NDLI: Occurrence of Transsulfuration in Synthesis of l-Homocysteine in an Extremely Thermophilic Bacterium, Thermus thermophilus HB8

Content Provider	PubMed Central
Author	Yamagata, Shuzo Ichioka, Kazuhito Goto, Koji Mizuno, Yasuko Iwama, Tomonori
Copyright Year	2001
Abstract	A cell extract of an extremely thermophilic bacterium, Thermus thermophilus HB8, cultured in a synthetic medium catalyzed cystathionine γ-synthesis with O-acetyl-l-homoserine and l-cysteine as substrates but not β-synthesis with dl-homocysteine and l-serine (or O-acetyl-l-serine). The amounts of synthesized enzymes metabolizing sulfur-containing amino acids were estimated by determining their catalytic activities in cell extracts. The syntheses of cysthathionine β-lyase (EC 4.4.1.8) and O-acetyl-l-serine sulfhydrylase (EC 4.2.99.8) were markedly repressed by l-methionine supplemented to the medium. l-Cysteine and glutathione, both at 0.5 mM, added to the medium as the sole sulfur source repressed the synthesis of O-acetylserine sulfhydrylase by 55 and 73%, respectively, confirming that this enzyme functions as a cysteine synthase. Methionine employed at 1 to 5 mM in the same way derepressed the synthesis of O-acetylserine sulfhydrylase 2.1- to 2.5-fold. A method for assaying a low concentration of sulfide (0.01 to 0.05 mM) liberated from homocysteine by determining cysteine synthesized with it in the presence of excess amounts of O-acetylserine and a purified preparation of the sulfhydrylase was established. The extract of cells catalyzed the homocysteine γ-lyase reaction, with a specific activity of 5 to 7 nmol/min/mg of protein, but not the methionine γ-lyase reaction. These results suggested that cysteine was also synthesized under the conditions employed by the catalysis of O-acetylserine sulfhydrylase using sulfur of homocysteine derived from methionine. Methionine inhibited O-acetylserine sulfhydrylase markedly. The effects of sulfur sources added to the medium on the synthesis of O-acetylhomoserine sulfhydrylase and the inhibition of the enzyme activity by methionine were mostly understood by assuming that the organism has two proteins having O-acetylhomoserine sulfhydrylase activity, one of which is cystathionine γ-synthase. Although it has been reported that homocysteine is directly synthesized in T. thermophilus HB27 by the catalysis of O-acetylhomoserine sulfhydrylase on the basis of genetic studies (T. Kosuge, D. Gao, and T. Hoshino, J. Biosci. Bioeng. 90:271–279, 2000), the results obtained in this study for the behaviors of related enzymes indicate that sulfur is first incorporated into cysteine and then transferred to homocysteine via cystathionine in T. thermophilus HB8.
Related Links	http://dx.doi.org/10.1128/jb.183.6.2086-2092.2001
Ending Page	2092
Page Count	7
Starting Page	2086
File Format	PDF
ISSN	00219193
e-ISSN	10985530
Journal	Journal of Bacteriology
Issue Number	6
Volume Number	183
Language	English
Publisher	American Society for Microbiology
Publisher Date	2001-03-15
Access Restriction	Open
Rights Holder	American Society for Microbiology
Subject Keyword	Molecular Biology Microbiology Research in Higher Education
Content Type	Text
Resource Type	Article
Subject	Molecular Biology Microbiology

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