Phenylalanine Conversion to tyrosine Tyrosine Biosynthesis Metabolic products Functions Disorders of tyrosine metabolism

1. PHENYLALANINE &
TYROSINE
METABOLISM
Dipesh Tamrakar
MSc. Clin. Biochemistry
1

2. OVERVIEW
 Phenylalanine
Conversion to tyrosine
 Tyrosine
Biosynthesis
Metabolic products
Functions
Disorders of tyrosine metabolism
2

3. Phenyl alanine
 Aromatic & essential amino acid
 Both Glucogenic & Ketogenic.
 Phenylalanine is converted to tyrosine.
 Phenomenon: 'sparing action' of tyrosine on
phenylanine.
 Predominant metabolism of phenylalanine occurs
through tyrosine and incorporated into various
biologically important compounds.
 epinephrine, norepinephrine, dopamine, thyroid
hormones & the pigment melanin.
3

4. Conversion to Tyrosine
 Degradation of phenylalanine mostly occurs
through tyrosine.
 Phenylalanine is hydroxylated at para-position by
phenylalanine hydroxylase to produce tyrosine.
 This reaction is irreversible, & requires specific
coenzyme biopterin, which is structurally related to
folate.
 Active form of biopterin is tetrahydrobiopterin.
 Tetrahydrobiopterin is oxidized to
dihydrobiopterin.
 Phenylalanine hydroxylase is present in liver.
4

5.  Tetrahydrobiopterin is then regenerated by an
NADPH-dependent dihydrobiopterin reductase.
 In the conversion of phenylalanine to tyrosine, the
reaction involves incorporation of one atom of
molecular oxygen into paraposition of
phenylalanine while the other atom O2 is reduced
to form water.
 Tetrahydrobiopterin supplies reducing
equivalents.
5

6. 6

7. TYROSINE
 aromatic amino acid
 synthesized from phenylalanine, and so is a
non-essential amino acid
 glucogenic and partly ketogenic.
Degradation of tyrosine
 Phenylalanine is converted to tyrosine, a single
pathway is responsible for the degradation of
both these amino acids.
 Occurs mostly in liver. Tyrosine first undergoes
transamination to Para-hydroxyphenylpyruvate,
catalyzed by tyrosine transaminase (PLP
dependent)
7

8. Production of homogentisic acid
 p-hydroxy phenylpyruvate hydroxylase (or
dioxygenase) is a copper-containing enzyme.
 It catalyzes oxidative decarboxylation as well as
hydroxylation of the phenyl ring of p-hydroxy
phenyl pyruvate to produce homogentisate.
 This reaction involves a shift in hydroxyl group
from para position to meta position &
incorporates a new hydroxyl group at para
position to give 2,5-dihydroxyphenylacetic acid or
homogentisic acid.
 This step requires ascorbic acid.
8

9.  Homogentisate oxidase (iron metalloprotein)
cleaves the benzene ring of homogentisate to
form 4-maleylacetoacetate.
 Molecular oxygen is required for this reaction to
break the aromatic ring.
 4-Maleylacetoacetate undergoes isomerization to
form 4-fumaryl acetoacetate. Catalyzed by
maleylacetoacetate isomerase.
 Fumaryl acetoacetase (fumaryl acetoacetate
hydrolase) brings about the hydrolysis of fumaryl
acetoacetate to liberate fumarate & acetoacetate.
 Fumarate is an intermediate in citric acid cycle &
can serve as precursor for gluconeogenesis.
9

10. 10

11. 11

12. Products from Tyrosine
1. MELANIN
2. THYROXINE
3. CATECHOLAMINES
12

13. 1. Synthesis of melanin
 Melanin is a pigment of skin, hair & eye.
 The synthesis of melanin occurs in melanosomes
present in melanocytes, the pigment producing
cells.
 Tyrosine is precursor for melanin & only one
enzyme, namely tyrosinase (a copper containing
oxygenase), is involved in its formation.
 Tyrosinase hydroxylates tyrosine to form 3,4
dihydroxy-phenylalanine (DOPA).
 DOPA can act as a cofactor for tyrosinase.
 DOPA is converted to dopaquinone by tyrosinase.
13

14.  Dopaquinone in subsequent couple of reactions
occur spontaneously, forming leucodopachrome
followed by 5,6-dihydroxy indole.
 The oxidation of 5,6-dihydroxyindole to indole
5,6-quinone is catalyzed by tyrosinase.
 DOPA serves as a cofactor.
 This reaction, inhibited by tyrosine regulates the
synthesis of melanin.
 Melanochromes are formed from indole quinone,
which on polymerization are converted to black
melanin.
14

15.  Another pathway:
 Cysteine condenses with dopaquinone & in the
next series of reactions results the synthesis of red
melanins.
 The skin color of the individual is determined by
the relative concentrations of black & red
melanins.
 This, in turn, is dependent on many factors, both
genetic & environmental.
 These include the activity of tyrosinase, the density
of melanocytes, availability of tyrosine etc.
 The presence of moles on the body represents a
localized severe hyperpigmentation due to
hyperactivity of melanocytes.
15

16.  Localized absence or degeneration of melanocytes
results in white patches on the skin commonly
known as leucoderma.
 Albinism is an inborn error with generalized lack of
melanin synthesis.
 Tyrosinase is present in melanoblasts and
produces DOPA ( useful in melanin synthesis)
 Tyrosine hydroxylases is present in adrenal medulla
and the DOPA thus generated is used for
catecholamine synthesis.
 Even in tyrosinase deficient person (albinism)
synthesis of the catecholamines is normal
16

17. 17

18. 2. Biosynthesis of thyroid hormones
 Thyroid hormones – Thyroxine
(tetraiodothyronine) & triiodithyronine – are
synthesized from the tyrosine residues of the
protein thyroglobulin & activated iodine.
 Iodination of tyrosine ring occurs to produce
mono & diiodotyrosine from which
triiodothyronine (T3) & thyroxine (T4) are
synthesized.
 The protein thyroglobulin undergoes proteolytic
breakdown to release the free hormones - T3 &
T4.
18

19. 3. Biosynthesis of catecholamines
 Catecholamines are derived from tyrosine.
 The name catechol refers to the dihydroxylated
phenyl ring (catechol nucleus).
 The amine derivatives of catechol are called
catecholamines.
 Tyrosine is the precursor for the synthesis of
catecholamines, namely dopamine,
norepinephrine (noradrinaline) & epinephrine
(adrinaline)
 Conversion of tyrosine to catecholamines occurs
in adrenal medulla & central nervous system.
19

20.  Tyrosine is hydroxylated to 3,4-
dihydroxyphenylalanine (DOPA) by tyrosine
hydroxylase.
 It is a rate limiting enzyme & requires
tetrahydrobiopterin as coenzyme.
 In contrast to this enzyme, tyrosinase present in
melanocytes converts tyrosine to DOPA.
 DOPA undergoes PLP-dependent
decarboxylation to give dopamine.
 Dopamine is a catecholamine.
 Dopamine is an inhibitor of prolactin secretion
 Dopamine is neurotransmitter in substantia
nigra, extrapyramidal tract, & striatal tract.
20

21.  In Parkinsonism, the dopamine content in
brain is reduced.
 As dopamine will not enter into the brain cells,
the precursor, L-DOPA is used as a drug in
Parkinsonism.
 Alpha methyl DOPA will inhibit DOPA
decarboxylase & prevent production of
epinephrine; so it is an antihypertensive drug.
21

22.  Dopamine is further hydroxylated to
norepinephrine or noradrenaline
 The term “nor” denotes that the molecule does
not contain the "R" or methyl group.
 Nor-epinephrine is methylated by the enzyme
N-methyl transferase to epinephrine or
adrenaline.
 S-adenosyl methionine (SAM) is the methyl
donor.
 It is mainly produced by adrenal medulla &
adrenergic nerve endings.
 Norepinephrine is produced in certain areas of
brain while dopamine is predominantly
synthesized in substantia nigra.
22

23. 23

24. Actions of Epinephrine
 increases in blood pressure
 Adrenaline also increases the rate & force of
myocardial contraction.
 Epinephrine causes relaxation of smooth
muscles of bronchi
 Adrenaline is anti-insulin in nature, it increases
glycogenolysis & stimulates lipolysis.
 Adrenaline is released from adrenal medulla in
response to flight, fight, exercise and
hypoglycemia
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25. Degradation of adrenaline
 The half-life of epinephrine is 2-5 minutes.
 Epinephrine is catabolized in tissues, by catechol-
O-methyl transferase (COMT) to metanephrine.
 It is then acted upon by mono amine oxidase
(MAO).
 MAO will oxidatively deaminate metanephrine.
 The major end product is 3-hydroxy-4- methoxy
mandelic acid or vanillyl mandelic acid (VMA).
 Normally VMA is excreated 2-6 mg/24 hrs
 VMA is Increased in pheochromocytoma and
neuroblastoma
25

26. Homovanillic acid (HVA) in Urine:
 It is also called methoxy hydroxy phenyl acetic
acid.
 HVA is the main urinary metabolite of DOPA &
dopamine.
26

27. 27

28. Disorders of tyrosine metabolism
1. Phenylketonuria (PKU)
2. Tyrosinemia type II
3. Neonatal tyrosinemia
4. Alkaptonuria
5. Tyrosinosis or tyrosinemia type I
6. Albinism
28

29. 1. Phenylketonuria (PKU)
 Most common metabolic disorder in aa
metabolism
 Autosomal recessive with Incidence of PKU is 1 in
10,000 births
 Due to deficiency of the hepatic enzymes,
phenylalanine hydroxylase
 Defect in dihydrobiopterin reductase is also
reported
 PKU primarily causes the accumulation of
phenylalanine in tissues and blood & excretion in
urine
 Disturbance in routine metabolism, phenylalanine
is diverted to alternative pathways resulting in
the excessive production of phenypyruvate,
phenylacetate, phenyllactate and
phenylglutamine: excreted in urine
29

30.  Alternatice pathway
for catabolism of
phenylalnine in
phenylaketonuria.
 phenypyruvate,
 phenylacetate,
 phenyllactate and
 phenylglutamine
30

31.  Phenylketouria is due to keto acids in urine
 Biochemical manifestation:
1. Effect on CNS
 Mental retardation, failure to walk or talk,
failure of growth, seizures and tremor
 Accumulation of phenylalanine in brain impairs
the transport & metabolism of other aromatic
a.a. (tryptophan & tyrosine)
 Impaired synthesis of serotonin
 Defect in myelin formation
31

32. 2. Effect on pigmentation
Melanin is the pigment synthesized from tyrosine
by tyrosinase
Accumulation of phenylalanine competitively
inhibits tyrosinase and impairs melanin formation
The result is hypopigmentation that causes light
skin color, fair hair, blue eyes
3. Elevated levels of phenylalnine, phenylpyruvate,
phenylactate and phenylacetate are found in
plasma & urine giving mousey odor
32

33. Diagnosis:
 Normal level in newborns: 1 -2 mg/dl
 PKU: 20 -65 mg/dl
 Guthrie test: performed after the baby is fed with
breast milk for a couple of days.
 Phenylpyruvate in urine can be detected by ferric
chloride test (green color) – non specific test
 immunoassays using fluorometric or photometric
detection
 amino acid measurement using tandem mass
spectrometry (MS/MS). Measurements done
using MS/MS determine the concentration of Phe
and the ratio of Phe to tyrosine, the ratio will be
elevated in PKU.
33

34. Guthrie test
 The Guthrie test, also called the PKU test, is a diagnostic
tool to test infants for phenylketonuria a few days after
birth.
 To administer the Guthrie test, doctors use Guthrie cards
to collect capillary blood from an infant's heel, and the
cards are saved for later testing.
 Robert Guthrie invented the test in 1962 in Buffalo, New
York.
 Phenylketonuria (PKU) is a congenital birth abnormality
in which toxic levels of the amino acid phenylalanine
build up in the blood, a process that affects the brains in
untreated infants.
 Guthrie's test detects phenylalanine in the blood of
newborns, enabling for early diagnosis of PKU.
 Early diagnoses of PKU prevent the development of
mental disabilities in the thousands of individuals
affected each year.
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35.  β-2-Thienylalanine, an amino acid, inhibits the growth of
the bacteria Bacillus subtilis.
 Guthrie found that phenylalanine, a chemical found in
blood of people with PKU, reversed β-2-Thienylalanine's
inhibition of B. subtilis growth.
 Thus, to conduct a PKU inhibition bacterial assay, Guthrie
coated a gel used to grow bacteria with β-2-
Thienylalanine.
 Guthrie then placed blood samples, dried on thick filter
paper, onto that gel. If the blood sample contained
phenylalanine, B. subtilis grew around the blood sample
and indicated that patient had PKU.
 If the sample of blood did not contain phenylalanine, B.
subtilis did not grow around the blood sample, indicating
that the patient did not have PKU.
 Guthrie noted that a single technician could test one to
200 blood samples in a single day and that each test
required only a finger prick of capillary blood on a filter
paper disc.
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36. 36

37. 2. Tyrosinemia type II
 Richner-Hanhart syndrome
 Defect in enzyme tyrosine transaminase
 Results in blockage in the routine degradative
pathway of tyrosine
 Accumulation and excretion of tyrosine and its
metabolites namely :
p-hydroxyphenylpyruvate,
p-hdroxyphenyllactate,
p-hydroxyphenylacetate,
N-acetyltyrosine and tyramine
 Characterized by skin and eye lesions and rarely
mental retardation
 Disturbed self-coordination is seen in these
patients
37

38. 38

39. 3. Neonatal tyrosinemia
 Caused by absence of enzyme p-hydoxyphenyl
pyruvate dioxygenase
 Mostly temporary condition and usually
responds to ascorbic acid
 Substrate inhibition of the enzyme is overcome
by the presence of ascorbic acid
39

40. 4. Alkaptonuria
 First described by Lusitanus in 1649
 Autosomal recessive disorder with 1 in 25,000
births
 Defective enzyme: homogentisate oxidase in
tyrosine metabolism
 Homogentisate accumulates in tissues and
blood and is excreted into urine
 On standing, Homogentisate gets oxidized to
corresponding quinones, which polymerize to
give black or brown color
 Urine resembles coke in color
 Chromatography for quanitification of
Homogentisate
40

41. Biochemical manisfestations:
 Homogentisate gets oxidized by polyphenol
oxidase to benzoquinone acetate which
undergoes polymerization to produce a
pigment called alkapton
 Deposition occurs in connective tissue, bones
and various organs (nose, ear) resulting in a
condition known as ochronosis
 Arthritis; due to deposition of pigment
alkaptons in the joints
 Treatment by consumption of protein diet
relatively low phenylalanine content
41

42. 42

43. 5. Tyrosinosis or tyrosinemia type I
 Due to deficiency of the enzymes:
fumarylacetoacetate hydroxylase and/or
maleylacetoacetate isomerase
 Rare but serious disorder
 Causes liver failure, rickets, renal tubular
dysfunction and polyneuropathy
 Tyrosine and its metabolites are excreted in
urine
 In acute tyrosinosis, the infant exhibits diarrhea,
vomiting and cabbage-like odor
 Death may seen due to liver failure within 1
year
 Treatment: diets low in tyrosine, phenylalanine
and methionine
43

44. 6. Albinism
 Albino – white
 Inborn error due to lack of synthesis of the
melanin pigment
 Defect in tyrosinase enzyme
 Autosomal recessive disorder with 1 in 20,000
 Biochemical basis:
1. Deficiency or lack of the enzyme tyrosinase
2. Decrease in melanosomes of melanocytes
3. Impairment in melanin polymerization
4. Limitation of substrate (tyrosine) availability
5. Lack of protein matrix in melanosomes
44

45.  Lack of melanin pigments makes skin sensitive
to sunlight
 Increased susceptibility to skin cancer
 Photophobia with lack of pigment in the eyes
45

46. Summary
46

47. Foods rich in phenylalanine
 The recommended daily intake for phenylalanine
and tyrosine is 25 mg per kilogram of body
weight,
1. Soy Foods (Roasted Soybeans)
2. Cheese
3. Seeds & Nuts (Pumpkin Seeds)
4. Lean Beef & Lamb (Roast Beef)
5. Chicken & Turkey (Chicken Breast, cooked)
6. Lean Pork (Chops, cooked)
7. Fish & Seafood (Tuna, cooked)
8. Eggs & Dairy (Eggs)
9. Beans & Lentils (Pinto Beans, cooked)
10.Wholegrains
47

48. Thank-you
48