Promiscuity comes at a price: Catalytic versatility vs efficiency in different metal ion derivatives of the potential bioremediator GpdQ |
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Authors: | Lena J Daumann Bianca Y McCarthy Kieran S Hadler Tracy P Murray Lawrence R Gahan James A Larrabee David L Ollis Gerhard Schenk |
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Institution: | 1. School of Chemistry and Molecular Biosciences, The University of Queensland, St. Lucia, QLD 4072, Australia;2. Research School of Chemistry, the Australian National University, Canberra, ACT, 0200, Australia;3. Department of Chemistry and Biochemistry, Middlebury College, Middlebury, VT 05753, USA;4. Department of Chemistry, National University of Ireland, Maynooth, Maynooth, Co. Kildare, Ireland |
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Abstract: | The glycerophosphodiesterase from Enterobacter aerogenes (GpdQ) is a highly promiscuous dinuclear metallohydrolase with respect to both substrate specificity and metal ion composition. While this promiscuity may adversely affect the enzyme's catalytic efficiency its ability to hydrolyse some organophosphates (OPs) and by-products of OP degradation have turned GpdQ into a promising candidate for bioremedial applications. Here, we investigated both metal ion binding and the effect of the metal ion composition on catalysis. The prevalent in vivo metal ion composition for GpdQ is proposed to be of the type Fe(II)Zn(II), a reflection of natural abundance rather than catalytic optimisation. The Fe(II) appears to have lower binding affinity than other divalent metal ions, and the catalytic efficiency of this mixed metal center is considerably smaller than that of Mn(II), Co(II) or Cd(II)-containing derivatives of GpdQ. Interestingly, metal ion replacements do not only affect catalytic efficiency but also the optimal pH range for the reaction, suggesting that different metal ion combinations may employ different mechanistic strategies. These metal ion-triggered modulations are likely to be mediated via an extensive hydrogen bond network that links the two metal ion binding sites via residues in the substrate binding pocket. The observed functional diversity may be the cause for the modest catalytic efficiency of wild-type GpdQ but may also be essential to enable the enzyme to evolve rapidly to alter substrate specificity and enhance kcat values, as has recently been demonstrated in a directed evolution experiment. This article is part of a Special Issue entitled: Chemistry and mechanism of phosphatases, diesterases and triesterases. |
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Keywords: | Binuclear metallohydrolases Bioremediation Glycerophosphodiesterase Metal ion replacement Reaction mechanism |
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