Catabolism of proteins

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Chapter 9

• Catabolism of proteins

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Section 9.1

• Nutritional function of proteins

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Nutritional requirement of proteins

• Nitrogen Balance
• 1. nitrogen balance (normal adult)
• intake N Losses N
• from the diet losses in urine and
  feces
• 2. Positive nitrogen balance (children,
  pregnant women, patients recovering)
• intake N gt Losses N
• 3. Negative nitrogen balance (starvation,
  malnutrition, patients with fever)
• intake N lt Losses N

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Protein requirement

• After 8-10 days diets without proteins, the
  nitrogen excretion is 53mg/kg body weight per
  day. ( 20g proteins per day 60kg man.)
• The World Health Organization recommend 0.75g/kg
  body wt day-1 proteins from food.
• To Chinese diet 80g per day a 65kg man.
• There is no storage of proteins in body, so
  proteins must be supplied every day.
• Excess diet protein can be used as energy supply.

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Nutritional quality of protein

• Essential Amino Acids are amino acids that cannot
  be synthesized by the body and must be obtained
  from diet.
• tryptophan phenylalanine
• lysine threonine
• valine leucine
• isoleucine methionine
• For infants and children
• histidine arginine

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• Non-essential Amino Acids are amino acids that
  can be synthesized by body. Which including the
  other 12 amino acids .
• Semi-essential Amino Acids can be synthesized in
  the body from essential amino acids.
• tyrosine phenylalanine
• cysteine methionine

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Complementary Effect of Dietary Proteins

• Quality of protein the essential amino acid
  composition.
• high quality appropriate concentration of
  essential amino acids. (animal proteins)
• plant proteins lack one or more of them.

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• Complementary effect of dietary proteins
• two or more plant proteins supplied together
  will complement each other to a higher quality.

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Complementary Effect of Dietary Proteins
Proteins origin amount of lysine amount of tryptophan
corn deficient rich
soybean rich deficient
together rich rich
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• Digestion, Absorption Putrefaction

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Digestion of dietary proteins

• In stomach pH of gastric juice lt 2
• function kill microorganisms
• denature protein
• activate pepsinogen to
  pepsin.
• Pepsin can
• hydrolyze peptide bonds to form large fragments
  and some free AA.
• coagulate milk (caseinogen casein)

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In small intestine (main)

• Proteolytic enzymes of pancreatic juice
• endopeptidases cleave the internal peptide
  bonds.
• trypsinase, chymotrypsin, elastase
• exopeptidases remove AAs from N- or C-terminal
  ends.
• carboxypeptidase A and B

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In small intestine

• Activation of zymogens from pancreas
• enterokinase
• trypsinogen trypsinase
• chymotrypsinogen chymotrypsin
• proelastase elastase
• procarboxypeptidase carboxypeptidase
  

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Products

• Pancreatic proteolytic enzymes produce free amino
  acids and small peptides (2-8 AA residues.)
• Aminopeptidase hydrolyzes amino-terminal AA from
  oligopeptide.
• Dipeptidase hydrolyzes dipeptide.
• The results of protein digestion
• free AAs, dipeptides, tripeptides.

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Carboxypeptidase
Aminopeptidase
endopeptidases
Dipeptidase
Digestion of protein
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Absorption and transportation of AA
Location intestine
free amino acids,dipeptides,tripeptides

• by transport systems
• seven transport systems

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Absorption and transportation of AA

• The AA is absorbed with Na, which has to be
  pumped out of the cell by a sodium pump. It is an
  ATP-requiring process.
• The absorbed dipeptides and tripeptides are
  hydrolyzed to free AAs before they be transport
  into portal vein.

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Putrefaction

• The undigested proteins and no absorbed AAs pass
  into the large intestine, where the decomposition
  of which by bacteria is called putrefaction.
• Products
• benefits vitamin K, B12, folic acid,
• toxicoids amines, phenol, indole, H2S,

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• Decarboxylation of AAs produces amines
• histidine histamine
• tyrosine tyramine
• lysine cadaverine
• Tyramine can raise blood pressure, histamine and
  cadaverine can decrease blood pressure
• Production of Phenol
• tyrosine phenol

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• Production of indole (odor of feces)
• tryptophan indole
• Production of H2S
• cysteine hydrogen sulfide
• Production of ammonia
• unabsorbed AA
• urea (from blood)

NH3
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• The toxic products of putrefaction are removed by
  liver.
• In liver disease, which may cause the hepatic
  coma.

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Section 9.3

• Degradation of Protein in Cells

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• Protein turnover
• the degradation and synthesis of protein.
• 1-2 of total body proteins turnover each day.

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• Half time (t1/2 ) the time required to reduce
  the proteins concentration to 50 of its initial
  value.
• HMG CoA reductase 0.5-1 hours
• plasma proteins 10 days
• collagen and histone several months

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There are two pathway to degrade protein in cells.

• Lysosomal pathway
• -extracellular membrane-associated long-lived
  proteins
• -ATP independent process
• -degraded by cathepsin (pH 5.0)

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There are two pathway to degrade protein in cells.

• Cytosol pathway
• -abnormal, damaged, short-lived proteins
• -require ATP and ubiquitin
• -degraded by proteosome (pH 7.8)

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• Ubiquitin
• -76 amino acid residues
• -presents in all eukaryotic cells
• -the primary structure is highly conserved,
• The process of ubiquitin pathway
• -ubiquitination chains of 4 or more ubiquitin
  combine to the protein
• -degradation of ubiqitinated protein

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ubiquitination
Ub
Ub
Ub
Ub
Ub
Ub
E1ubiquitin activating enzyme, E2
ubiquitin-conjugating enzyme, E3
ubiquitin-protein ligase
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Degradation
Ubn-CO-NH-protein
proteosome
Amino acids
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Section 9.4

• Amino Acid Catabolism General

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• Amino acid metabolic pool
• the amino acids coming from digestion and
  absorption of dietary proteins or from
  degradation of body proteins will be used equally
  in the body.

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utilization
Source
Body protein
synthesis
Amino acid metabolic pool
Degradation
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Deamination of AAs

• Term removal of the amino groups of AAs
• Including
• -Oxidative deamination
• -Non-oxidative deamination
• -Transamination
• -Coupling the transamination with
• deamination of glutamate
• -Purine nucleotide cycle

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1. Oxidative deamination

• L-amino acid oxidase
• -in the liver and kidney,
• -the activity is low ( unimportant)
• D- amino acid oxidase
• -in liver, kidney and other tissues
• -important for glycin and D- amino acid

L-amino acid oxidase
L-amino acid
a-imino acid
a-Keto acid NH3
2H
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1. Oxidative deamination

• L-glutamate Dehydrogenase
• -wide distribution, high activity (except muscle)
• -the major enzyme in the metabolism of AAs
• -inhibitors GTP, ATP activators GDP, ADP

NAD(P)HH
NAD(P)
NH3
H2O
L-glutamate Dehydrogenase
a-ketoglutarate
L-glutamate
a- aminoglutarate
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2.Non-oxidative deamination

• Serine and threonine
• -the a-amino group of which can be removed
  nonoxidatively
• -special dehydratases
• Cysteine
• -cysteine desulfhydrase

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3.Transamination

• The transfer of a-amino group from a a-amino acid
  to a a-keto acid, then the a-amino acid forms a
  corresponding a-keto acid, the a-keto acid forms
  a corresponding a-amino acid. (except Lys, Thr
  and Pro )

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Transami-nase
a-amino acid 1
a-keto acid 1
a-amino acid 2
a-keto acid 2
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3.Transamination

• The ?-amino group of most amino acids is
  transferred to ?-ketoglutarate to form glutamate
• ? -NH2 is only transferred from one of amino
  acids to a ? - keto acid, not really removed.
• Aminotransferases (transaminases)
• -Different transaminating reactions are
  catalyzed by different transaminases

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• Alanine transaminase (ALT, or GPT) and Aspartate
  transaminase (AST, or GOT)

Alanine ?-ketoglutarate
pyruvate Glutamate
ALT
Aspartate ?-ketoglutarate
Oxaloacetate Glutamate
AST
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• The serum levels of GOT, GPT are very low
  normally.
• Measure the serum level of special transaminase
  has diagnostic significance.
• Increase of GPT liver damage (hepatitis)
• Increase of GOT heart damage (myocardial
  infarction)

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Mechanism of transamination

• The cofactor of transaminase is pyridoxal
  phosphate

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1
Amino acid
Pyridoxal phosphate
a-ketoacid
Aldimeine (Schiff base )
1
1
Ketimine (Schiff base )
pyridoxamine phosphate
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Transaminase
Pyridoxal phosphate
pyridoxamine phosphate
?-keto acid
?-amino acid
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4.Coupling the transamination with deamination of
glutamate

• The ?-amino group of most amino acids is
  transferred to ?-ketoglutarate to form glutamate
  by transamination.
• Then glutamate deaminated to ammonia and
  ?-ketoglutarate by glutamate dehydrogenase.
• Which is called coupling deamination.

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coupling deamination.
?-ketoglutarate
?- amino acid
transaminase
?-keto acid
glutamate
H2ONAD
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5.Purine Nucleotide Cycle

• The activity of L-glutamate dehydrogenase is low
  in the skeletal muscle and heart, where the major
  deamination process is purine nucleotide cycle.

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?-keto-glutarate
a-amino acid
Adenylo-succinate
3 Adenylosuccinate synthetase
1 Transaminase
4 AMP deaminase
2 Aspartate transaminase(AST)
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Metabolism of Ammonia

• Source of ammonia in blood
• --Endogenous source
• deamination of amino acids (major)
• catabolism of other nitrogen compounds
• --Exogenous source (4g/day)
• production of putrefaction
• degradation of urea in large intestine by
• bacteria

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Transport of ammonia in blood

• Only traces of ammonia (NH3) exist in blood.
• NH3 is toxic to the central nervous system. So it
  must be transport in nontoxic type.
• transport as glutamine
• Alanine-glucose cycle

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1.Fixes ammonia as glutamine

• catalyzed by glutamine synthetase.
• glutamine is the temporary non-toxic storage and
  transport form of NH3
• Synthesized in brain and muscle,
• degraded in kidney and liver

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NH3ATP
ADPPi
Glutamine synthetase
glutaminase
NH3
H2O
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2.Alanine-glucose cycle
Muscle
Blood
Liver
protein
urea
glucose
glucose
glucose
Amino acid
NH3
NH3
Glutamate
pyruvate
pyruvate
a-ketoglutarate
Glutamate
Alanine
Alanine
a-keto- glutarate
Alanine
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Formation of Urea

• Urea is the major product of nitrogen.
• Synthesized in liver, excreted by kidney.
• The process is called urea cycle or ornithine
  cycle found by Hans Krebs.
• To 1mol urea
• 1mol ammonia
• 1mol a-amino nitrogen from aspartate
• 1mol CO2
• 3mol ATP

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Process of urea cycle

• 1. synthesis of carbamoyl phosphate (in
  mitochondria)

Carbamoyl phosphate synthetase?(CPS?) (N-acetylglu
tamate,Mg2)
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Process of urea cycle
2. Formation of citrulline (in mitochondria)
Ornithine carbamoyl- transferase (OCT)

H3PO4
Carbamoyl phosphate
Ornithine
Citrulline
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Process of urea cycle
3. Synthesis of Arginine (in cytosol)
NH
COOH
2
C
C
argininosuccinate synthesase (ASS)
N
H
CH
NH
2

COOH
(CH
)
Mg2
2
3
ATP
H2O
AMPPPi
CH
NH
2
COOH
Aspartate
Citrulline
argininosuccinate
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Process of urea cycle
3. Synthesis of Arginine


Argininosuccin-ate lyase (AST)
??????????
Fumarate
Arginine
argininosuccinate
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Reutilization of aspartate
citrulline
Aspartate
Amino acid
a-ketoglutarate
argininosuccinate
glutamate
oxaloacetate
Arginine
a-ketoacid
Fumarate
Malate
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Process of urea cycle
4. Hydrolysis of arginine to release urea


Arginase
Urea
ornithine
Arginine
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Formation fo NO
Nitric oxide synthase (NOS)
NO
O2
NADPHH NADP
Arginine
Citrulline
Nitric oxide is the muscle relaxant and gas
signal molecule.
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Summary of Ornithine cycle

P
NH3
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• Urea synthesized will be excreted by kidney, some
  of the urea will enter the intestine and are
  degraded to ammonia by bacteria.
• To the two nitrogen atoms of urea, one come from
  ammonia, which come from the degradation of amino
  acid,
• the other one come from Aspartate, which can
  get the amino group again from other amino acid.
  So two nitrogen all come from amino acid.

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Regulation in urea biosynthesis

• Dietary nitrogen intake
• high protein diet synthesis
• starvation synthesis
• CPS-? activated by
• N-acetylglutamic acid
• (AGA) and arginine

N-acetylglutamate
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Enzymes in urea cycle

• Key enzyme Argininosuccinate synthetase

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Hyperammonemia

• High level of ammonia in the blood.
• Reasons
• -inborn errors of enzymes in urea cycle,
• - liver failure
• Damage (ammonia poisoning)
• coma and irreversible brain damage

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Mechanism of the brain damage
NH3
NH3
a-ketoglutarate
Glutamate
glutamine
Quantity of a-ketoglutarate in brain?
TAC ?
ATP deficiency
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Other metabolic pathway of ammonia

• Excretion in urine as NH4 (in kidney)

Glutami-nase

NH3
NH4 is excreted in the urine, which can maintain
the acid-base balance of the body.
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Other metabolic pathway of ammonia

• Synthesis of amino acids
• non-essential amino acids
• Biosynthesis of Pyrimidine

Carbamoyl phosphate synthetase? (CPS?)
Pyrimidine
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Metabolism of the Carbon Skeleton of Amino acids
AAs
glucose FA and ketone bodies TCA
Carbon skeletons
Amino acid
CO2 , H2O and ATP
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Metabolism of the Carbon Skeleton of Amino acids

• Ketogenic amino acids can be degraded to acetyl
  CoA or acetoacetyl CoA , which are precursor of
  producing ketone bodies.
• Leu, Lys
• Glucogenic amino acids can be degraded to the
  carbon skeletons which can be converted into
  glucose.
• Ketogenic and glucogenic amino acids
• Ile, Phe, Trp, Tyr, Thr

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Amino Acid Catabolism Individual

• Decarboxylation of Amino Acid

Physiological effects
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• ?-Aminobutyric Acid (GABA)
• GABA is inhibitory neurotransmitter.

L-glutamate decarboxylase
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• Histamine
• vasodilation
• stimulate the secretion of pepsin and
  hydrochloric acid

Bronchial asthma, allergic reaction
Histidine decarboxylase
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• 5-Hydroxytryptamine (5-HT)
• inhibitory neurotransmitter in brain
• cause contraction of smooth muscle of
  arterioles and bronchioles

Tryptophan
5-Hydroxytryptamine
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• Polyamine important to cell proliferation and
  tissue growth.

Ornithine decarboxylase
Ornithine
Putrescine
CO2
S-adenosylmethionine (SAM )
Decarboxylased SAM
Methythio-adenosine
CO2
spermine
spermidine
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Metabolism of One Carbon Units

• One Carbon Units One carbon containing groups
  produced in catabolism of some amino acids.

-CH3
methyl
methylene
-CH2-
methenyl
-CH
formyl
-CHO
formimino
-CHNH
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Carrier of one carbon unit tetrahydrofolate (FH4)
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N5, N10 nitrogen atoms involved in the transfer
of one carbon units.
N5Methyl FH4
N5?N10Methylene FH4
N5?N10-MethenylFH4
N10FormylFH4
N5FormiminoFH4
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Production of one carbon unit
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Conversion of one carbon units
N10CHOFH4
H
H2O
NH3
N5, N10CHFH4
N5CHNHFH4
NADPHH
NADP
N5, N10CH2FH4
NADHH
NAD
N5CH3FH4
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Function of one carbon units
As donors of one carbon units in purine and
pyrimidine synthesis.
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Metabolism of Methionine, Cysteine and Cystine
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SAM
Macrocytic anemia
Homocysteine methyltransferase (Vitamine B12)
Methionine cycle
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Creatine and creatine phosphate
Storage of high energy phosphate of ATP
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Cysteine and Cystine

• Conversion of cysteine to cystine

Cysteine
Cystine
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• Systhesis of Taurine

1.Form conjugant with bile acid. 2.Involved in
brain development
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Formation of PAPS
Sulfate is produced mostly from cysteine.
AMP - SO3-
(adenosine -5-phosphosulfate)
3-PO3H2-AMP-SO3-
(3-phosphoadenosine-5-phosphosulfate,PAPS)
PAPS is the active sulfate group for biosynthesis.
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Glutathione (GSH)
?-glutamylcysteinylglycine

• transport amino acids across membranes.
• Protect erythrocytes from oxidative damage.

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?-glutamyl cycle
Cell membrane
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Metabolism of aromatic amino acids
aromatic amino acids
phenylalanine
Tyrosine
Tryptophan
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Phenylalanine and Tyrosine
The main catabolism pathway of phenylalanine is
hydroxylated to tyrosine first.
Phenylalanine hydroxylase
O2
H2O
phenylalanine
Tyrosine
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Phenylketonuria (PKU)

• Normally, small amount of phenylalanine can be
  transaminated to form phenylpyruvate.
• If phenylalanine hydroxylase is genetic
  deficiency, transamination become the main
  catabolism pathway, which results in high level
  of phenylpyruvate and phenyllacetate in the
  urine, called PUK

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transamination
p
phenyllacetate
phenylalanine
phenylpyruvate
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Metabolism of tyrosine

• Production of Dopamine, Epinephrine and
  Norepinephrine.

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Parkinsons disease
character
slowing of emotional and voluntary movement,
muscular rigidity, postural abnormality and
tremor
Reasons

• deficiency of Dopamine

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Metabolism of tyrosine

• Synthesis of Melanin
• Tyrosine Dopa
  Melanin
• Genetic lack of tyrosinase will cause Albinism.
• --lack of pigment in the skin and eyes.
• --sensitive to sunlight
• --photophobia

tyrosinase
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• Production of thyroid Hormone
• thyroxine (T4) and triiodothyronine (T3)

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Metabolism of tryptophan
One carbon units
tryptophan
pyruvate acetoacetyl CoA
Melatonin
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Degradation of branched-chain amino acid
branched-chain amino acid
isoleucine
valine
leucine
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Succinyl CoA
Succinyl CoA

Acetyl CoA
Acetyl CoA

Acetoacetate