Amino acid metabolism 1Dr.S.Chakravarty,MD

Learning objectives

• Explain the steps in synthesis of various non-essential amino acids in the body

• List the molecules derived from aromatic amino acids and their uses

• Discuss the enzyme deficiencies of aromatic amino acid metabolism and their clinical features

• Differentiate various types of phenylketonuria and its diagnosis

• Discuss the clinical features of Alkaptonuria and its treatment

Biosynthesis of

Non Essential Amino-acids

1. Glutamate :

Alpha keto Glutarate Glutamate

Glutamate Dehydrogenase

NH3

2. Glutamine:

Glutamate Glutamine

NH3

Glutamine synthase

Glutamate dehydrogenase

Glutaminesyhthetase

3. Alanine:

Pyruvate Alanine ALT

Glutamate Alpha keto glutarate PLP

4. Aspartate :

Oxaloacetate Aspartate

Glutamate Alpha keto glutarate

AST

PLP

Transamination reactions

5. Asparagine:

Aspartate Asparagine

NH3

Asparagine synthase

6. Tyrosine :

Tyrosine Phenylalanine Phenyl alanine Hydroxylase

THB DHB

7. Glycine - Glycine amidotransferases synthesize glycine from glyoxylate and glutamate or alanine.Two other reactions make glycine :-

serine hydroxymethytransferase reaction(freely reversible)

• 8. Serine :- Two ways to make it – Reversal of serine hydroxymethytransferase

reaction

9. Proline – from Glutamate -reversal reaction of proline catabolism

10. Cysteine:

Methionine

S- Adenosyl Methionine

S- Adenosyl Homocystiene

Homocysteine

Cystathionine

Cysteine

Serine

Alpha-keto butyrate

Methyl THF

THF

(CH3 1-carbon )B12

B6

B6

Cystathionine β synthase

Cystathioninase

Homocysteine methyl Transferase

Methionine adenosyl transferase

Methyl transferase

ATP

Acceptor

CH3-acceptor

Metabolism of Aromatic amino acids

Metabolism of Aromatic amino acids:

• Phenylalanine – essential • Tyrosine – non essential

• Tryptophan – essential

PHENYLALANINE AND

TYROSINE

TYROSINE

SYNTHESIS OF THYROID

HORMONES

SYNTHESIS OF CATECHOLAMINES eg

Epinephrine and Norepinephrine

MELANIN SYNTHESIS

CATABOLISM TO ACETOACETIC ACID

(KETOGENIC ) + FUMARIC ACID

FATES OF TYROSINE IN BODY

P h e n y l a l a n i n e h y d r o x y l a s e r e a c ti o n

SYNTHESIS OF TYROSINE FROM PHENYLALANINE

Dihydrobiopterin reductase

Phenylalanine Tyrosine

DOPA

Dopamine

Nor-Epinephrine

Epinephrine

Tyrosine Hydroxylase

DOPA Decarboxylase

Dopamine β oxidase

Phenylethanolamine N-methyl Transferase (NMT)

THB

DHB

Ascorbate(Vitamin C)

Dehydro-Ascorbate

S- Adenosyl Methionine (SAM)

S- Adenosyl Homocysteine (SAH)

Catecholamine synthesis

Dihydrobiopterin reductase

B6

O2,

Cu2+

Important – alpha methyl DOPA inhibits dopa decarboxylase and prevents hypertension by decreasing epinephrine

metanephrine Vanillyl mandelic acid

Phenylalanine hydroxylase

Catechol-o-methyl Transferase (COMT) mono amine oxidase(MAO)

Arvid CarlssonM.D. Nobel Prize 2000 alongwithEric Kandel and Paul Greengard.

CNS and ADRENAL MEDULLA

co2

Diseases associated with catecholamine synthesis:

• Schizophrenia – Dopamine overproduction

• Parkinson’s disease :

Damage to Nigro-striatal tract - Dopamine

Treatment: – Levo-DOPA + Carbidopa Carbidopa is PERIPHERAL DOPA-DECARBOXYLASE INHIBITOR it increases the plasma half-life of

levodopa from 50 minutes to 1½ hours. Carbidopa cannot cross the blood brain barrier, so it inhibits only peripheral DDC.

It thus prevents the conversion of L-DOPA to dopamine peripherally

• Pheochromocytoma • Neuroblastoma

Increased catecholamine production

IMPORTANCE OF VMA estimation

• Some tumors like Pheochromocytoma (epinephrine excess ) or Neuroblastoma

• Excess of VMA in urine Lab analysis

Formation of Melanin Tyrosine

DOPA

Dopaquinone

Melanin

Tyrosinase(Melanoblasts )

Copper

Copper

TYROSINASE IS

ABSENT IN

ALBINISM

NO MELANINSeveral steps

Tyrosinase(Melanoblasts )

THIS ALSO EXPLAINS HYPOPIGMENTATION IN PHENYLKETONURIA !!

Formation of thyroid hormones

• T3 Triiodothyronine

• T4 Thyroxine

Phenylalanine

Tyrosine

Parahydroxyphenyl pyruvate

Homogentisic acid

Fumarate Acetoacetate

Phenyl alanine Hydroxylase

Tyrosine Aminotransferase

Homgentisate oxidase

Fumaryl acetoacetate hydrolase

Phenylketonuria

Tyrosinemia- II

Alkaptonuria

Tyrosinemia- I

Catabolism of phenylalanine and tyrosine

PLP

P-Hydroxyphenyl pyruvate hydroxylaseCu, Vit C

USMLE !!

CATABOLISM OF TYROSINE

1

2

3

4

Type II TYROSINEMIA

NEONATAL TYROSINEMIA

ALKAPTONURIA

TYPE I TYROSINEMIA

Tyrosinemia Type 1

• Defect in fumarylacetoacetate hydrolase

• Plasma tyrosine levels elevated (6-12mg/dl)]– ACUTE FORM – FATAL BY 6-8 MONTHS – CHRONIC FORM- 10 YEARS

• DIARRHOEA• VOMITING• CABBAGE LIKE ODOR• LIVER FAILURE

• URINE :-tyrosine, p -hydroxyphenyl pyruvate,P- hydroxyphenyl lactate , p- hydroxyphenyl acetate

Tyrosinemia Type II(Richner Hanhart Syndrome)

DEF. OF TYROSINE AMINOTRANSFERASE• Mental retardation• Keratosis of palmar surface• Painful corneal lesions• Photophobia

NEONATAL TYROSINEMIA • Def . Of p-hydroxyphenyl pyruvate hydroxylase

Phenylketonuria Phenylalanine

Phenylpyruvate Tyrosine

(-)

Phenylalanine hydroxylase defect

Dihydrobiopterin reductase defect

Phenyl acetate Phenyl Lactate

Phenyl acetyl Glutamine

ReductionDecarboxylation

Conjugation with glutamine

Transaminase

ALTERNATE ROUTES OF METABOLISM OF PHENYLALANINE

Phenylketonuria • Autosomal recessive disease – MC disorder of amino acid metabolism

• Def of phenylalanine hydroxylase or Dihydrobiopterin reductase.

• Increased phenylalanine in the blood

• Saturates – LNAAT (large neutral aminoacid transporter system of brain).

• mental retardation, seizures-– Poor protein and neurotransmitter synthesis in brain – Toxicity from accumulating alternate metabolites like phenylketones

• Decreased pigmentation of skin and eyes.

USMLE !!

Cont..• Mousy/ Musty odor of urine – phenyl acetate, phenyl lactate and phenylpyruvate

in urine. • National biochemical screening programme • Blood sample – Heel filter paper analytical laboratory ( PCR + HPLC or TANDEM

MASS SPECTROMETRY)– Screens diseases like :-

• Cystic fibrosis • PKU• Congenital hypothyroidism• Medium chain acyl CoA dehydrogenase deficiency

• FeCl3 test – Ferric chloride test.

• Guthrie test: Gold standard of the past– Certain strains of Bacillus Subtilis need Phe as essential growth factor.Bacterial growth

cannot occur in medium devoid of Phe.– So, bactera will grow if blood containing Phe is added = PHENYLKETONURIA

Amino Acid DisordersArgininosuccinic aciduria (ASA) Citrullinemia, type I (CIT) Classic phenylketonuria (PKU) Homocystinuria (HCY) Maple syrup urine disease (MSUD) Tyrosinemia, type I (TYR I) Tyrosinemia, type II (TYR II) Endocrine DisordersCongenital adrenal hyperplasia (CAH) Primary congenital hypothyroidism (CH)

Fatty Acid Oxidation DisordersCarnitine acylcarnitine translocase deficiency (CACT) Carnitine palmitoyltransferase I deficiency (CPT-IA) Carnitine palmitoyltransferase type II deficiency (CPT-II) Carnitine uptake defect (CUD) Glutaric acidemia, type II (GA-2) Long-chain L-3 hydroxyacyl-CoA dehydrogenase deficiency (LCHAD) Medium-chain acyl-CoA dehydrogenase deficiency (MCAD) Short-chain acyl-CoA dehydrogenase deficiency (SCAD) Trifunctional protein deficiency (TFP) Very long-chain acyl-CoA dehydrogenase deficiency (VLCAD)

NEONATAL SCREENING IN FLORIDA(SOURCE CDC website and http://www.babysfirsttest.org)

Hemoglobin DisordersS, Beta-thalassemia (Hb S/ßTh) S, C disease (Hb S/C) Sickle cell anemia (Hb SS) Organic Acid Conditions3-Hydroxy-3-methylglutaric aciduria (HMG) 3-Methylcrotonyl-CoA carboxylase deficiency (3-MCC) Beta-ketothiolase deficiency (BKT) Glutaric acidemia type I (GA1) Holocarboxylase synthetase deficiency (MCD) Isovaleric acidemia (IVA) Methylmalonic acidemia (cobalamin disorders) (Cbl A,B) Methylmalonic acidemia (methymalonyl-CoA mutase deficiency) (MUT) Propionic acidemia (PROP) Other DisordersBiotinidase deficiency (BIOT) Classic galactosemia (GALT) Cystic fibrosis (CF) Hearing loss (HEAR) Severe combined immunodeficiency (SCID)

Treatment • Early detection is VERY IMPORTANT !!

• Diet containing low phenylalanine– ( but NEVER ZERO Phe!!)– FOOD BASED ON TAPIOCA (CASSAVA ) IS HELPFUL

• SPECIAL DIET TILL 5YEARS OF AGE

• SPECIAL DIET AGAIN IF PERSON IS PREGNANT LATER ON Excess Phe affects brain development of fetus .

Phenylketonuria

Phenylalanine Hydroxylase def

• Normal levels of dopamine

• Normal levels of prolactin

• Normal levels of catecholamines

• Normal levels of tryptophan and serotonin

Dihydrobiopterin reductase def

• Low levels of dopamine • High levels of prolactin

• Low levels of cathecolamines

• Increased tryptophan and decreased serotonin

Alkaptonuria • Autosomal Recessive

• Deficient enzyme: Homogentisate 1,2-dioxygenase/ (Oxidase)– conversion of homogentisic acid (product of tyrosine metabolism) to

maleylacetoacetate (→ acetoacetate → Fumarate → TCA)

• Pathology– Homogentisic acid accumulates, auto-oxidizes– Oxidized homogentisate polymerizes, forms dark-colored pigment– Purplish black color of urine on standing

• Precipitates of dark homogentisic acid (Alkaptan bodies) deposit in connective tissue discoloration (ochronosis)– e.g. in cartilage, joints, ear wax– vertebrae– deposits cause Arthralgia (joint pain)– sometimes associated with degenerative arthritis

Main Fates of Tryptophan

Tryptophan

Synthesis of Serotonin and Melatonin

Catabolized to Acetoacetyl CoA (KETOGENIC )

+alanine (Glucogenic )

INDICAN

Niacin

Tryptophan

5-HydroxyTryptophan

5-HydroxyTryptamine (Serotonin)

N-acetyl Serotonin

Melatonin

THB

DHB

PLP

SAM

SAH

Tryptophan Hydroxylase

MELATONIN SYNTHESIS

NADPH + H +

NADP

Acetyl CoA 5 HYDROXY INDOLE ACETIC ACID ( HIAA)

Monoaminooxidase-A(MAO)

A.A. decarboxylase

Acetylation

Acetylation

methylation

Serotonin

5- Hydroxy indole acetic acid (HIAA)

Monoaminooxidase-A(MAO)

MAO-A inhibitors(Anti-Depressants) (-)

Catabolism of serotonin

MAO- mono amino oxidase

• Epinephrine, norepinephrine, serotonin and melatonin are metabolised by MAO- A enzymes

• Dopamine, Tyramine and tryptamine are metabolised by both MAO-A and MAO-B

• Tyramine mimics catecholamines in their actions

Cheese reaction • A patient presents with headaches, palpitations, nausea and

vomiting and elevated blood pressure. These symptoms appear after the person has eaten a large meal containing aged cheeses and wine. The patient’s history indicates that he is on some medicaton for a different condition. Assuming that the medication is in some way involved in these symptoms, which enzyme might be the target of this drug?

A. Glutamate decarboxylase B. Monoamine oxidaseC. Tyrosine hydroxylaseD. DOPA decarboxylaseE. COMT (catechol O-methyl transferase)

Tryptophan

N-formyl kynurenine

3-hydroxykynurenine

PLP3-hydroxy Anthranilic acid(HIAA )

Acetoacetyl Co-ANAD, NADP (Niacin)

Xanthurenic acid

Acetyl Co-A

TCA cycle

Tryptophan catabolism

60mg tryptophan = 1 mg NIACIN

Tryptophan pyrrolase

Kynurenine formylaseTHFA

Formyl THFA 1 CARBON POOL

Kynureninase H2O

Alanine

Diseases associated with tryptophan Metabolism

A. Carcinoid syndrome :- (Argentaffinomas )1. Neuroendocrine tumors – Midgut, bronchus2. Excessive serotonin and kallikrenin.3. Diarrhoea , flushing, abdominal cramps, 4. Heart failure – damage to valves5. Diagnosis : HIAA in urine

• Pellagra like synptoms : Def of B6 or Tryptophan

Diarrhoea, Dementia and Dermatitis• Remember – Hartnups disease

• Melatonin: promotes sleep – sleep wake cycle1. Hormone of the dark – blue light inhibits

melatonin synthesis.2. Lowers Leptin levels

• Tryptophan load test – B6 deficiency

Depression

• Decrease in serotonin levels in CNS• Treatment : 1. MAO-A inhibitors2. SSRIs – selective serotonin reuptake

inhibitors

Mcq

• The non essential amino acid that becomes essential in PKU is :-

• A. Phenylanaline• B.Tyrosine• C.Tryptophan • D. ALANINE• E. Cysteine

Mcq

• The cause of light skin color in PKU is • A. decreased synthesis of melanin from Phe• B. decreased synthesis of melanin from Tyr• C. excess melanin synthesis from Phe• D. excess of phenylketones• E.mental retardation causes decreased

melatonin

Thank you