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AMINO ACID METABOLISM
Jana Novotná
Department of the Medical Chemistry and Biochemistry
The 2nd Faculty of Medicine, Charles Univ.
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Amino acid structure
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The 20 common amino acids of proteins
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Metabolic relationship of amino acids
BODY PROTEINS
Proteosynthesis
Degradation
NONPROTEIN
DERIVATIVES
GLYCOLYSIS
KREBS CYCLE
UREA
Conversion
(Carbon skeleton)
AMINO ACIDS
DIETARY
PROTEINS
GLUCOSE
250 – 300
g/day
NH3
ACETYLDownloaded
CoA
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Porphyrins
Purines
Pyrimidines
Neurotransmitters
Hormones
Komplex lipids
Aminosugars
KETONBODIES
Enzymes cleaving the peptide bond
Endopeptidases – hydrolyse the peptide bond inside a
chain: pepsin, trypsin, chymotrypsin
Exopeptidases – split the peptide bond at the end of a
protein molecule: aminopeptidase, carboxypeptidases
Dipeptidases
pepsin (pH 1.5 – 2.5) – peptide bond derived from Tyr, Phe,
bonds between Leu and Glu
trypsin (pH 7.5 – 8.5) – bonds between Lys a Arg
chymotrypsin (pH 7.5 – 8.5) – bonds between Phe a Tyr
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Essential amino acids in humans
Arginine*
Histidine*
Isoleucine
Leucine
Valine
Lysine
Methionine
Threonine
Phenylalanine
Tryptophan
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from and/or sometimes during illness.
*Required to some degree in young growing
period
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Non-essential and nonessential
amino acids in humans
Can be formed from a-keto acids by transamination and
subsequent reactions.
Alanine
Asparagine
Aspartate
Glutamate
Glutamine
Glycine
Proline
Serine
Cysteine (from Met*)
Tyrosine (from Phe*)
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* Essential
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amino acids
General reactions of amino acid catabolism
O
+ NH4+
R
deamination
COO-
C
O
NH2
R
CH
transamination
R
COO-
COO-
C
NH2
R
CH
COO-
oxidative
decarboxylation
NH3+
R
CH2
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+
CO2
The fate of the amino group during amino acid catabolism
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Transamination reaction
The first step in the catabolism of most amino acids is
removal of a-amino groups by enzymes transaminases
or aminotransferases
All aminotransferases have the same prostethic group and
the same reaction mechanism.
The prostethic group is pyridoxal phosphate (PPL),
the coenzyme form of pyridoxine (vitamin B6)
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Biosynthesis of amino acid:
transamination reactions
amino acid1 +a-keto acid2
amino acid2 +a-keto acid1
NH3 +
-
O 2 CCH 2 CH 2 CHCO 2
-
Glutamate
O
R-CCO 2 -
+
Keto-acid
Pyridoxal phosphate (PLP)dependent aminotransferase
O
O 2 CCH 2 CH2 CCO 2 -
a-Ketoglutarate
+
NH2
R-CHCO 2 -
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Amino acid
Active metabolic form of vitamin B6
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Mechanism of transamination reaction: PPL complex with enzyme accept
an amino group to form pyridoxamine phosphate, which can donate its amio
group to an a-keto acid.
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All amino acids except threonine, lysine, and
proline can be transaminated
Transaminases are differ in their specificity for L-amino
acids.
The enzymes are named for the amino group donor.
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Clinicaly important transaminases
Alanine-a-ketoglutarate transferase ALT
(also called glutamate-pyruvate transaminase – GPT)
Aspartate-a-ketoglutarate transferase AST
(also called glutamate-oxalacetate transferase – GOT)
Important in the diagnosis of heart and liver damage caused by heart
attack, drug toxicity, or infection.
ALT
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Glucose-alanine cycle
Alanine plays a special role in
transporting amino groups to liver.
Ala is the carrier of ammonia and of the
carbon skeleton of pyruvate from muscle to
liver.
The ammonia is excreted and the pyruvate is
used to produce glucose, which is returned to
the muscle.
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L. Nelson, M. M. Cox :LEHNINGER. PRINCIPLES OF BIOCHEMISTRY Fifth edition
Glutamate releases its amino group as
ammonia in the liver
The amino groups from many of the a-amino acids are collected in the
liver in the form of the amino group of L-glutamate molecules.
 Glutamate undergoes oxidative deamination catalyzed by L-glutamate
dehydrogenase.
 Enzyme is present in mitochondrial matrix.
 It is the only enzyme that can use either NAD+ or NADP+ as the acceptor of reducing
equivalents.
 Combine action of an aminotransferase and glutamate dehydrogenase referred to as
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transdeamination.
Ammonia transport in the form of glutamine
Excess ammonia is added to
glutamate to form glutamine.
Glutamine
synthetase
Glutamine enters the liver and NH4+
is liberated in mitochondria by the
enzyme glutaminase.
Ammonia is remove by urea
synthesis.
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Relationship between glutamate, glutamine
and a-ketoglutarate
NH3
NH3
glutamate
a-ketoglutarate
NH3
glutamine
NH3
A. Glutamate dehydrogenase
glutamate
+
NAD+
+
H2O
a-ketoglutarate
B. Glutamine synthetase (liver)
ATP
+
NH3
ADP
glutamine
C. Glutaminase (kidney)
glutamine
+
NH3
To urea cycle
From transamination
reactions
glutamate
+
H2O
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glutamate
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+
NH3
+
NADH
Oxidative deamination
A. Oxidative deamination
Amino acids
+
FMN
•L-amino acid oxidase produces
+
H2O
L-amino acid oxidase
a-keto acids
+ FMNH2 +
NH3
O2
catalse
FMN
B. Nonoxidative deamination
H2O2
ammonia and a-keto acid directly,
using FMN as cofactor.
•The reduced form of flavin must be
regenerated by O2 molecule.
•This reaction produces H2O2
molecule which is decompensated by
catalase.
H2O
+
O2
Is possible only for hydroxy amino acids
serine
threonine
Serin-threonin dehydratase
pyruvate +
NH3
a-ketoglutate
+
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NH3
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Amino acid metabolism and central
metabolic pathways
20 amino acids are converted
to 7 products:
 pyruvate
 acetyl-CoA
 acetoacetate
 a-ketoglutarate
 succynyl-CoA
 oxalacetate
 fumarate
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Glucogenic Amino Acids
formed: a-ketoglutarate, pyruvate,
oxaloacetate, fumarate, or succinyl-CoA
Aspartate
Asparagine
Arginine
Phenylalanine
Tyrosine
Isoleucine
Methionine
Valine
Glutamine
Glutamate
Proline
Histidine
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Alanine
Serine
Cysteine
Glycine
Threonine
Tryptophan
Ketogenic Amino Acids
formed acetyl CoA or acetoacetate
Lysine
Leucine
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Both glucogenic and ketogenic amino
acids
formed: a-ketoglutarate, pyruvate,
oxaloacetate, fumarate, or succinyl-CoA in
addition to acetyl CoA or acetoacetate
Isoleucine
Threonine
Tryptophan
Phenylalanine
Tyrosine
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The C3 family: alanine, serine, cysteine and
threonine are converted to pyruvate
Alanine
Pyruvate
Serine
Cysteine
Threonine
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The C4 family: aspartate and asparagine are
converted into oxalacetate
Asparagine
Aspartic acid
Oxalacetate
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The C5 family: several amino acids are converted into
a-ketoglutarate through glutamate
Glutamine
a-ketoglutarate
Proline
Arginine
Histidine
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Interconversion of amino acids and intermediates of
carbohydrate metabolism and Krebs cycle
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Metabolism of some selected
amino acids
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Serine biosynthesis from glycolytic
intermediate 3-phosphoglycerate
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Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
Glycine biosynthesis from serine
Reaction involves the transfer of the hydroxymethyl group from serine to the cofactor
tetrahydrofolate (THF), producing glycine and N5,N10-methylene-THF.
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Glycine oxidation to CO2
Glycine produced from serine or from the diet can also be oxidized by glycine
decarboxylase (also referred to as the glycine cleavage complex, GCC) to yield a
second equivalent of N5,N10-methylene-tetrahydrofolate as well as ammonia and
CO2.
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Cysteine and methionine are metabolically
related
The sulfur for cysteine synthesis comes from the essential amino acid
methionine.
SAM
Condensation of ATP and methionine
yield S-adenosylmethionine (SAM)
SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the
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conversion of norepinephrine to epinenephrine).
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Cysteine synthesis
Conversion of homocysteine back to Met. N5methyl-THF is donor of methyl group.
*
*folate + vit B12
1.
Conversion of SAM to
homocysteine.
2.
Condensation of
homocysteine with serine to
cystathione.
3.
Cystathione is cleavaged to
cysteine.
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Homocystinuria
Genetic defects for both the synthase and the lyase.
Missing or impaired cystathionine synthase leads to homocystinuria.
High concentration of homocysteine and methionine in the urine.
Homocysteine is highly reactive molecule.
Disease is often associated with mental retardation, multisystemic
disorder of connective tissue, muscle, CNS, and cardiovascular
system.
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Biosynthesis of Tyrosine from Phenylalanine
Phenylalanine hydroxylase is a mixed-function oxygenase: one atom of oxygen is
incorporated into water and the other into the hydroxyl of tyrosine. The reductant is the
tetrahydrofolate-related cofactor tetrahydrobiopterin, which is maintained in the reduced
state by the NADH-dependent enzyme dihydropteridine reductase
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Phenylketonuria
Hyperphenylalaninemia - complete deficiency of phenylalanine
hydroxylase (plasma level of Phe raises from normal 0.5 to 2 mg/dL to
more than 20 mg/dL).
The mental retardation is caused by the accumulation of
phenylalanine, which becomes a major donor of amino groups in
aminotransferase activity and depletes neural tissue of α-ketoglutarate.
Absence of α-ketoglutarate in the brain shuts down the TCA cycle and
the associated production of aerobic energy, which is essential to
normal brain development.
Newborns are routinelly tested for blood concentration of Phe.
The diet with low-phenylalanine diet.
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Catabolism of branched amino acids
valine
isoleucine
a-ketoglutarate
a-ketoisovalerate
leucine
glutamate (transamination)
a-keto-b-methylbutyrate
a-ketoisokaproate
NAD+
oxidative decarboxylation
Dehydrogenase of a-keto acids*
CO2
isobutyryl CoA
a-methylbutyryl CoA
NADH + H+
isovaleryl CoA
Dehydrogenation etc., similar to fatty acid b-oxidation
propionyl CoA
acetyl CoA
+ from
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acetyl CoA
+
acetoacetate
Branched-chain aminoaciduria
Disease also called Maple Syrup Urine Disease (MSUD) (because
of the characteristic odor of the urine in affected individuals).
Deficiency in an enzyme, branched-chain α-keto acid
dehydrogenase leads to an accumulation of three branchedchain amino acids and their corresponding branched-chain α-keto
acids which are excreted in the urine.
There is only one dehydrogenase enzyme for all three amino
acids.
Mental retardation in these cases is extensive.
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Histidine Metabolism:
Histamine Formation
H
N
+
NH3
CH2 CHCO2 -
Histidine
decarboxylase
H
N
CH2 CH2 NH2
N
Histidine
CO2
N
Histamine
Histamine:
Synthesized in and released by mast cells
Mediator of allergic response: vasodilation,
bronchoconstriction
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Tryptophan catabolism
Tryptophan has complex catabolic pathway:
1. the indol ring is ketogenic
2. the side chain forms the glucogenic products
Kynurenate and xanthurenate are excrete in the urine.
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Enzymes which metabolised amino acides
containe vitamines as cofactors
THIAMINE B1 (thiamine diphosphate)
oxidative decarboxylation of a-ketoacids
RIBOFLAVIN B2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD)
oxidses of a-aminoacids
NIACIN B3 – nicotinic acid (nikotinamide adenine dinucleotide NAD+
nikotinamide adenine dinukleotide phosphate NADP+)
dehydrogenases, reductase
PYRIDOXIN B6 (pyridoxalphosphate)
transamination reaction and decarboxylation
FOLIC ACID (tetrahydropholate)
Meny enzymes of amino acid metabolism
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Helpful website
http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
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