sulfurcontainingaMethionine metabolism-171225170610.pptx

sulfur-containing Amino acid metabolism
Methionine metabolism

1. sulfur-containing Amino acid metabolism MSc biotechnology 1

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

3. METHIONINE (MET) (M) 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: Activation of methionine and transmethylation Conversion of methionine to cysteine 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: Creatine Epinephrine Choline 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: Impaired utilization Excessive remethylation of homocysteine 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. CYSTEINE (CYS) (C) 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 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. The sulfur may be removed either as H2S or elemental sulfur or as sulfite. 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). Glutamate + Cysteine → gamma glutamyl cysteine 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: Maleylacetoacetate → fumarylacetoacetate Cysteic acid → taurine (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. organophosphorus compounds halogenated compounds nitrogenous substances ( chlorodinitrobenzene ) heavy metals 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 H 2 S derived from cysteine may be oxidized to sulfites and thiosulfates and further oxidized to sulfate. The excretory forms of sulfur in urine are: Inorganic sulfates, Organic or ethereal sulfates, and Neutral sulfur. 28

29. Formation of Active Sulfate 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. cystinuria 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: high urinary levels of Hcy , high plasma levels of Hcy and methionine and low plasma levels of cysteine

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

37. Homocysteine causes activation of Hageman’s factor. This may lead to increased platelet adhesiveness and life-threatening intravascular thrombosis. Cyanide-nitroprusside test will be positive in urine. Urinary excretion of homocystine is more than 300 mg/24 h. Plasma homocysteine and methionine levels are increased. 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 Nutritional deficiency of vitamins, such as cobalamin, folic acid and pyridoxine. Metabolic: Chronic renal diseases, hypothyroidism. Drug induced: Folate antagonists, vitamin B12 antagonists; pyridoxine antagonists; estrogen antagonists, nitric oxide antagonists. 41

42. Homocysteine and heart attacks 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

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46. Thank-you 46