department of the medical chemistry and clinical
TRANSCRIPT
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Amino acid metabolism I
Jana Novotná, Bruno Sopko
Department of the Medical Chemistry and Clinical Biochemistry
The 2nd Faculty of Medicine, Charles Univ.
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Metabolic relationship of amino acids
Body proteins
Proteosynthesis Degradation
Amino acid
poolDietary
proteins
NONPROTEIN
DERIVATIVESPorphyrins
Purines
Pyrimidines
Neurotransmitters
Hormones
Komplex lipids
Aminosugars
UREA NH3
Carbon skeleton
conversion
250 – 300
g/day
Acetyl CoACarbohydrates
LipidsCO2
H2O
Glycolysis
Krebs cycle
Conversion to
Ketonbodies
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http://pharmaxchange.info/press/2013/08/metabolism-of-amino-acids-%E2%80%93-bimolecular-ping-pong-mechanism-of-transamination/
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Digestive tract:
Endopeptidases – hydrolysis of peptide bond inside a polypeptide chain:
pepsin (stomach), trypsin, chymotrypsin, elastase (pancreas)
Exopeptidases – split the peptide bond at the end of a protein molecule:
aminopeptidase, carboxypeptidases, dipeptidases (small intestine)
Hydrolysis of proteins polypeptides oligopeptides amino acids
intestinal lumen transport to target tissues
Pepsin (pH 1.5 – 2.5) – hydrolysis of peptide bond before Tyr, Phe and
between Leu and Glu.
Trypsin (pH 7.5 – 8.5) – peptide bond after Lys a Arg.
Chymotrypsin (pH 7.5 – 8.5) – peptide bond after Trp, Phe,Tyr, Met, Leu.
Pancreatic elastase (pH 7.5 – 8.5) - peptide bond after Ala, Gly and Ser
Degradation of amino acids intracellularly the first step is deamination,
transamination, oxidative decarboxylation
Enzymes cleaving the peptide bond
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Absorption of amino acids
• Absorption from the lumen of small
intestine by transepitelial transport
• Semispecific Na+-dependent
transport system
• Na+-dependent carriers transport
both Na+ and an amino acid.
• At least six different Na+-dependent
carriers:
- neutral AA
- proline and hydroxyproline
- acidic AA
- basic AA (Lys, Arg) and cistine
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Clinical note:
Genetically determined defect in the transport of amino acids across
the brush border membranes of cells in both small intestine and
renal tubules
Cystinuria – AR disease, caused by mutation in two genes for
transporter proteins in the kidney proper reabsorption of basic, or
positively charged, amino acids (Lys, Arg and ornithine) and
cysteine into bloodstream is prevent Cys is oxidized to insoluble
cystine formation of kidney stones renal colic.
Hartnup disease – relatively rare AR disease – defect in tranport of
neutral AA including essential (Ile, Leu, Val, Phe, Thr, Trp -
availability of essential AA may cause a variety clinical disorders
The urine of newborns is routinely screening.
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g-Glutamyl cycle and amino acid transport
▪
▪ Gamma-glutamyl transferase
(gamma-glutamyl transpeptidase,
GGT)
▪ Found in many tissues, mainly in
the liver.
▪ Diagnostic marker for liver
disease - elevations in GGT in
patients with chronic viral hepatitis
infections.
▪Transport of AA across cell
membrane by reacting with
glutathion to for g-glutamyl amino
acid
▪ AA is released into the cell.
▪ Glutathion is resinthesized.
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Transamination - exchange of NH2 group with C=O
General reactions of amino acid catabolism
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General reactions of amino acid catabolism
Deamination
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General reactions of amino acid catabolism
Decarboxylation
Decarboxylation of AA gives amines having a variety of functions.
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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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Active metabolic form of vitamin B6
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Mechanism of transamination reaction: PLP complex with enzyme accept
an amino group to form pyridoxamine phosphate, which can donate its amino
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
individual L-a-amino acid.
The enzymes are named for the amino group donor.
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Clinicaly important transaminases
ALT
Alanine transaminase ALT(previously called serum glutamate-pyruvate transaminase – SGPT)
Predominantly found in the liver.
Important in the diagnosis of liver (viral hepatitis drug toxicity), ALT is a
more specific indicator of liver inflammation than AST.
Aspartate transaminase AST(previously called serum glutamate-oxaloacetate transaminase – SGOT).
- Found in the liver, heart, skeletal muscles, kidneys, brain, and red blood
cells.
- Elevated in liver diseases, myocardial infarction, acute pancreatitis, acute
hemolytic anemia, severe burns, acute renal diseases, musculoskeletal
diseases, and trauma (in 1954 defined as a biochemical marker for the
diagnosis of acute myocardial infarction)
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Deamination
Amino acids FMN H2O+ +
a-keto acids FMNH2 NH3
L-a-amino acid oxidase
A. Oxidative deamination
FMN
H2O2 H2O + O2
+ +
O2
catalse
B. Nonoxidative deamination
serine
pyruvate
threonine
a-ketobutyrate
+ +
Serin-threonin dehydratase
• L-a-amino acid oxidase produces
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.
Reaction is possible only for hydroxy amino acids
NH3 + H2O NH3 + H2O
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Decarboxylation
• process is catalysed by enzymes decarboxylase – cofaktor is pyridoxalphosphate
• R-CHNH2-COOH R-CH2NH2 + CO2
• takes place only in small quantities
• primary amines
• biologically active amines
• hormones (neurotransmitters, coenzymes)
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Synthesis of non-essential amino acids
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Overview of the synthesis of non-
essential amino acids
The carbon of 10 AA may be
produced from glucose through
intermediates of glycolysis or the
TCA cycle.
Tyrosine from phenylalanine.
The sulphur of cysteine – from
methionine.
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Amino acids derived from
intermediates of glycolysis
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The major pathways for serine synthesis from
glucose and serine degradation
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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.
Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
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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.
Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
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Tetrahydrofolate acts as a carrier of reactive
single C units
Copy from: http://www.chembio.uoguelph.ca/educmat/chm452/lectur25.htm
Serine glycine – formation of N5,N10-methylen THF
Glycine CO2 - formation of N5,N10-methylen THF
Homocysteine methionine – donor of C is N5-methyl
THF
Histidine degradation – formation of N5-formiminoTHF;
N5,N10-metnhenyl a N10-formyl THF
Tryprophane degradation – formation of N10-formyl THF
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Metabolism of glycine
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Cysteine synthesis
Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
1. Conversion of SAM to
homocysteine.
2. Condensation of
homocysteine with serine to
cystathione.
3. Cystathione is cleavaged to
cysteine.
Conversion of homocysteine back to Met. N5-methyl-THF is donor of methyl group.
*
*folate + vit B12
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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.
Clinical note
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Relationship between glutamate, glutamine
and a-ketoglutarate
a-ketoglutarate glutamate glutamine
NH3
NH3
NH3
NH3
Glutamate + NAD+ + H2O a-ketoglutarate NH3+ + NADH
Glutamate NH3+ glutamine
ATP ADP
Glutamine H2O+ glutamate NH3+
A. Glutamate dehydrogenase
B. Glutamine synthetase (liver)
C. Glutaminase (kidney)
From transamination
reactions
To urea cycle
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Amino acid degradation
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Degradation of AA
20 amino acids are converted to
7 products:
➢ pyruvate
➢ acetyl-CoA
➢ acetoacetate
➢ a-ketoglutarate
➢ succinyl-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
Alanine
Serine
Cysteine
Glycine
Threonine
Tryptophan
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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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Amino acids that form acetyl-CoA and
acetoacetate
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Amino acids related through glutamate
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Synthesis and degradation
of proline
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Histidine degradation
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Amino acids that form
succinyl-CoA
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Amino acids related to oxalacetate
Aspartate and asparagine
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The sulfur for cysteine synthesis comes from the essential amino acid
methionine.
SAM serves as a precurosor for numerous methyl transfer reactions (e.g. the
conversion of norepinephrine to epinenephrine).
Cysteine and methionine are metabolically related
Condensation of ATP and methionine
yield S-adenosylmethionine (SAM)
SAM
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valine isoleucine leucine
a-ketoglutarate glutamate (transamination)
a-ketoisovalerate a-keto-b-methylbutyrate a-ketoisokaproate
oxidative decarboxylation
Dehydrogenase of a-keto acids*CO2
NAD+
NADH + H+
isobutyryl CoA a-methylbutyryl CoA isovaleryl CoA
Dehydrogenation etc., similar to fatty acid b-oxidation
propionyl CoA acetyl CoA
acetoacetate
acetyl CoA
propionyl CoA+ +
Degradation of branched amino acids
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Branched-chain aminoaciduriaDisease 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 branched-
chain 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.
Clinical note
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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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Tetrahydrobiopterin as a cofactor of hydroxylases
Dihydrobiopterin
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• Hyperphenylalaninemia, phenylketonuria -
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.
Clinical note
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Tryptophan catabolism
Tryptophan has complex
catabolic pathway:
1. the indol ring is
ketogenic
2. the side chain
alanin
gluconeogenesis
Xanthurenic acid is
excrete in the urine.
Nicotinamide NAD and
NADP.
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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-amino acids
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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Pictures were taken from textbooks:
Marks´ Basic Medical Biochemistry A Clinical Approach. Four edition M. Lieberman,
A.D. Marks ed., 2013.
Essentials of Medical Biochemistry With Clinical Cases. First edition. N.V. Bhagavan,
Chung-Eun Ha ed., 2011.