Carbonyl Sulfide Hydrolase from Thiobacillus thioparus Strain THI115 Is One of the 尾-Carbonic Anhydrase Family Enzymes

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• 作者：Takahiro Ogawa ; Keiichi Noguchi ; Masahiko Saito ; Yoshiko Nagahata ; Hiromi Kato ; Akashi Ohtaki ; Hiroshi Nakayama ; Naoshi Dohmae ; Yasuhiko Matsushita ; Masafumi Odaka ; Masafumi Yohda ; Hiroshi Nyunoya ; Yoko Katayama
• 刊名：The Journal of the American Chemical Society
• 出版年：2013
• 出版时间：March 13, 2013
• 年：2013
• 卷：135
• 期：10
• 页码：3818-3825
• 全文大小：594K
• 年卷期：v.135,no.10(March 13, 2013)
• ISSN：1520-5126

文摘

Carbonyl sulfide (COS) is an atmospheric trace gas leading to sulfate aerosol formation, thereby participating in the global radiation balance and ozone chemistry, but its biological sinks are not well understood. Thiobacillus thioparus strain THI115 can grow on thiocyanate (SCN^{鈥?/sup>) as its sole energy source. Previously, we showed that SCN鈥?/sup> is first converted to COS by thiocyanate hydrolase in T. thioparus strain THI115. In the present work, we purified, characterized, and determined the crystal structure of carbonyl sulfide hydrolase (COSase), which is responsible for the degradation of COS to H₂S and CO₂, the second step of SCN鈥?/sup> assimilation. COSase is a homotetramer composed of a 23.4 kDa subunit containing a zinc ion in its catalytic site. The amino acid sequence of COSase is homologous to the 尾-class carbonic anhydrases (尾-CAs). Although the crystal structure including the catalytic site resembles those of the 尾-CAs, CO₂ hydration activity of COSase is negligible compared to those of the 尾-CAs. The 伪5 helix and the extra loop (Gly150鈥揚ro158) near the N-terminus of the 伪6 helix narrow the substrate pathway, which could be responsible for the substrate specificity. The k_cat/K_m value, 9.6 脳 105 s鈥? M鈥?, is comparable to those of the 尾-CAs. COSase hydrolyzes COS over a wide concentration range, including the ambient level, in vitro and in vivo. COSase and its structurally related enzymes are distributed in the clade D in the phylogenetic tree of 尾-CAs, suggesting that COSase and its related enzymes are one of the catalysts responsible for the global sink of COS.}