AMINO ACID METABOLISM Jana Novotná Department of the Medical Chemistry and Biochemistry The 2nd Faculty of Medicine, Charles Univ.
Jan 02, 2016
AMINO ACID METABOLISM
Jana NovotnáDepartment of the Medical Chemistry and Biochemistry
The 2nd Faculty of Medicine, Charles Univ.
Amino acid structure
The 20 common amino acids of proteins
Metabolic relationship of amino acids
BODY PROTEINS
Proteosynthesis Degradation
AMINO ACIDSDIETARYPROTEINS
GLYCOLYSISKREBS CYCLE
Digestion
Transa
mination
NONPROTEINDERIVATIVESPorphyrinsPurinesPyrimidinesNeurotransmittersHormonesKomplex lipidsAminosugars
UREA NH3Con
vers
ion
(Carb
on
skele
ton
)
250 – 300 g/day
ACETYL CoAGLUCOSE CO2 KETONBODIES
Endopeptidases – hydrolyse the peptide bond inside a chain: pepsin, trypsin, chymotrypsinExopeptidases – split the peptide bond at the end of a protein molecule: aminopeptidase, carboxypeptidasesDipeptidases
Enzymes cleaving the peptide bond
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
Essential amino acids in humans
Arginine* Histidine* Isoleucine Leucine Valine
Lysine Methionine Threonine Phenylalanine Tryptophan
*Required to some degree in young growing period and/or sometimes during illness.
Non-essential and nonessential amino acids in humans
Alanine Asparagine Aspartate Glutamate Glutamine
Glycine Proline Serine Cysteine (from Met*) Tyrosine (from Phe*)
* Essential amino acids
Can be formed from -keto acids by transamination and subsequent reactions.
C
O
R COO-
+ NH4+
deamination
transamination C
O
R COO-
CH
NH2
R COO-
CH
NH2
R COO-
oxidativedecarboxylation
CH2
NH3+
R CO2+
General reactions of amino acid catabolism
The fate of the amino group during amino acid catabolism
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)
Biosynthesis of amino acid: transamination reactions
amino acid1 keto acid2 amino acid2 +-keto acid1
NH3+
-O2CCH 2CH2CHCO 2-
Glutamate
OR-CCO 2
-+
O-O2CCH 2CH2CCO 2
-
-Ketoglutarate
NH2
R-CHCO 2-
+
Pyridoxal phosphate (PLP)-dependent aminotransferase
Keto-acid
Amino acid
Active metabolic form of vitamin B6
Mechanism of transamination reaction: PPL complex with enzyme accept an amino group to form pyridoxamine phosphate, which can donate its amio group to an -keto acid.
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.
Clinicaly important transaminases
ALT
Alanine--ketoglutarate transferase ALT(also called glutamate-pyruvate transaminase – GPT)
Aspartate--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.
Glucose-alanine cycle
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.
Alanine plays a special role in transporting amino groups to liver.
According to D. 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
transdeamination.
Ammonia transport in the form of glutamine
Glutamine synthetase
Excess ammonia is added to glutamate to form glutamine.
Glutamine enters the liver and NH4+
is liberated in mitochondria by the enzyme glutaminase.
Ammonia is remove by urea synthesis.
Relationship between glutamate, glutamine and -ketoglutarate
-ketoglutarate glutamate glutamine
NH3
NH3
NH3
NH3
glutamate + NAD+ + H2O -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
Oxidative deamination
Amino acids FMN H2O+ +
keto acids FMNH2 NH3
L-amino acid oxidase
A. Oxidative deamination
FMN H2O2
H2O O2+
+ +
O2
catalse
B. Nonoxidative deamination
serine
pyruvate
threonine
-ketoglutateNH3+
+
NH3
Serin-threonin dehydratase
•L-amino acid oxidase produces
ammonia and -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.
Is possible only for hydroxy amino acids
Amino acid metabolism and central metabolic pathways
20 amino acids are converted
to 7 products:
pyruvate
acetyl-CoA
acetoacetate
-ketoglutarate
succynyl-CoA
oxalacetate
fumarate
Glucogenic Amino Acids
formed: -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
Ketogenic Amino Acids
formed acetyl CoA or acetoacetate
Lysine
Leucine
Both glucogenic and ketogenic amino acids
formed: -ketoglutarate, pyruvate, oxaloacetate, fumarate, or succinyl-CoA in
addition to acetyl CoA or acetoacetate
IsoleucineThreonineTryptophanPhenylalanineTyrosine
Alanine
Serine
Cysteine
Threonine
The C3 family: alanine, serine, cysteine and threonine are converted to pyruvate
Pyruvate
The C4 family: aspartate and asparagine are converted into oxalacetate
Aspartic acid Asparagine
Oxalacetate
The C5 family: several amino acids are converted into -ketoglutarate through glutamate
Glutamine
Proline
Histidine
Arginine
ketoglutarate
Interconversion of amino acids and intermediates of carbohydrate metabolism and Krebs cycle
Metabolism of some selected amino acids
Serine biosynthesis from glycolytic intermediate 3-phosphoglycerate
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.
Copy from: http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
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
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
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
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.
Homocystinuria
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
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.
Phenylketonuria
valine isoleucine leucine
-ketoglutarate glutamate (transamination)
-ketoisovalerate -keto--methylbutyrate -ketoisokaproate
oxidative decarboxylationDehydrogenase of -keto acids*
CO2
NAD+
NADH + H+
isobutyryl CoA -methylbutyryl CoA isovaleryl CoA
Dehydrogenation etc., similar to fatty acid -oxidation
propionyl CoA acetyl CoA
acetoacetate
acetyl CoA
propionyl CoA+ +
Catabolism of branched amino acids
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 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.
Histidine Metabolism: Histamine Formation
N
NH
CH2CHCO2-
NH3
+
N
NH
CH2CH2NH2
Histidine Histamine
Histidinedecarboxylase
CO2
Histamine: Synthesized in and released by mast cells
Mediator of allergic response: vasodilation, bronchoconstriction
Tryptophan catabolism
Tryptophan has complex catabolic pathway: 1. the indol ring is ketogenic2. the side chain forms the glucogenic productsKynurenate and xanthurenate are excrete in the urine.
Enzymes which metabolised amino acides containe vitamines as cofactors
THIAMINE B1 (thiamine diphosphate) oxidative decarboxylation of -ketoacids
RIBOFLAVIN B2 (flavin mononucleotide FMN, flavin adenine dinucleotide FAD)oxidses of -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
http://themedicalbiochemistrypage.org/amino-acid-metabolism.html
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