Cystathionine-7-lyase

Pyridoxine (vitamin B6) Transsulfuration pathway cofactor for cystathionine-(3-synthase and cystathionine "/-lyase Folate cycle methylation of tetrahydrofolate cofactor for serine hydroxymethyltransferase... 

Deficiencies of methionine adenosyltransferase, cystathionine 8-synthase, and cystathionine )/-lyase have been described. The first leads to hypermethioninemia but no other clinical abnormality. The second leads to hypermethioninemia, hyperhomocysteinemia, and homo-cystinuria. The disorder is transmitted as an autosomal recessive trait. Its clinical manifestations may include skeletal abnormalities, mental retardation, ectopia lentis (lens dislocation), malar flush, and susceptibility to arterial and venous thromboembolism. Some patients show reduction in plasma methionine and homocysteine concentrations and in urinary homocysteine excretion after large doses of pyridoxine. Homocystinuria can also result from a deficiency of cobalamin (vitamin B12) or folate metabolism. The third, an autosomal recessive trait, leads to cystathioninuria and no other characteristic clinical abnormality. 

Selenomethionine metabolism to selenide and the incorporation into selenium-specific proteins may occur by two pathways metabolism to methane selenol and selenide or via selenocysteine. Evidence that the incorporation of selenium from selenomethionine into protein is by the transsulfuration pathway (methionine to cysteine) comes from studies of selenomethionine metabolism in lymphoblast cell lines deficient in cystathionine lyase and cystathionine synthetase, enzymes of the transsulfuration pathway (Beilstein and Whanger 1992). Deficiency in these enzymes greatly reduces the incorporation of selenomethionine into glutathione peroxidase. 

H2S can be produced via the metabolism of sulfhydryl-bearing amino acids, specifically by several enzymes found in the methionine-homocysteine-cysteine pathway such as cystathionine 3 synthase (CBS) and cystathionine lyase (CGL) (Fig. 8.1) [6, 10, 11]. The sequence of CBS has been identified in genomes from bacteria to humans [12-14], and a gene similar to the sulfide quinone oxidoreductase gene has been identified in the genome of flies, worms, mice, rats, and humans [15], indicating that cellular H2S and its regulation may be widespread and essential. 

Homoserine kinase 2 homoserine acyltransferase 3 0-succinylhomoserine (thiol)-lyase 4 cystathionine -lyase 5 tetrahydropteroylglutamate methyltransferase... 

L-homoserine + thiol (s) cystathionine-a-lyase Ervi carotovora 133... 

Pyridoxamine phosphate serves as a coenzyme of transaminases, e.g., lysyl oxidase (collagen biosynthesis), serine hydroxymethyl transferase (Cl-metabolism), S-aminolevulinate synthase (porphyrin biosynthesis), glycogen phosphoiylase (mobilization of glycogen), aspartate aminotransferase (transamination), alanine aminotransferase (transamination), kynureninase (biosynthesis of niacin), glutamate decarboxylase (biosynthesis of GABA), tyrosine decarboxylase (biosynthesis of tyramine), serine dehydratase ((3-elimination), cystathionine 3-synthase (metabolism of methionine), and cystathionine y-lyase (y-elimination). 

Hydrogen sulfide is a well known general metabolite produced on sulfate reduction by certain bacteria. Moreover, organic forms of sulfur can give rise to HS , hence H2S in certain bacteria. Thus, cysteine desulfhydrase (EC 4.4.1.1, cystathionine y-lyase) converts L-cysteine to H2S, pyruvate, and NH3. This enzyme shows a requirement for pyridoxal phosphate and the unstable ami-noacrylic acid is an intermediate (Equation 1) in the reaction ... 

Inhibitor of cystathionine-processing enzymes (cystathionine 78, 189 /3-lyase) and cystalysin... 

Inhibitor of cystathionine /3-lyase from both Salmonella 78... 

This enzyme [EC 4.4.1.6] catalyzes the pyridoxal-phos-phate-dependent hydrolysis of an 5-alkyl-L-cysteine to generate an alkyl thiol, ammonia, and pyruvate. The reaction is an a,/3-elimination. In yeast, the enzyme may be identical to cystathionine /3-lyase. See also Alliin Lyase... 

This pyridoxal-phosphate-dependent enzyme [EC 4.4.1.8], also referred to as /3-cystathionase and cystine lyase, catalyzes the hydrolysis of cystathionine to yield homocysteine, pyruvate, and ammonia. 

CYSTATHIONINE yS-LYASE CYSTATHIONINE y-LYASE CYSTATHIONINE yS-SYNTHASE Oystathionine y-synthase,... 

CYSTATHIONINE /3-LYASE CYTIDINE DEAMINASE CYTIDYLATE KINASE Cytochrome be, complex,... 

CYSTATHIONINE y-LYASE HOMOSERINE DEHYDROGENASE ASPARTATE KINASE HOMOSERINE SUCCINYLTRANSFERASE HomotopIc,... 

CYSTATHIONINE /3-LYASE CYSTATHIONINE /3-SYNTHASE CYSTEINE CONJUGATE /3-LYASE CYSTEINE LYASE... 

Cystathionine (3-lyase (cystathionase) O-Acetylserine sulfhydrylase (cysteine synthase)... 

Aspartate y-decarboxylase Selenocysteine lyase NifS protein of nitrogenase Gamma elimination and replacement Cystathionine y-synthase Cystathionine y-lyase Threonine synthase... 

Cyclohexanedione, reaction with guanidinium groups, 126 Cyclophilin 488 human 488s D-Cycloserine 739s Cyclosporin 488, 488s p Cylinders 65, 66, 686 Cystathionine, 746s formation 746 Cystathionine p lyase 742 Cystathionine p-synthase 744 Cystathionine y-synthase 743, 746 Cystatins 622, 629... 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

Thiocysteine can also arise in a similar manner through action of cystathionine (3 lyase on cystine. Thiocysteine is eliminated with production of pyruvate and ammonia from the rest of the cystine molecule 467 One of the nifS-like proteins of E. coli is thought to transfer a selenium atom from selenocysteine (pp. 823-827) into selenophosphate 466a f The latter can be formed by transfer of a phospho group from ATP to selenide HSe-. The other products of ATP cleavage are AMP and P . Reduction of Se° to HSe- is presumably necessary. 

Cysteine Methionine S-Adenosylmethionine synthase a-Methyltransferase S-Adenosylhomocysteinase Cystathionine- /3-synthase Cystathionine- y- lyase... 

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