PuSH - Publication Server of Helmholtz Zentrum München: Characteristic isotope fractionation patterns in <em>s</em>-triazine degradation have their origin in multiple protonation options in the <em>s</em>-triazine hydrolase trzn.

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Schürner, H.K.V. ; Seffernick, J.L.* ; Grzybkowska, A.* ; Dybała-Defratyka, A.* ; Wackett, L.P.* ; Elsner, M.

Characteristic isotope fractionation patterns in s-triazine degradation have their origin in multiple protonation options in the s-triazine hydrolase trzn.

Environ. Sci. Technol. 49, 3490-3498 (2015)

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s-Triazine herbicides (atrazine, ametryn) are groundwater contaminants which may undergo microbial hydrolysis. Previously, inverse nitrogen isotope effects in atrazine degradation by Arthrobacter aurescens TC1 (i) delivered highly characteristic (13C/12C, 15N/14N) fractionation trends for pathway identification and (ii) suggested that the s-triazine ring nitrogen was protonated in the enzyme s-triazine hydrolase (TrzN) where (iii) TrzN crystal structure and mutagenesis indicated H+-transfer from the residue E241. This study tested the general validity of these conclusions for atrazine and ametryn with purified TrzN and a TrzN-E241Q site-directed mutant. TrzN-E241Q lacked activity with ametryn; otherwise, degradation consistently showed normal carbon isotope effects (εcarbon = -5.0‰ ± 0.2‰ (atrazine/TrzN), εcarbon = -4.2‰ ± 0.5‰ (atrazine/TrzN-E241Q), εcarbon = -2.4‰ ± 0.3‰ (ametryn/TrzN)) and inverse nitrogen isotope effects (εnitrogen = 2.5‰ ± 0.1‰ (atrazine/TrzN), εnitrogen = 2.1‰ ± 0.3‰ (atrazine/TrzN-E241Q), εnitrogen = 3.6‰ ± 0.4‰ (ametryn/TrzN)). Surprisingly, TrzN-E241Q therefore still activated substrates through protonation implicating another proton donor besides E241. Sulfur isotope effects were larger in enzymatic (εsulfur = -14.7‰ ± 1.0‰, ametryn/TrzN) than in acidic ametryn hydrolysis (εsulfur = -0.2‰ ± 0.0‰, pH 1.75), indicating rate-determining C-S bond cleavage in TrzN. Our results highlight a robust inverse 15N/14N fractionation pattern for identifying microbial s-triazine hydrolysis in the environment caused by multiple protonation options in TrzN.

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