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
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
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
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
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
24
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
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.
34
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.
35
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
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
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
Tyrosine intake can reduce the dietary requirement of phenylalanine is the phenomenon
Addition of OH group to the aromatic ring. Co enzymes NADPH, NADH and tetrahydrobiopterine
Phenylalnine hydroxylase also k/a phenylalanine-4-monooxygenase: class of mixed-function oxidase
1A enzyme is dihydrobiopterin reductase.
Acetoacetate is ketone body
Greek word Melan means black
Melanin-the color pigment:
The difference between epinephrine & norepinephrine is only a methyl group. In adrenal medulla, synthesis of the hormones, norepinephrine & epinephrine is prominent.
Catabolic pathway for phenylalanine and tyrosine. normally these a.a converted to acetoacetyl CoA and fumarate
Fumarylactone gets converted to succinylatone in tyrosinemia type 1 and ALA also increased
Treatment:
Food selection with low phenylalanine content with periodic adjustment by measuring plasma levels.
Administration of 5-hydroxytryptophan and dopa to restore the synthesis of serotonin and catecholamines
Guthrie inhibition test in detail
Arabic word alkapton and Greek word “to suck up O2 to alkali
Succinylactone increased along with ALA for new born screening in urine.