH22
C E
H
CR
2
CH
CO
N= E2
2NH
CH CO2
PLPenzyme
O=C E
H
Aldimine
C +R1 CO2
PMPenzyme
EH N-CH22
OCR CO
2O
2
R+R
1
2NH
CH CO2
R1
2
CH
CO
N =
Ketimine
CR1
2
CH
CO
N= E2
O=C E
HPLPenzyme
CR CO2O
2
1CR CO2O
C E
H
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Catabolism of Amino AcidsCatabolism of Amino AcidsCatabolism of Amino AcidsCatabolism of Amino Acids
• Amino acids provide about 15% of human metabolic energy.Amino acids provide about 15% of human metabolic energy.• Peripheral tissues are the primary sites of amino acid Peripheral tissues are the primary sites of amino acid
catabolism, converting the carbon skeletons into pyruvate, catabolism, converting the carbon skeletons into pyruvate, acetyl CoA, acetoacetyl CoA, acetyl CoA, acetoacetyl CoA, -ketoglutarate, succinyl CoA, -ketoglutarate, succinyl CoA, fumarate or oxaloacetate.fumarate or oxaloacetate.
• Amino groups are removed, converted to urea and excreted. Amino groups are removed, converted to urea and excreted. • When necessary, peripheral tissue amino groups can be When necessary, peripheral tissue amino groups can be
transported to the liver as part of alanine (or glutamine).transported to the liver as part of alanine (or glutamine).• In the liver, alanine is converted back into pyruvate (suitable In the liver, alanine is converted back into pyruvate (suitable
for gluconeogenesis), and the amino group is transferred to for gluconeogenesis), and the amino group is transferred to glutamate and then to the urea cycle.glutamate and then to the urea cycle.
• Amino acids provide about 15% of human metabolic energy.Amino acids provide about 15% of human metabolic energy.• Peripheral tissues are the primary sites of amino acid Peripheral tissues are the primary sites of amino acid
catabolism, converting the carbon skeletons into pyruvate, catabolism, converting the carbon skeletons into pyruvate, acetyl CoA, acetoacetyl CoA, acetyl CoA, acetoacetyl CoA, -ketoglutarate, succinyl CoA, -ketoglutarate, succinyl CoA, fumarate or oxaloacetate.fumarate or oxaloacetate.
• Amino groups are removed, converted to urea and excreted. Amino groups are removed, converted to urea and excreted. • When necessary, peripheral tissue amino groups can be When necessary, peripheral tissue amino groups can be
transported to the liver as part of alanine (or glutamine).transported to the liver as part of alanine (or glutamine).• In the liver, alanine is converted back into pyruvate (suitable In the liver, alanine is converted back into pyruvate (suitable
for gluconeogenesis), and the amino group is transferred to for gluconeogenesis), and the amino group is transferred to glutamate and then to the urea cycle.glutamate and then to the urea cycle.
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-amino acid
-keto acid
pyruvate
alanine
Peripheral tissue
pyruvate
alanine -ketoglutamate
glutamate
Liver
Common Amino Common Amino Group CarriersGroup Carriers
Common Amino Common Amino Group CarriersGroup Carriers
• Alanine aminotransferase and glutamate Alanine aminotransferase and glutamate aminotransferase are the catalysts.aminotransferase are the catalysts.
-ketoglutarate
glutamate
amino acid
-keto acidR
R
OO
NH3+
H
OO
O
OO
CH2
CH2
NH3+
O O
H
OO
CH2
O
CH2
O O
pyruvate
alanine
amino acid
-keto acid
R
OO
NH3
+H
OO
CH3
NH3
+H
R
OO
O
OO
CH3
O
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Clinical Measures of Tissue DamageClinical Measures of Tissue DamageClinical Measures of Tissue DamageClinical Measures of Tissue Damage
Alanine aminotransferase (ALT), also known as serum Alanine aminotransferase (ALT), also known as serum glutamic pyruvic transaminase (SGPT) is normally more glutamic pyruvic transaminase (SGPT) is normally more concentrated in the liver, compared to other locatons. It is concentrated in the liver, compared to other locatons. It is released into the bloodstream as the result of liver injury and released into the bloodstream as the result of liver injury and serves as a fairly specific indicator of liver status. serves as a fairly specific indicator of liver status.
Aspartate aminotransferase (AST), also known as serum Aspartate aminotransferase (AST), also known as serum glutamic oxaloacetic transaminase (SGOT) is, by contrast, glutamic oxaloacetic transaminase (SGOT) is, by contrast, normally found in a diversity of tissues including liver, heart, normally found in a diversity of tissues including liver, heart, muscle, kidney, and brain. It is released into serum when any muscle, kidney, and brain. It is released into serum when any one of these tissues is damaged. For example, its level in one of these tissues is damaged. For example, its level in serum rises with heart attacks and with muscle disorders. serum rises with heart attacks and with muscle disorders.
Alanine aminotransferase (ALT), also known as serum Alanine aminotransferase (ALT), also known as serum glutamic pyruvic transaminase (SGPT) is normally more glutamic pyruvic transaminase (SGPT) is normally more concentrated in the liver, compared to other locatons. It is concentrated in the liver, compared to other locatons. It is released into the bloodstream as the result of liver injury and released into the bloodstream as the result of liver injury and serves as a fairly specific indicator of liver status. serves as a fairly specific indicator of liver status.
Aspartate aminotransferase (AST), also known as serum Aspartate aminotransferase (AST), also known as serum glutamic oxaloacetic transaminase (SGOT) is, by contrast, glutamic oxaloacetic transaminase (SGOT) is, by contrast, normally found in a diversity of tissues including liver, heart, normally found in a diversity of tissues including liver, heart, muscle, kidney, and brain. It is released into serum when any muscle, kidney, and brain. It is released into serum when any one of these tissues is damaged. For example, its level in one of these tissues is damaged. For example, its level in serum rises with heart attacks and with muscle disorders. serum rises with heart attacks and with muscle disorders.
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Web linksWeb links
Nitrogen FixationNitrogen Fixation. A summary of the topic.. A summary of the topic.
Nitrogen CycleNitrogen Cycle. The biological big picture.. The biological big picture.
Amino Acid MetabolismAmino Acid Metabolism. Reviews reactions.. Reviews reactions.
Next topic:Next topic: Urea cycle Urea cycle