Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound   Target Concepts: Gene/Protein Disease Symptom Drug Enzyme Compound

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Query: EC:4.2.1.22 (cystathionine beta-synthase)
965 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

Globular proteins often appear to consist of distinct compact "domains," and the assumption is frequently implicitly made that these domains correspond to intermediate structures in the folding process. If this assumption is correct, the polypeptide fragment that builds up a domain should be able to spontaneously fold into its native conformation even when isolated. In an attempt to isolate and study such a fragment, the beta2 subunit of tryptophan synthetase [tryptophan synthase, L-serine hydro-lyase (adding indoleglycerol-phosphate), EC 4.2.1.20] has been subjected to controlled proteolysis. The resulting protein is shown to be a dimer, the protomer of which contains two nonoverlapping polypeptide chains of molecular weights 12,000 and 29,000. Though inactive, the nicked protein is shown to be in a conformation that closely resembles that of the original enzyme, since it still can form an enzyme-bound intermediate of the catalytic reactions. The fluorescence of this intermediate is used to characterize the binding sites for the cofactor (pyridoxal-P) and substrates, which are shown to exist on the nicked protein. The possibility is discussed of using the fragments isolated from the nicked protein to study individual steps of the enzymatic reaction, intracistronic complementation, and the folding process in the normal beta2 subunit.
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PMID:Preparation and characterization of a modified form of beta2 subunit of Escherichia coli tryptophan synthetase suitable for investigating protein folding. 32 25

The steady-state kinetics of the two substrate reaction of L-cysteine desulfation in the presence of 2-mercaptoethanol catalyzed by serine sulfhydrase from bakers yeast -- a pyridoxal phosphate-containing enzyme of the beta -- substituting lyase type -- were studied. Highly purified enzyme preparations (approximately 90% purity) of Saccharomyces cerevisiae with specific activity of 25 mumoles of H2S per 1 hr per mg of protein were used. The values of V, KS1, KS2 and alpha were calculated from the initial rates of the reaction under constant concentration of L-cysteine (S1) and variable concentration of 2-mercaptoethanol (S2) and vice versa. The data obtained suggest that under conditions of a two-substrate reaction catalyzed by serine sulfhydrase and in case of beta-cyanoalanine synthase of blue lupin the substrate binding to the enzyme is interdependent and obeys a unordered mechanism with o formation of a ternary aminosubstrate-pyridoxal phosphateenzyme-cosubstrate complex (alpha = 2.6).
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PMID:[Steady-state kinetics of reactions catalyzed by serine sulfhydrases of Saccharomyces serevisiae]. 38 Jun 61

Theoretic and experimental arguments are surveyed which justify the setting up, within the family of pyridoxal-P-dependent lyases, of a special subgroup that comprizes several enzymes catalyzing exclusively beta-replacement reactions of alpha-aminoacids with electronegative substituents in the beta-position. The authors and their associates have studied the physico-chemical and catalytic properties of four high purity enzymes belonging to this subgroup, namely: cysteine lyase (EC 4.1.1.10) from embryonic chicken yolk-sac, serine sulfhydrase from chicken liver and the closely analogous or synonymic cystathionine beta-synthase (EC 4.4.1.8) from rat liver, and beta-cyanoalanine synthase (EC 4.4.1.9) from lupine seedlings, in comparison with some pyridoxal-P-requiring lyases differing in reaction specificity, for example, gamma-specific, alphabeta-eliminating or plurifunctional lyases such as gamma-cystathionase (EC 4.4.1.1) of animal tissues. The results of these studies, relating to subtrate and cosubstrate specificities of the enzymes mentioned, their interactions with some selective inhibitors, catalysis of isotopic exchange of hydrogen atoms in substrates and substrate analogs, etc., indicate that lyases of the exclusively beta-replacing type substantially differ in reaction mechanism from other subgroups of this enzyme family. Thus, it appears highly improbable that transient formation of an alphabeta-unsaturated, coenzyme-substrate imine, considered as an obligatory step in the action of lyases in the alphabeta-eliminating and other subgroups, should occur in the sequences of reaction intermediates in the case of beta-replacing lyases. Suggested features of the presumable catalytic mechanism of these lyases are discussed, such as : fixed conformation of the aminoacid substrate in the ES complex (protein-bound pyridoxal-P aldimine), with beta-substituent in orientation cis (rather than trans) to the Halpha atom ; role of the binding of appropriate cosubstrates (nucleophilic replacing agent, Cs) inducing essential electronic and/or steric transitions in the catalytic site of the ternanry CsES complexes, etc.
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PMID:The pyridoxal-phosphate-dependent enzymes exclusively catalyzing reactions of beta-replacement. 78 60

O-Acetylserine (thiol) lyase, the last enzyme in the cysteine biosynthetic pathway, was purified to homogeneity from spinach leaf chloroplasts. The enzyme has a molecular mass of 68,000 and consists of two identical subunits of Mr 35,000. The absorption spectrum obtained at pH 7.5 exhibited a peak at 407 nm due to pyridoxal phosphate, and addition of O-acetylserine induced a considerable modification of the spectrum. The pyridoxal phosphate content was found to be 1.1 per subunit of 35,000, and the chromophore was displaced from the enzyme by O-acetylserine, leading to a progressive inactivation of the holoenzyme. Upon gel filtration chromatography on Superdex 200, part of the chloroplastic O-acetylserine (thiol) lyase eluted in association with serine acetyltransferase at a position corresponding to a molecular mass of 310,000 (such a complex called cysteine synthase has been characterized in bacteria). The activity of O-acetylserine (thiol) lyase was optimum between pH 7.5 and 8.5. The apparent Km for O-acetylserine was 1.3 mM and for sulfide was 0.25 mM. The calculated activation energy was 12.6 kcal/mol at 10 mM O-acetylserine. The overall amino-acid composition of spinach chloroplast O-acetylserine (thiol) lyase was different than that determined for the same enzyme (cytosolic?) obtained from a crude extract of spinach leaves. A polyclonal antibody prepared against the chloroplastic O-acetylserine (thiol) lyase exhibited a very low cross-reactivity with a preparation of mitochondrial matrix and cytosolic proteins suggesting that the chloroplastic isoform was distinct from the mitochondrial and cytosolic counterparts.
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PMID:Purification and characterization of O-acetylserine (thiol) lyase from spinach chloroplasts. 137 15

Severe homocysteinemia due to genetic defects either of pyridoxal 5-phosphate (PLP)-dependent cystathionine beta-synthase (CBS) or of enzymes in vitamin B12 and folate metabolism is associated with very early-onset vascular disease. Therefore, we studied homocysteine metabolism in 72 patients presenting before the age of 55 years with occlusive arterial disease of cerebral, carotid, or aorto-iliac vessels. Twenty patients (28%) had basal homocysteinemia; and 26 patients (36%) had abnormal increases of plasma homocysteine after peroral methionine loading, which exceeded the highest value for 46 comparable controls and was within the range for 20 obligate heterozygotes for homocystinuria due to CBS deficiency. Basal plasma homocysteine content was strongly and negatively correlated to vitamin B12 and folate concentrations. Plasma PLP was depressed in most patients but there was no correlation between PLP and homocysteine values. In 20 patients, treatment with pyridoxine hydrochloride (240 mg/day) and folic acid (10 mg/day) reduced fasting homocysteine after 4 weeks by a mean of 53%, and methionine response by a mean of 39%. These data show that a substantial proportion of patients with early-onset vascular disease have impaired homocysteine metabolism, which may contribute to vascular disease, and that the impaired metabolism can be improved easily and without side effects.
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PMID:Impaired homocysteine metabolism in early-onset cerebral and peripheral occlusive arterial disease. Effects of pyridoxine and folic acid treatment. 240 53

Pyridoxine metabolism is summarised and speculation on possible defects leading to disease is made. Inherited deficiencies of PLP enzymes, which are known to respond in vivo to pharmacologic doses of pyridoxine are listed. The mechanism of pyridoxine responsiveness in homocystinuria due to cystathionine beta-synthase deficiency is discussed. There is a correlation in most (but not all) cases between the presence of residual CS activity, which is often stimulated by pyridoxal phosphate much more than control enzyme, in cultured fibroblasts and pyridoxine responsiveness in vivo. Exceptional patients have been found and are discussed in the light of more detailed studies on their cell lines. Clearly defined abnormalities of pyridoxal phosphate binding to mutant enzyme have been demonstrated and evidence of reduced intracellular stability of mutant CS and possible modulation by pyridoxal phosphate is presented. Preliminary findings suggest that the tissue level of pyridoxal phosphate achieved following pyridoxine treatment could be one other factor in determining pyridoxine responsiveness.
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PMID:Recent advances in the mechanism of pyridoxine-responsive disorders. 393 Aug 45

Homocystinuria due to cystathionine beta-synthase deficiency may be responsive to pyridoxine, a precursor of the cofactor pyridoxal phosphate, and the amount of residual enzyme activity present is the probable determinant of this. In six treated pyridoxine-responsive patients whose biochemical control of fasting plasma amino acid levels appeared optimal, we assessed the effects on plasma amino acids of standard oral methionine loads (4g/m2 of body area) before and after adding betaine (trimethylglycine) 6 g/d, to the treatment regimen of pyridoxine and folic acid. Our aim was to define the capacity of these patients to metabolize methionine and to determine whether betaine would effect a reduction in postload homocysteine levels. During the 24 hours after the methionine challenge all patients had higher plasma methionine and homocysteine and lower cysteine than did 17 normal subjects. After betaine these homocysteine responses were reduced to near normal, and there was a trend toward increased methionine. There was a direct correlation between premethionine fasting homocysteine and mean homocysteine responses during the 24 hours following the methionine load, both before (r = 0.79) and after betaine (r = 0.71). Betaine also increased plasma cysteine levels in patients with the more severe biochemical abnormalities. After betaine there were modest increases in plasma serine (mean increase 25%; P less than 0.025). Since the vascular complications of homocystinuria are related to increased plasma homocysteine, betaine therapy may reduce this risk in patients receiving a standard pyridoxine and folic acid regimen in whom there are abnormal homocysteine responses after a standard methionine load.
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PMID:Homocystinuria due to cystathionine beta-synthase deficiency--the effects of betaine treatment in pyridoxine-responsive patients. 393 99

It has been shown that yeast tryptophan synthase (L-serine hydro-lyase (adding indoleglycerol-phosphate) EC 4.2.1.20) catalyses tritium exchange reactions between protons on the alpha-carbon of L-serine of L-tryptophan, and water. The absolute rates of these reactions and indole-serine condensation (reaction B), all of which are pyridoxal phosphate-dependent, were measured. L-Serine exchange was resolved into two components, a high-affinity, slow, Michaelian reaction (KmS,H = 0.06 mM, kcats,H 3 X 10(-3) s-1) and a faster reaction (kcat greater than 2.5 S-1) which was not saturated even at 100 mM L-serine. Hydrogen exchange by tryptophan was a Michaelian process (KmT,H = 2.9 mM; kcatT,H = 0.6 s-1). Indole did not inhibit either exchange reaction. A plausible explanation of the results, that reaction B has a ping-pong mechanism with serine as first substrate and water and L-tryptophan as first and second products, respectively, was inadequate because of the observations that L-tryptophan is as first and second products, respectively, was inadequate because of the observations that L-tryptophan is synthesised with less than 1 mol of exchanged proton per mol amino acid, and that the ratio kcat/Km for serine changes between enzyme reactions. A branched modification with two enzyme-serine complexes, only one of which will exchange protons with water, will fit all the results.
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PMID:Hydrogen exchange kinetics and the mechanism of reaction B of yeast tryptophan synthase. 393 73

S-Sulfocysteine synthase was isolated from Salmonella typhimurium LT-2 to homogeneous form with polyacrylamide gel electrophoresis. The molecular weight of this enzyme was determined to be ca. 55,000. The enzyme consisted of two identically sized subunits, and it contained one pyridoxal phosphate per subunit. The enzyme catalyzed the biosynthesis of cysteine or S-methylcysteine from sulfide or methanethiol and O-acetylserine, respectively, in addition to the formation of S-sulfocysteine from thiosulfate and O-acetylserine. The enzyme is identical to cysteine synthase B. The intracellular level of this enzyme was regulated by lesser extents of the same factors as those effective for cysteine synthase A.
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PMID:Enzymatic proof for the identity of the S-sulfocysteine synthase and cysteine synthase B of Salmonella typhimurium. 637 37

Homocyst(e)ine [H(e)], the sum of homocysteine, homocystine, and the homocysteine-cysteine mixed disulfide, free and protein-bound, has been shown to be associated in retrospective case control studies, and in one prospective study, with vascular disease, including coronary artery disease (CAD), cerebrovascular disease, and peripheral vascular disease. Elevated levels of homocyst(e)ine severe enough to cause homocystinuria are seen in severe nutritional deficiencies of vitamin B12, folic acid and vitamin B6. Rare genetic disorders of vitamin B12 synthesis of 5'-10'-methylene tetrahydrofolate reductase, or the pyridoxal phosphate-dependent enzyme cystathionine beta-synthase may cause severe hyperhomocyst(e)inemia and homocystinuria. The clinical manifestation of these disorders are mental retardation, neurological disorders, and widespread thromboembolic phenomena. The measurement of H(e) is currently performed using high-pressure liquid chromatography with fluorescence detection. Other methods, especially mass spectroscopy, are also used. Internal standards using increasing concentrations of homocystine and acetylcysteine and several external standards are used to ensure accuracy of the assay. Milder elevations of H(e) have recently been associated with vascular disease, in both men and women. The strength of this association appears to be stronger for peripheral and cerebrovascular disease than for CAD. Nevertheless, several case control studies in Europe, Canada, and the United States have shown that H(e) levels are elevated in CAD patients compared with controls, and H(e) levels are independent of the conventional cardiovascular risk factors (age, gender, lipid and lipoprotein cholesterol levels, hypertension, or cigarette smoking). One prospective study, the Physicians' Health Study, has shown that H(e) levels are slightly but significantly higher in CAD cases vs controls in a population of US physicians.(ABSTRACT TRUNCATED AT 250 WORDS)
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PMID:Measurement of homocyst(e)ine in the prediction of arteriosclerosis. 762 74


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