Structure and mechanism of a nonhaem-iron SAM-dependent C-methyltransferase and its engineering to a hydratase and an O-methyltransferase

Xiao Wei Zou, Yu Chen Liu, Ning Shian Hsu, Chuen Jiuan Huang, Syue Yi Lyu, Hsiu Chien Chan, Chin-Yuan Chang, Hsien Wei Yeh, Kuan Hung Lin, Chang Jer Wu, Ming Daw Tsai, Tsung Lin Li*

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

15 Scopus citations

Abstract

In biological systems, methylation is most commonly performed by methyltransferases (MTs) using the electrophilic methyl source S-adenosyl-l-methionine (SAM) via the SN2 mechanism. (2S,3S)-β- Methylphenylalanine, a nonproteinogenic amino acid, is a building unit of the glycopeptide antibiotic mannopeptimycin. The gene product of mppJ from the mannopeptimycin-biosynthetic gene cluster is the MT that methylates the benzylic C atom of phenylpyruvate (Ppy) to give βMePpy. Although the benzylic C atom of Ppy is acidic, how its nucleophilicity is further enhanced to become an acceptor for C-methylation has not conclusively been determined. Here, a structural approach is used to address the mechanism of MppJ and to engineer it for new functions. The purified MppJ displays a turquoise colour, implying the presence of a metal ion. The crystal structures reveal MppJ to be the first ferric ion SAM-dependent MT. An additional four structures of binary and ternary complexes illustrate the molecular mechanism for the metal ion-dependent methyltransfer reaction. Overall, MppJ has a nonhaem iron centre that bind, orients and activates the α-ketoacid substrate and has developed a sandwiched bi-water device to avoid the formation of the unwanted reactive oxo-iron(IV) species during the C-methylation reaction. This discovery further prompted the conversion of the MT into a structurally/functionally unrelated new enzyme. Through stepwise mutagenesis and manipulation of coordination chemistry, MppJ was engineered to perform both Lewis acid-assisted hydration and/or O-methyltransfer reactions to give stereospecific new compounds. This process was validated by six crystal structures. The results reported in this study will facilitate the development and design of new biocatalysts for difficult-to-synthesize biochemicals.

Original languageEnglish
Pages (from-to)1549-1560
Number of pages12
JournalActa Crystallographica Section D: Biological Crystallography
Volume70
Issue number6
DOIs
StatePublished - 1 Jan 2014

Keywords

  • MppJ
  • methyltransferases

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