Amino Acid Metabolism

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Amino Acid Metabolism

• Hanley N. Abramson
• Professor of Pharmaceutical Sciences
• Wayne State University
• November 2008

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Dynamics of Protein And Amino Acid Metabolism
Dietary Proteins Digestion to Amino
Acids Transport in Blood to Cells
Protein Synthesis Functional
Proteins Protein Degradation In
Proteasomes Following Tagging With
Ubiquitin
Amino Acids Metabolites
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Digestion of Proteins
Stomach Pepsinogen Pepsin (max. act. pH
2) Small Intestine Trypsinogen
Trypsin Trypsin cleaves Chymotrypsinogen
to chymotrypsin Proelastase to elastase Procarboxy
peptidase to carboxypeptidase Aminopeptidases
(from intestinal epithelia)
Enteropeptidase
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Intestinal Absorption
Amino Acids Oligopeptides
Lumen
Transport Protein
Oligopeptides
Peptidases
Amino Acids
Blood
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Incorporation of NH4 Into Organic Compounds
Carbamoyl Phosphate Synthase I (CPS-I)
1) NH4 HCO3- 2 ATP
NH2CO2PO3-2 2 ADP
Carbamoyl Phosphate Pi 2 H 2)
NH4
TCA Cycle
mitochondria
Glutamate dehydrogenase
a-Ketoglutarate
Glutamate
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Incorporation of NH4 Into Organic Compounds
(Cont.)
3)
NH4 2 ATP
Glutamate
Glutamine Synthase Mg
Glutamine
N of glutamine donated to other compounds in
synthesis of purines, pyrimidines, and other
amino acids
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Biosynthesis of Amino Acids Transaminations
Amino Acid1 a-Keto Acid2 Amino Acid2
a-Keto Acid1

Glutamate
Pyridoxal phosphate (PLP)- Dependent
Aminotransferase

a-Ketoglutarate
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Transaminations Role of PLP
Tautomerization
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Transaminations
Glutamate-Pyruvate Aminotransferase (Alanine
Transferase ALT)
Glutamate a-Ketoglutarate
Pyruvate
Alanine Glutamate a-Ketoglutarate
Oxaloacetate
Aspartate
Glutamate-Oxaloacetate Aminotransferase (Aspartate
Transferase AST)
Blood levels of these aminotransferases, also
called transaminases, are important indicators of
liver disease
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Metabolic Classification of the Amino Acids

• Essential and Non-essential
• Glucogenic and Ketogenic

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Non-Essential Amino Acids in Humans

• Not required in diet
• 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)

Essential amino acids
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Essential Amino Acids in Humans

• Required in diet
• Humans incapable of forming requisite
• carbon skeleton

• Arginine
• Histidine
• Isoleucine
• Leucine
• Valine

• Lysine
• Methionine
• Threonine
• Phenylalanine
• Tryptophan

Essential in children, not in adults
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Glucogenic Amino Acids

• Metabolized to a-ketoglutarate, pyruvate,
  oxaloacetate, fumarate, or succinyl CoA
• Phosphoenolpyruvate Glucose

• 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

• Metabolized to acetyl CoA or acetoacetyl CoA
• Animals cannot convert acetyl CoA or
• acetoacetyl CoA to pyruvate

• Isoleucine
• Leucine
• Lysine
• Threonine

• Tryptophan
• Phenylalanine
• Tyrosine

Leucine and lysine are only ketogenic
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Amino Acids Formed From a-Ketoglutarate
4 Steps
a-Keto- glutarate
Transamination or Glutamate dehydrogenase
Proline
5 Steps
Glutamate
Ornithine
Glutamine synthase
Urea Cycle
Arginine
Glutamine
Guanidino group
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GABA Formation
Glutamate decarboxylase
Gamma-aminobutyrate (GABA)
Glutamate
CO2
GABA is an important inhibitory
neurotransmitter in the brain Drugs (e.g.,
benzodiazepines) that enhance the effects of GABA
are useful in treating epilepsy
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Arginine Synthesis The Urea Cycle
N-Acetylglutamate synthase
CoASAc
Glutamate
N-Acetylglutamate
Activates
4 Steps
Carbamoyl phosphate
CPS-I
NH4 HCO3- NH2CO2PO3-2
Ornithine
Ornithine Transcarbamoylase (OTC) (mitochondria)
Citrulline
Ureido group
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The Urea Cycle (Contd.)
Asp
Citrulline
Arginosuccinate synthase
Arginosuccinate
Ornithine Transcarbamoylase (mitochondria)
Fumarate
Urea
H2NCONH2
Argino- succinase
Ornithine
TCA Cycle
Arginase
These reactions occur in the cytosol
Arginine
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Urea Formation

• Occurs primarily in liver excreted by kidney
• Principal method for removing ammonia
• Hyperammonemia
• Defects in urea cycle enzymes (CPS, OTC, etc.)
• Severe neurological defects in neonates
• Treatment
• Stop protein intake
• Dialysis
• Increase ammonia excretion Na benzoate, Na
  phenylbutyrate, L-arginine, L-citrulline

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Blood Urea Nitrogen

• Normal range 7-18 mg./dL
• Elevated in amino acid catabolism
• Glutamate N-acetylglutamate
• CPS-1 activation
• Elevated in renal insufficiency
• Decreased in hepatic failure

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Synthesis of Nitric Oxide
Arginine
Nitric oxide synthase (NOS)
NO
Citrulline
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Nitric Oxide

• Cell messenger
• Implicated in a wide range of physiological
• and pathophysiological events
• Vasodilation
• Activates guanylyl cyclase cGMP
• Nitroglycerin Glycerin NO
• Sildenafil (Viagra) in vascular smooth muscle
• NO cGMP GMP

Blocks
Phospho- diesterase-5
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Formation of Serine
Dehydrogenase
Glycolysis
Glucose
NAD NADH H
3 Steps
3-Phospho- glycerate
3-Phospho- hydroxypyruvate
Pyruvate
Inhibits
Glutamate
Transaminase
a-Ketoglutarate
Phosphatase
Serine (Ser)
3-Phosphoserine
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Conversion of Serine to Glycine
Dihydrofolate reductase
Folate
Serine
Tetrahydrofolate (FH4)
Serine hydroxymethyl transferase (PLP-dep.)
Key intermediate in biosynthesis of purines
and formation of thymine
Glycine
Important in biosynthesis of heme, porphyrins,
and purines
N5, N10-Methylene FH4
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Sulfur-Containing Amino Acids
Methionine Synthase (Vit. B12-dep.)
5-Methyl FH4
FH4
L-Homocysteine
Methionine (Essential)
Cystathionine b-synthase (PLP-dep.)
Serine
Cystathionine lyase

Cystathionine
b-Hydroxy- butyrate
Cysteine (Non-essential)
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Homocysteine

• Homocysteinuria
• Rare deficiency of cystathionine b-synthase
• Dislocated optical lenses
• Mental retardation
• Osteoporosis
• Cardiovascular disease death
• High blood levels of homocysteine associated with
• cardiovascular disease
• May be related to dietary folate deficiency
• Folate enhances conversion of
• homocysteine to methionine

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Methionine Metabolism Methyl Donation
S-Adenosyl methionine synthase
ATP
Methionine
SAM Decarboxylase
S-Adenosyl Methionine (SAM)
Decarboxylated SAM
CO2
R-H
Methyl- transferases

R-CH3
S-Adenosyl homocysteine
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Polyamine Biosynthesis
Ornithine decarboxylase (ODC) (PLP-dep.)
Ornithine (from urea cycle)
Putrescine
CO2
Decarboxylated SAM
Spermidine synthase
5-Methylthio- adenosine
Spermine synthase
Spermine
5-Methylthio- adenosine
Spermidine
Decarboxylated SAM
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Polyamines

• Spermidine and spermine found in virtually
• all procaryotic and eucaryotic cells
• Precise role undefined
• Bind to nucleic acids
• Inhibition of biosynthetic pathway

a-Difluoromethyl- ornithine (DFMO) (Eflornithine)
- inhibits ODC used to treat Pneumocystis
carinii infectons
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Creatine and Creatinine
Arginine-glycine transamidinase (Kidney)
Glycine Ornithine
Arginine
Guanidoacetate
SAM ATP S-Adenosyl- homocysteine ADP
Guanidoacetate Methyltransferase (Liver)
Creatinine (Urine)
Non-enzymatic (Muscle)
Creatine kinase (Muscle)
ADP Pi
Phosphocreatine
Creatine
ATP
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Creatine and Creatinine

• Creatine
• Dietary supplement
• Used to improve athletic performance
• Creatinine
• Urinary excretion generally constant
• proportional to muscle mass
• Creatinine Clearance Test
• Compares the level of creatinine in urine (24
  hrs.)
• with the creatinine level in the blood
• Used to assess kidney function
• Important determinant in dosing of several drugs
• in patients with impaired renal function

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Histidine Metabolism Histamine Formation
Histidine decarboxylase
Histidine
Histamine
CO2

• Histamine
• Synthesized in and released by mast cells
• Mediator of allergic response vasodilation,
  bronchoconstriction
• (H1 receptors)
• H1 blockers Diphenhydramine (Benadryl)
• Loratidine (Claritin)
• Stimulates secretion of gastric acid (H2
  receptors)
• H2 blockers Cimetidine (Tagamet) ranitidine
  (Zantac)

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Phenylalanine and Tyrosine
Phenylalanine (Essential)
Tetrahydrobiopterin (BH4)

O2
Phenylalanine-4- Monooxygenase (Phenylalanine hydr
oxylase)
NADP
H2O
NADPH H
Dihydrobiopterin

Tyrosine (Non-essential)
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Normal Utilization of Phenylalanine
Protein (25)
Phenylalanine
Tyrosine (75)
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Phenylketonuria (PKU) Disease

• Deficiency of Phe hydroxylase
• Occurs in 120,000 live births in U.S.
• Seizures, mental retardation, brain damage
• Treatment limit phenylalanine intake
• Screening of all newborns mandated in all states

Tyr
Phe
Phenylpyruvate (urine)
Transamination
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Sapropterin (Kuvan)
Indicated for reduction of phenylalanine
blood levels in patients with BH4-responsive PKU
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Catecholamine Biosynthesis
Catechol
Tyr hydroxylase
O2
Tyrosine
Dihydroxyphenylalanine (DOPA)
DOPA decarboxylase
Epinephrine (Adrenaline)
CO2
Dopamine hydroxylase
Methyl transferase
S-Adenosyl- homocysteine
Dopamine
SAM
Norepinephrine
DOPA, dopamine, norepinephrine, and epinephrine
are all neurotransmitters
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L-DOPA in Parkinsonism
Blood Brain
L-DOPA L-DOPA Dopamine
Blocks
Parkinsonism associated with dopamine in
brain through loss of neurons in basal
ganglia. Carbidopa L-DOPA
Carbidopa
Dopamine
Blood Brain Barrier
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Monoamine Oxidase (MAO)
MAO (in mitochondria)
R R OH H Norepi OH CH3
Epi H H Dopamine
Aldehyde dehydrogenase
Urinary metabolite
MAO inhibitors (e.g., tranylcypromine) are useful
in the treatment of depression Brain levels
of dopamine and norepi. also serotonin
ROH Vanillylmandelic acid (VMA) RH Homovanillic
acid (HVA)
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Tyramine
MAO
Tyramine
( blood pressure)

• Tyramine found naturally in several types of
  cheese
• also beer and red wine.
• Tyramine intake can cause hypertensive crisis in
• persons taking a MAO inhibitor ( norepi
  release)

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Catechol-O-Methyl Transferase (COMT)
COMT
SAM
S-Adenosyl- homocysteine
Active catecholamine
Inactive metabolite

• COMT found in cytoplasm
• Terminates activity of catecholamines
• Catecholamine excretion products result from
• combined actions of MAO and COMT
• Inhibitors of COMT (e.g., tolcapone) useful
• in Parkinsons disease

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Homogentisic Acid Formation
Transamination
Tyrosine
p-Hydroxyphenyl- pyruvate
O2
Deficient in alkaptonuria
p-Hydroxyphenyl- pyruvate dioxygenase (ascorbate-d
ep.)
Homogentisate dioxygenase
Cleavage of aromatic ring
CO2
O2
Fumarate acetoacetate
Homogentisate
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Alkaptonuria

• Deficiency of homogentisate dioxygenase
• Urine turns dark on standing
• Oxidation of homogentisic acid
• Asymptomatic in childhood
• Tendency toward arthritis in adulthood

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Melanin Formation
Tyr hydroxylase
O2
Tyrosine
DOPA
Tyrosinase
Highly colored polymeric intermediates
Melanin (Black polymer)
Melanin formed in skin (melanocytes), eyes, and
hair In skin, protects against sunlight Albinism
genetic deficiency of tyrosinase
Dopaquinone
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Tryptophan Metabolism Serotonin Formation
Indole ring
Trp hydroxylase
Decarboxylase
O2
5-Hydroxy- tryptophan
Tryptophan (Trp)
CO2
5-Hydroxy- tryptamine (5-HT) Serotonin
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Serotonin

• Serotonin formed in
• Brain (neurotransmitter regulation of sleep,
  mood, appetite)
• Platelets (platelet aggregation,
  vasoconstriction)
• Smooth muscle (contraction)
• Gastrointestinal tract (enterochromaffin cells -
  major storage site)
• Drugs affecting serotonin actions used to treat
  
• Depression
• Serotonin-selective reuptake inhibitors (SSRI)
• Migraine
• Schizophrenia
• Obsessive-compulsive disorders
• Chemotherapy-induced emesis
• Some hallucinogens (e.g., LSD) act as serotonin
  agonists

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L-Tryptophan

• Food supplement promoted for serotonin effects
• L-Tryptophan disaster (1989)
• Eosinophilia-myalgia syndrome (EMS)
• Severe muscle and joint pain
• Weakness
• Swelling of the arms and legs
• Fever
• Skin rash
• Eosinophilia
• Many hundreds of cases several deaths
• Traced to impurities

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Serotonin Metabolism 5-HIAA
MAO
Serotonin
Dehydrogenase

• Carcinoid tumors
• Malignant GI tumor type
• Excretion of large amounts of 5-HIAA

5-Hydroxyindole acetic acid (5-HIAA) (Urine)
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Serotonin Metabolism Melatonin
2 Steps
Serotonin
Melatonin

• Melatonin
• Formed principally in pineal gland
• Synthesis controlled by light, among other
  factors
• Induces skin lightening
• Suppresses ovarian function
• Possible use in sleep disorders

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Tryptophan MetabolismBiosynthesis of Nicotinic
Acid
Several steps
Tryptophan
Nicotinic acid (Niacin)
Nicotinamide adenine dinucleotide (NAD)