This document summarizes metabolism of the sulfur-containing amino acids methionine and cysteine. It discusses: 1) Activation of methionine to S-adenosyl methionine (SAM) which is used in transmethylation reactions. 2) Conversion of methionine to cysteine through transsulfuration pathways involving homocysteine. 3) Metabolic functions of cysteine including glutathione synthesis and roles in redox reactions, amino acid transport, and metal detoxification.

1. Dipesh Tamrakar
MSc. Clin. Biochemistry
1

2.  Methionine metabolism
A. Activation of methionine and transmethylation
B. Conversion of methionine to cysteine
C. Degradation of cysteine.
 Cysteine metabolism
A. Formation
B. Metabolic Function
 Metabolism Disorders of Sulfur containing amino
acid
2

3.  It is sulfur-containing, essential, glucogenic amino
acid.
 Forms succinyl CoA
 It is required for the initiation of protein biosynthesis.
 Degradation of methionine results in the synthesis of
cysteine and cystine.
 Metabolism of sulfur-containing amino acids may be
studied under the following major headings:
A. Activation of methionine and transmethylation
B. Conversion of methionine to cysteine
C. Degradation of cysteine.
3

4. 1. ACTIVATION OF
METHIONINE TO SAM
 In the major pathway, methionine is activated to
‘active methionine’ or S-adenosyl methionine (SAM).
 The synthesis of S-adnosylmethionine occurs by the
transfer of an adenosyl group from ATP to sulfur
atom of methionine.
 This is done by the enzyme, methionine adenosyl
transferase (MAT).
 There are 3 isoenzymes for MAT, out of which 1 and 3
are of hepatic origin.
 SAM is the main source of methyl groups in the body.
 The activation of methionine is unique as the sulfur
becomes a sulfonium atom by addition of 3 Carbon
 3ATP are consumed in the formation SAM.
4

5. 2. Methyl Transfer:
 In methionine, the thio-ether linkage (C–S–C) is
very stable.
 In SAM, due to the presence of a high energy bond,
the methyl group is labile, and may be transferred
easily to other acceptors
 SAM transfer methyl group to acceptor and gets
itself converted to SAH
3. Homocysteine:
 S-adenosyl homocysteine (SAH) is hydrolyzed to
homocysteine and adenine, which is the higher
homologue of cysteine
5

6. 4. Methionine synthesis:
Homocysteine can be converted to methionine by
addition of a methyl group.
This methyl group is donated from one-carbon pool
with the help of vitamin B12.
5. Homocysteine degradation:
 Homocysteine condenses with serine to form
cystathionine.
 This is catalyzed by pyridoxal phosphate dependent
cystathionine-beta synthase.
 Absence of this enzyme leads to homocystinuria.
6

7. 6. Cysteine synthesis:
 In the next step, cystathionine is hydrolyzed by
cystathionase to form cysteine and homoserine.
 Net result is that the SH group from methionine is
transferred to serine to form cysteine.
 This is called trans-sulfuration reaction
7. Final oxidation:
 Homoserine is deaminated and then decarboxylated
to propionyl CoA.
 It finally enters into the TCA cycle as succinyl CoA,
which is converted to glucose.
7

8. Formation of active methionine 8

9. METHIONINE IN
TRANSMETHYLATION
REACTIONS
 Many compounds become functionally active after
methylation
 Protein methylation helps to control protein turnover
and protects from immediate degration
 In Plants, SAM is the precursor for the synthesis of a
plant hormone, ethylene ( for growth and development)
 Some important products are:
1. Creatine
2. Epinephrine
3. Choline
4. Melatonin
9

10.  These reactions are called methyl transfer reactions,
and these are carried out with the help of S-adenosyl
methionine (SAM).
 Methyl groups are originally derived from the one
carbon pool.
 The methyl-THFA can transfer the methyl group to
homocysteine.
 Vitamin B12 is the co-enzyme for the reaction.
 This would account for the deficiency of folic acid
associated with B12 deficiency (folate trap).
 SAM is the methyl donor for all the transmethylation
reactions.
10

11. Hypermethioninemias
 Causes of hypermethioninemia are:
1. Impaired utilization
2. Excessive remethylation of homocysteine
3. Secondary to hepatic dysfunction.
 Oasthouse syndrome is due to malabsorption of
methionine.
 Such children excrete methionine, aromatic amino
acids and branched chain amino acids in urine.
11

12.  It is non-essential and glucogenic.
 Cysteine is present in large quantity in keratin of
hair and nails.
 Formation of Cysteine is by using the carbon
skeleton contributed by serine and sulfur
originating from methionine.
 Methionine → SAM → SAH → Homocysteine →
Cystathionine → Cysteine
 This sulfur containing amino acid undergoes
desulfurization to yield pyruvate.
12

13. Cysteine formation
13

14. DEGRADATION OF
CYSTEINE
1. Transamination:
 Cysteine is transaminated to form beta mercapto
pyruvic acid and finally pyruvate.
 The beta mercapto pyruvate can transfer the S to
CN to form thiocyanate (SCN).
14

15. 2. The sulfur may be removed either as H2S or
elemental sulfur or as sulfite.
3. Cysteine on decarboxylation gives beta
mercaptoethanolamine.
 This is used for synthesis of co-enzyme A .
15

16. METABOLIC FUNCTION OF
CYSTEINE
Formation of Glutathione:
 Glutathione is gamma glutamyl cysteinyl glycine
 Glutathione is generally abbreviated as GSH, to indicate
the reactive SH group.
 It was isolated in 1921 by Sir Frederick Hopkins (Nobel
prize, 1929).
1. Glutamate + Cysteine → gamma glutamyl cysteine
2. Glutamyl cysteine + glycine → glutathione
 Both steps need hydrolysis of each ATP.
16

17. Amino Acid Transport:
 The role of glutathione in the absorption of amino acid
is as:
17

18. Co-enzyme Role:
 Metabolic role of GSH is mainly in reduction reactions
2GSH → GS-SG + H2
(Reduced) (Oxidized)
 The hydrogen released is used for reducing other
substrates.
 A few examples are shown below:
i. Maleylacetoacetate → fumarylacetoacetate
ii. Cysteic acid → taurine
iii. (Iodine) I2 + 2GSH → 2HI + GS-SG
18

19. RBC Membrane Integrity:
 Glutathione is present in the RBCs.
 This is used for inactivation of free radicals formed
inside RBC.
 The enzyme is glutathione peroxidase, a selenium
containing enzyme.
 The glutathione is regenerated by an NADPH
dependent glutathione reductase.
 The NADPH is derived from the glucose- 6-phosphate
(GPD) shunt pathway.
 The occurrence of hemolysis in GPD deficiency is
attributed to the decreased regeneration of reduced
glutathione
19

20. Free radical scavenging
20

21. Met-hemoglobin:
 The met-Hb is unavailable for oxygen transport.
 Glutathione is necessary for the reduction of met-
hemoglobin (ferric state) to normal Hb (ferrous
state).
2Met-Hb-(Fe3+) + 2GSH → 2Hb-(Fe2+) + 2H+ +GS-
SG
21

22. Conjugation for Detoxification:
 Glutathione helps to detoxify several compounds by
transferring the cysteinyl group, e.g.
a. organophosphorus compounds
b. halogenated compounds
c. nitrogenous substances (chlorodinitrobenzene)
d. heavy metals
e. drug metabolism.
 The reaction is catalyzed by glutathione-S-transferase
(GST)
22

23.  GST is seen in all tissues,especially in liver.
 GST is a dimer; and each chain may beany one out
of 4 polypeptides; so there are 6 iso-enzymes.
 These are named as A, B, C, D, E and AA. Moreover,
many polymorphic forms of GST are also described.
23

24. Activation of Enzymes:
 Many enzymes having SH groups in the active site are
kept in the active form by the glutathione.
 Such enzymes are active in the reduced form.
 Glutathione keeps the enzymes in reduced, active state.
Formation of Taurine:
 Cysteine is oxidized to cysteic acid and then
decarboxylated to form taurine.
 Alternatively cysteine is oxidized to cysteine sulfinic
acid.
 It is then decarboxylated by a decarboxylase to
hypotaurine which in turn is oxidized to taurine.
 Taurine is used for conjugation of bile acids.
24

25. Formation of taurine from cysteine
25

26.  Taurine + Cholyl CoA → Taurocholate + CoA-SH
 Taurine is a modulator of calcium fluxes, calcium
binding and movement.
 In the CNS it is an inhibitory neurotransmitter.
 Taurine is widely distributed in animal tissues.
 It is found in bile and large intestine.
 Taurine has multiple functions in the body including
conjugation of bile acids, antioxidant role,
osmoregulation, membrane stability and calcium
signaling.
 It is important for the development of cardiovascular
system, development and function of skeletal system,
eyes and central nervous system.
 Some reports suggest it can be used as treatment for
cirrhosis and essential hypertension in experimental
animals.
26

27. Keeping the Correct Structure of Proteins
 Cysteine residues in polypeptide chains form
disulfide bridges to make active proteins, e.g.
insulin and immunoglobulins.
27

28. METABOLISM OF SULFUR
 The sulfur present in body may be either organic sulfur
as a component of proteins (sulfur-containing amino
acids) or as part of sulfatides and glycosaminoglycans
(GAG).
 Inorganic sulfur is derived from the sulfur-containing
amino acids by trans-sulfuration or desulfuration
reactions.
 The H2S derived from cysteine may be oxidized to
sulfites and thiosulfates and further oxidized to sulfate.
 The excretory forms of sulfur in urine are:
a. Inorganic sulfates,
b. Organic or ethereal sulfates, and
c. Neutral sulfur.
28

29.  Active sulfate or Phosphoadenosine phospho-5’-sulfate
(PAPS) is formed by the reaction between ATP and
SO4 and the sulfate is attached to the ribose-5’-
phosphate.
 PAPS is used for various sulfuration reactions, e.g.
synthesis of sulfatides, glycosaminoglycans, etc.
29

30. CYSTINUR
IA
 Cystinuria is one of the inborn errors of metabolism.
 It is an autosomal recessive condition.
 The disorder is attributed to the deficiency in transport of
amino acids
 Signs and symptoms include:
i. Abnormal excretion of cystine and to a lesser extent
lysine, ornithine and arginine. Hence the condition is also
called Cystine-lysinuria.
ii. Crystalluria and calculi formation. In acidic pH, cystine
crystals are formed in urine.
30

31. iii. Obstructive uropathy, which may lead to renal
insufficiency.
iv. Treatment is to increase urinary volume by increasing
fluid intake. Solubility of cystine is increased by
alkalanization of urine by giving sodium bicarbonate.
31

32. Cyanide-Nitroprusside Test:
 It is a screening test. Urine is made alkaline with
ammonium hydroxide
 and sodium cyanide is added. Cystine, if present, is
reduced to cysteine.
 Then add sodium nitroprusside to get a magenta-red
colored complex.
 Specific aminoaciduria
 may be confirmed by chromatography.
32

33. CYSTINOSIS
 It is a familial disorder characterized by the
widespread deposition of cystine crystals in the
lysosomes.
 Cystine accumulates in liver, spleen, bone marrow,
WBC, kidneys, cornea and lymph nodes.
 There is an abnormality in transport of cystine which
is responsible for the accumulation.
 It is an autosomal recessive condition.
 Microscopy of blood shows cystine crystals in WBCs.
 Treatment policies are to give adequate fluid so as to
increase urine output, alkalinization of urine by
sodium bicarbonate, as well as administration of D-
penicillamine.
33

34. HOMOCYSTINURIAS
 First described in 1962, these are the latest in the
series of inborn errors of metabolism.
 All of them are autosomal recessive conditions with
Incidence of 1 in 200,000 births.
 Normal homocysteine level in blood is 5–15
micromol/L.
 In diseases, it may be increased to 50 to 100 times.
 Moderate increase is seen in aged persons, vitamin
B12 or B6 deficiency, tobacco smokers, alcoholics and
in hypothyroidism..
 In plasma, homocysteine (with -SH group) and
homocysteine (disulfide, -S-S- group) exist. Both of
them are absent in normal urine; but if present, it will
be the homocysteine (disulfide) form.
34

35.  If homocysteine level in blood is increased, there is
increased risk for coronary artery diseases.
 Other important diseases associated with
hyperhomocysteinemia are neurological disorders
(stroke), pre-eclampsia of pregnancy, chronic
pancreatitis, etc.
35
Enzyme deficiency in homocystinurias (pyridoxal phosphate co enzyme)
Characterized by:
a. high urinary levels of
Hcy,
b. high plasma levels of
Hcy and methionine
and
c. low plasma levels of
cysteine

36. CYSTATHIONINE BETA SYNTHASE
DEFICIENCY
1. It causes elevated plasma levels of methionine
and homocysteine. There is increased excretion of
methionine and homocystine in urine. Plasma
cysteine is markedly reduced.
2. General symptoms are mental retardation and
Charley Chaplin gait. Skeletal deformities are
also seen.
3. In eyes, ectopia lentis (subluxation of lens),
myopia and glaucoma may be observed.
36

37. 4. Homocysteine causes activation of Hageman’s factor.
This may lead to increased platelet adhesiveness and
life-threatening intravascular thrombosis.
5. Cyanide-nitroprusside test will be positive in urine.
Urinary excretion of homocystine is more than 300
mg/24 h.
6. Plasma homocysteine and methionine levels are
increased.
7. Treatment is a diet low in methionine and rich in
cysteine. Sometimes the affinity of apo-enzyme to the
co-enzyme is reduced. In such cases, pyridoxal
phosphate, the co-enzyme given in large quantities
(500 mg) will correct the defect.
37

38. COBALAMIN DEFICIENCY
 The enzyme, N5-methyl-THFA-homocysteine-
methyl-transferase is dependent on vitamin B12.
 Therefore, vitamin B12 deficiency may produce
alteration in methionine metabolism.
 Blood contains increased level of homocysteine, but
methionine level is low. Urine contains
homocysteine.
38

39. DEFICIENT N5, N10-
METHYLENE THFA
REDUCTASE
 This enzyme catalyzes the reaction N5, N10-methylene-
THFA to N5-methyl-THFA
 Deficiency of this enzyme leads to reduced methionine
synthesis with consequent increase in homocystine level
in urine.
 Behavioral changes and vascular abnormalities may be
observed.
 Folate supplementation is beneficial. MTHFR gene
polymorphism (MTHFR C677T) is seen in
hyperhomocysteinemia.
39

40. CYSTATHIONINURIA
 It is due to cystathionase deficiency.
 It is an autosomal recessive condition.
 Mental retardation, anemia, thrombocytopenia, and
endocrinopathies accompany this condition.
 Less severe forms may be seen in conditions interfering
with homocysteine remethylation, in B12 deficiency
and in impaired folate metabolism.
 Acquired Cystathioninuria may be due to pyridoxine
deficiency.
 It may also be seen in liver diseases and after
thyroxine administration.
 Diagnosis rests on cyanide-nitroprusside test
(negative) and detection of cystathionine in urine.
 Large quantities of pyridoxine (200–400 mg) may be
beneficial.
40

41. ACQUIRED
HYPERHOMOCYSTEINEMIAS
a. Nutritional deficiency of vitamins, such as
cobalamin, folic acid and pyridoxine.
b. Metabolic: Chronic renal diseases,
hypothyroidism.
c. Drug induced: Folate antagonists, vitamin B12
antagonists; pyridoxine antagonists; estrogen
antagonists, nitric oxide antagonists.
41

42.  An increase of 5 micromol/L of homocysteine in serum
elevates the risk of coronary artery disease by as
much as cholesterol increase of 20 mg/dL.
 Homocysteine interacts with lysyl residues of collagen
interfering with collagen cross linking.
 Homocysteine interacts with lysyl aldehyde groups on
collagen and bind to fibrillin producing endothelial
dysfunction.
 Many patients with homocysteinemia also have
Marfanoid features since the protein fibrillin is
defective.
 It forms homocysteine thiolactone, a highly reactive
free radical which thiolates LDL particles.
42

43.  These particles tend to aggregate, are endocytosed
by macrophages and increase the tendency for
atherogenesis.
 Providing adequate quantity of pyridoxine, vitamin
B12 and folic acid will keep homocysteine in blood
at normal levels.
 Maternal hyperhomocysteinemia is known to
increase the chances of neural tube defects in fetus.
So, high doses of folic acid are advised in pregnancy.
43

44. SUMMARY
A summary of methionine metabolism is shown with
the roles played by vitamins. 44

45. 45

46. Thank-you
46