Mg2+ binding and catalytic function of Sphingomyelinase from Bacillus cereus

Shinobu Fujii, Bunpei Inoue, Hiroki Yamamoto, Kenji Ogata, Tomohiro Shinki, Seiji Inoue, Masahiro Tomita, Hiro Omi Tamura, Kikuo Tsukamoto, Hiroh Ikezawa, Kiyoshi Ikeda

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Abstract

The modes of Mg2+ binding to SMase from Bacillus cereus were studied on the basis of the changes in the tryptophyl fluorescence intensity. This enzyme was shown to possess at least two binding sites for Mg2+ with low and high affinities. The effects of Mg2+ binding on the enzymatic activity and structural stability of the enzyme molecule were also studied. The results indicated that the binding of Mg2+ to the low-affinity site was essential for the catalysis, but was independent of the substrate binding to the enzyme. It was also indicated that the alkaline denaturation of the enzyme was partly prevented by the Mg2+ binding, whereas no significant protective effect was observed against the denaturation by urea. The pH dependence of the kinetic parameters for the hydrolysis of micellar HNP and mixed micellar SM with Triton X-100 (1:10), catalyzed by SMase from B. cereus, was studied in the presence of a large amount of Mg2+ to saturate both the low- and high-affinity sites. The pH dependence curves of the logarithm of 1/K(m) for these two kinds of substrates were similar in shape to each other, and showed a single transition. On the other hand, the shapes of the pH dependence curves of the logarithm of k(cat) for these two kinds of substrates were different from each other. The pH dependence curve for micellar HNP showed three transitions and, counting from the acidic end of the pH region, the first and third transitions having tangent lines with slopes of +1 and -1, respectively. On the other hand, the curve for mixed micellar SM with Triton X-100 showed one large transition with a slope of +1 (the first transition) and a very small transition (the third transition). On the basis of the present results and the three-dimensional structure of bovine pancreatic DNase I, which has a primary structure similar to that of B. cereus SMase, we proposed a catalytic mechanism for B. cereus SMase based on general-base catalysis.

Original languageEnglish
Pages (from-to)1178-1187
Number of pages10
JournalJournal of biochemistry
Volume124
Issue number6
DOIs
Publication statusPublished - 1998

Keywords

  • Catalytic mechanism
  • Denaturation
  • Enzyme kinetics
  • Sphingomyelinase

ASJC Scopus subject areas

  • Biochemistry
  • Molecular Biology

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    Fujii, S., Inoue, B., Yamamoto, H., Ogata, K., Shinki, T., Inoue, S., Tomita, M., Tamura, H. O., Tsukamoto, K., Ikezawa, H., & Ikeda, K. (1998). Mg2+ binding and catalytic function of Sphingomyelinase from Bacillus cereus. Journal of biochemistry, 124(6), 1178-1187. https://doi.org/10.1093/oxfordjournals.jbchem.a022236