Cystathionine

L-cystathionine L-homoserine + L-cysteine cystathionine-y- synthase Streptomyces phaeochromogenes ... 

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

Cystamine dihydrochloride, S,5-(L,L)-Cystathionine, Cysteamine, Cysteamine hydrochloride, ( )-Cysteic acid, S-Cysteic acid (H2O), L-Cysteine hydrochloride (H2O), ( )-Cysteine hydrochloride and L-Cystine, Cytidine, see entries in Chapter 6. 

The amino acid cysteine, C3H7NO2S, is biosynthesized from a substance called cystathionine by a multistep pathway. 

Cyclopentenones. from 1.4-diketones. 886-887 Cyclopropane, angle strain in, 115 bent bonds in. 115 from alkenes. 227-229 molecular model of, 111. 115 strain energy of, 114 torsional strain in, 115 Cystathionine, cysteine from. 1177 Cysteine, biosynthesis of, 1177 disulfide bridges from, 1029 structure and properties of, 1018 Cytosine, electrostatic potential map of, 1104... 

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). 

S., Simultaneous determination of urinary cystathionine, lanthionine, S-(2-aminoethyl)-L-cysteine and their cyclic compounds using liquid chromatography-mass spectrometry with atmospheric pressure chemical ionization, /. Chromatogr. B, 698, 301, 1997. 

S. Taoka, S. Ohja, X. Shan, W. D. Kruger, and R. Banerjee, Evidence for heme-mediated redox regulation of human cystathionine beta-synthase activity. J. Biol. Chem. 273, 25179-25184 (1998). 

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 ... 

Chen, X., Jhee, K.-H. and Kruger, W. D. Production of the neuromodulator H2S by cystathionine beta-synthase via the condensation of cysteine and homocysteine. /. Biol. Chem. 279 52082-52086, 2004. 

The transsulfuration pathway is the major route for the metabolism of the sulfur-containing amino adds 674 Homocystinuria is the result of the congenital absence of cystathionine synthase, a key enzyme of the transsulfuration pathway 676 Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin B6), which is a cofactor for the cystathionine synthase reaction 676... 

Homocystinuria Usually a failure of cystathionine synthase (Fig. 40-2 reaction 6). Rarely associated with aberrant vitamin B12 metabolism (Fig. 40-2) Thromboembolic diathesis, marfanoid habitus, ectopia lentis. Mental retardation is frequent. Diet low in methionine Vitamin B6 in pyridoxine-responsive syndromes Vitamin B12 in responsive syndromes Anticlotting agents... 

The most common cause of homocystinuria is a congenital deficiency of cystathionine-p-synthase, a pyridoxine-dependent enzyme that condenses homocysteine and... 

Cleavage of cystathionine is accomplished by cystathi-onase, another pyridoxine-dependent enzyme that is coded on human chromosome 16 (Fig. 40-4 reaction 6). The enzyme functions almost entirely to produce cysteine, there being virtually no reversal of the reaction. 

Homocystinuria can be treated in some cases by the administration of pyridoxine (vitamin Bs), which is a cofactor for the cystathionine synthase reaction. Some patients respond to the administration of pharmacological doses of pyridoxine (25-100 mg daily) with a reduction of plasma homocysteine and methionine. Pyridoxine responsiveness appears to be hereditary, with sibs tending to show a concordant pattern and a milder clinical syndrome. Pyridoxine sensitivity can be documented by enzyme assay in skin fibroblasts. The precise biochemical mechanism of the pyridoxine effect is not well understood but it may not reflect a mutation resulting in diminished affinity of the enzyme for cofactor, because even high concentrations of pyridoxal phosphate do not restore mutant enzyme activity to a control level. 

Biochemical findings are variable. The blood cobala-min and folate levels often are normal. Patients often have homocysteinemia with hypomethioninemia, the latter finding discriminating this group from homocystinuria secondary to cystathionine- P-synthase deficiency. Urinary excretion of methylmalonic acid may be high, reflecting the fact that vitamin B12 serves as a cofactor for the methyl-malonyl-CoA (coenzyme A) mutase reaction. 

Kluijtmans, L. A., Boers, G. H., Stevens, E. M. et al. Defective cystathionine beta-synthase regulation by S-adenosyl-methionine in a partially pyridoxine responsive homocys-tinuria patient./. Clin. Invest. 98 285-289,1996. 

CBS Domain in cystathionine /3-synthase and other proteins. E(MFP)AB 9(20) 9(20) 1B30... 

Phe Tyr (phenylalanine hydroxylase, biopterin cofactor) Met homoCys + Ser cystathionine Cys... 

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