Metabolism of sulfur containing amino acids, branch chain amino acid with their clinical significance, Tryptophan and its specilized product

1. Amino Acid Metabolism
Dr. Abhra Ghosh

2.  Sulphur containing amino acids:
(i) L-Methionine (Essential amino acid)
(ii) L-Cysteine
(iii) L-Cystine
(Non-essential amino acid)
 Other sources of sulphur in the body:
• Thiamine (Vit B1)
• Lipoic acid and
• Biotin

3. • Methionine, cysteine and cystine are the principal sources of
sulphur in the body.
• Demethylation of methionine produces homocysteine
• Cystine is reversibly convertible to cysteine and homocystine by
oxidation-reduction.
• Both methionine and cysteine can undergo transamination
reaction.
• Methionine is an essential amino acid. Cysteine is not essential
and can be synthesized in the body from methionine.

4. • The presence of cysteine and cystine in the diet reduces the
requirement of methionine (sparing action).
• Methionine before catabolized has to be activated first to
“Active” methionine (S-adenosyl methionine), which can act as
–CH3 group donor in the body

5.  Metabolic Fate of L-Methionine:
• Stage 1: Activation of methionine and its demethylation to
form L-Homocysteine.
• Stage 2: Conversion of L-homocysteine to L-homoserine.
• Stage 3: Degradation of L-homoserine to end products L -
propionyl-CoA and α-amino butyrate.

6. Stage 1: Activation of L-methionine and its demethylation to form L-homocysteine
L -methionine S – adenosylmethionine
(Active methionine)
L – methionine adenosyl
transferase
ATP Pi + PPi
Mg+2
G-SH
Acceptor
Acceptor – CH3
S - adenosylhomocysteineL - homocysteine
Hydrolysis of
S-C bond
Adenosine H2O

7. Stage 2: Conversion of L-homocysteine to L-Homoserine
L – homocysteine
Serine H2O
Cystathionine
B6-P
L - homoserine
Thionase
L - cysteine
+ H2
Cystathionine synthetase

8. Stage 3: Degradation of L-Homoserine
L - homoserine
α – keto butyrate
Propionyl CoA
Succinyl CoA
α – amino butyrate
Excreted in urine
Deaminase

9. METABOLIC ROLE OF METHIONINE
• Methionine is “glucogenic”
• Cysteine formation
• Lipotropic function
• Polyamine synthesis
• Formation of methyl mercaptan
• Transmethylation

11. Metabolism of Cystine
One molecule of cystine is reduced to form two molecules of cysteine
and vice-versa
Cystine
Cysteine
+2H- 2H
NADH dependent
oxydoreductase
Pyruvic acid

12. L - cysteine Cysteine-sulphinic acid
Liver enzyme
Transminase
Sulphinyl-pyruvic acidPyruvic acid
Desulphinase
SO2
-
B6-P
Mg+
α-KA
α-AA

13. L-cysteine
Desulfhydrase
Imino acid
NH3
Pyruvic acid Imino acid
Rearrangement
H2O
H2S
B6-P

14. L-cysteine
Transaminase
Thiol pyruvic acid
Trans sulphurase
Pyruvic acid
α-KA α-AA
B6-P
2H
H2S

15. L-cysteine
Liver enzyme
Dehydrogenase
L-cysteine-sulphinic acid
Cysteic acid
Transminase
Sulphonyl pyruvate
Desulphinase
Pyruvic acid
ATP, Mg+2
α-KA
α-AA
B6-P
SO4
-2

16.  Metabolic Role of Cysteine:
• Glucogenic
• Formation of taurine
• Formation of glutathione
• Formation of mercaptoethanolamine
• Role of cysteine in detoxication
L-glutamic acid + L-cysteine
𝛾-L-glutamyl-L-cysteine

17. Inherited disorders associated with S-containing amino acids
 Cystinuria:
• ↑ excretion of cystine
• ↑ excretion of other dibasic aa – lysine, arginine, ornithine
Defect:
• Involvement of a single reabsorptive site
• Defect in renal transportation of the four aminoacids
Complications:
• Formation of cystine calculi in renal tubules, ureters, bladder
Diagnosis:
• Hexagonal, flat crystals in urinary deposit
• Cyanide-nitroprusside test (Lewis)

18.  Cystinosis:
• Also known as cystine storage disease
• Accumulation cystine in liver, spleen, bone marrow, peripheral
leucocytes, lymph nodes, kidney and cornea
Defect:
• Deficiency of cystine reductase
• Impaired conversion of cystine to cysteine in the involved tissues

19. Clinical features:
• Nephropathic
• Juvenile type
• Adult type
Diagnosis:
• Cystine crystals in cornea by slit lamp microscopy
• Cystine crystals in unstained blood/biopsies of rectal mucosa

20.  Homocystinuria Type-1 (classical type):
• IEM involving the catabolism of methionine or more specifically its
metabolic intermediates, homocysteine/and homocysteine
• Normal homocysteine level: 5–15 micromol/L
Defect:
• Genetic deficiency of the enzyme cystathionine synthetase
Clinical features:
• Mental retardation
• Tall stature, with long extremities, frequently with flat feet with toes
out (Charlie Chaplin gait)
• Hepatomegaly

21. • Skeletal deformities: kyphosis, scoliosis, arachnodactyly. Premature
osteoporosis which also accounts to above deformities. X-ray spine
shows cod fish Vertebrae.
• Ectopia lentis: Curious dislocation of lens of the eye. Not seen at
birth, may show at the age of 2 to 3 years.
• Life-threatening arterial/venous thrombosis.
• Most of the patients show abnormal EEG
Diagnosis:
• Urine: Sodium cyanide-nitroprusside test is positive
• Urinary excretion of homocystine is more than 300 mg/24 h.

22.  Homocystinuria Type-2
• Inheritance: Autosomal recessive
• Enzyme deficiency: N5 – methyl - Tetrahydrofolatehomocysteine
methyl transferase
• Clinical feature:
 Mental retardation
 No ectopia lentis or thrombotic episodes
• Blood: Shows increased level of homocysteine.
• Urine: Homocysteine is excreted in urine. Nitroprusside test +ve.

23.  Homocystinuria Type-3
• Inheritance: Autosomal recessive.
• Enzyme deficiency: N5, N10 - methylene tetrahydrofolate
reductase deficiency
• Clinical features:
 Mental retardation
 No ectopia lentis or thrombotic episodes
• Blood: Shows increase homocysteine.
• Urine: Excretion of homocystine, nitroprusside test +ve

24. Homocysteine and Heart Attack:
• Homocysteine interacts with lysyl aldehyde groups on collagen and
bind to fibrillin
• Homocysteine thiolactone, a highly reactive free radical thiolates
LDL particles
• These particles tend to aggregate, endocytosed by macrophages
• Sufficient amounts of folic acid, B12 and B6 reduce the blood levels
of homocysteine

25. Glutathione
• Glutathione is a tripeptide of three amino acids:
(i) glutamic acid, (ii) cysteine and (iii) glycine.
• Chemically it is α-L-Glutamyl-cystinyl-glycine.
Synthesis:
• It is synthesized in cytosol outside the ribosomes
• Synthesis requires ATP

26. Functions
• Helps in destruction of H2O2
• Coenzyme of Liver enzyme Glutathione insulin transhydrogenase
• Reductive cleavage of S-S linkages in thyroglobulin glycoprotein
• SH group containing enzymes are protected against the oxidation of
their –SH groups
• Coenzyme with formaldehyde dehydrogenase
• It also acts as coenzyme with Maleyl-acetoacetate isomerase
• Glutathione takes part in γ-glutamyl cycle for absorption of amino
acids from gut

27. Tryptophan
• It is an essential amino acid.
• It is both glucogenic and ketogenic.
• Tryptophan can synthesize niacin (nicotinic acid)
• It is a heterocyclic amino acid and chemically it is “α-amino-β-3-
indole propionic acid”.

28. Metabolic fate
L-tryptophan N-formyl kynurenine
Tryptophan pyrrolase
Kynurenine
formylase
Kynurenine
3-OH-kynurenine3-OH-anthranilate
Kynurenine
hydroxylase
Kynureninase
B2
B6
Xanthurenic acid
Alanine to PA

29. 3-OH-anthranilate
Nicotinic acid Acetyl CoA
 Metabolic role:
• Glucogenic and Ketogenic
• Nicotinic acid formation
• Formantion of tryptamine
• Formation of xanthurenic acid
• Formation of serotonin
• Formation of indole acetic acid

30.  Hartnup’s Disease
• Defective tryptophan metabolism
• Biochemical defect: Impaired formation of “transport proteins” for
tryptophan and neutral amino acids in intestinal mucosal, renal
tubular epithelial cells and the brain.
Clinical features:
• Mental retardation.
• Intermittent cerebellar ataxia and other neurological symptoms
• Pellagra-like skin rash

31. Blood: Tryptophan and other neutral amino acids ↓
Faeces and Urine: The neutral amino acids, including tryptophan are ↑
Urine: ↑ Indoleacetic acid.
T/t: Patients improve when put on a high protein diet with
supplementation of niacin and minimum exposure to sunlight.
↓

32. Serotonin
• It is 5-OH tryptamine (5-HT)
• Vasoconstrictor substance
• Present in the blood and produced in tissues like gastric mucosa,
intestine, brain, mast cells and platelets
• These cells take up silver staining hence also called Argentaffin cells

33. L-tryptophan
Hydroxylase
NADPH, O2
5-OH-tryptophan
B6-PO4
CO2
5-OH-tryptophan
decarboxylase
5-OH-tryptamine

34.  Functions:
• It is a potent vasoconstrictor
• Produces contraction of smooth muscles
• ↑ in brain tissues produces stimulation of cerebral activity
• ↓ serotonin produces depressant effect
 Catabolism of Serotonin:
• Monoamine oxidase (MAO)
• Converts serotonin to 5-HIAA (5-OH-Indole acetic acid)
• 5-HIAA is excreted in urine
• Normal adults excrete about 7 mg HIAA per day

35.  Carcinoids:
A malignant tumor of serotonin producing cells
Clinical features:
• Cutaneous vasomotor episodes of flushing
• Occasionally cyanotic appearance
• Chronic diarrhoea
• Respiratory distress and bronchospasm
• Right-sided heart failure
Urinary findings:
• Urinary excretion of 5-HIAA >400 mg per day

36. Histidine
• Nutritionally semi-essential amino acid.
• Histidine is required in the diet in growing animals and in pregnancy
and lactation.
• It is α-amino-β-imidazole propionic acid.

37. Histidine Imidazole PAHistamine
CO2
β imidazole acetaldehyde
β imidazole acetic acid
Urocanic acid
Formiminoglutamic acid (FIGLU)
Glutamic acid Ketoglutarate
Carnosine
Anserine
Folic acid deficiency
Histidase

38.  Histidinaemia
It is an inherited disorder.
Enzyme deficiency: Inadequate activity of Liver histidase
Symptoms: There may be mental retardation. Speech development may
be retarded
Biochemical findings:
• ↑ levels of histidine in blood and urine
• Presence of imidazole pyruvic acid, imidazole lactic acid and
imidazole acetic acid.
Treatment: Histidine free diet

39.  Maple Syrup Urine Disease (MSUD)
• Branched chain ketonuria
• Characteristic smell of urine (similar to burnt sugar or maple sugar) due
to excretion of branched chain keto acids.
Defect:
• Deficient decarboxylation of branched chain keto acids
Clinical findings:
• Convulsions, severe mental retardation, vomiting, acidosis, coma and
death within the first year of life.

40. Laboratory findings:
• Urine contains branched chain keto acids of valine, leucine and
isoleucine.
• Rothera’s test is positive
Treatment
• Diet with low branched chain amino acids.

41. Synthesis of Nitric Oxide

42.  Endothelial NOS (eNOS):
• Present in endothelial cells.
• Constant production of NO
• Depends on elevated Ca2+ ions
 Neuronal NOS (nNOS):
• Present in central and peripheral neurons, cerebellum
• Depends on elevated Ca2+ ions
 Macrophage NOS (iNOS):
• Present in macrophages
• Independent of elevated Ca2+

43. • It acts as a vasodilator
• Regulate blood flow and maintains blood pressure.
• Acts as a neurotransmitter
• Role in relaxation of skeletal muscles.
• Inhibits adhesion, activation and aggregation of platelets.
• May constitute part of a primitive immune system and may
mediate bactericidal actions of macrophages
• Low level of nitric oxide may be involved in causation of
pylorospasm of infantile hypertrophic pyloric stenosis

44.  Clinical aspects:
• Use of Nitroglycerine
• In septic shock
• In eclampsia and pre-eclampsia
• Iron supplements