A Water‐Bridged H‐Bonding Network Contributes to the Catalysis of the SAM‐Dependent C‐Methyltransferase HcgC. Issue 36 (2nd August 2017)

• Record Type: Journal Article
• Authors: Bai, Liping; Wagner, Tristan; Xu, Tao; Hu, Xile; Ermler, Ulrich; Shima, Seigo
• Is Part Of: Angewandte Chemie international edition. Volume 56:Issue 36(2017)
• Page Start: 10806
• Page End: 10809
• Publication Date: 2017-08-02
• Subjects: biosynthesis -- enzyme mechanisms -- methyltransferases -- mutagenesis -- protein structures; Chemistry -- Periodicals; 540
• DOI: 10.1002/anie.201705605
• Languages: English
• ISSNs: 1433-7851

Abstract

Abstract: [Fe]‐hydrogenase hosts an iron‐guanylylpyridinol (FeGP) cofactor. The FeGP cofactor contains a pyridinol ring substituted with GMP, two methyl groups, and an acylmethyl group. HcgC, an enzyme involved in FeGP biosynthesis, catalyzes methyl transfer from S ‐adenosylmethionine (SAM) to C3 of 6‐carboxymethyl‐5‐methyl‐4‐hydroxy‐2‐pyridinol (2 ). We report on the ternary structure of HcgC/ S ‐adenosylhomocysteine (SAH, the demethylated product of SAM) and2 at 1.7 Å resolution. The proximity of C3 of substrate2 and the S atom of SAH indicates a catalytically productive geometry. The hydroxy and carboxy groups of substrate2 are hydrogen‐bonded with I115 and T179, as well as through a series of water molecules linked with polar and a few protonatable groups. These interactions stabilize the deprotonated state of the hydroxy groups and a keto form of substrate2, through which the nucleophilicity of C3 is increased by resonance effects. Complemented by mutational analysis, a structure‐based catalytic mechanism was proposed. Abstract : Wet Wet Wet : The enzyme HcgC catalyzes methyl transfer from S ‐adenosylmethionine to C3 of 6‐carboxymethyl‐5‐methyl‐4‐hydroxy‐2‐pyridinol, which is involved in the biosynthesis of the [Fe]‐hydrogenase cofactor. Structures of the HcgC/pyridinol/ S ‐adenosylhomocysteine (SAH) complex revealed a water‐bridged H‐bonding network at the pyridinol‐binding site, which suggests a unique activation strategy involving water‐balanced keto/enolate resonance stabilization.