FIGURE 1.

A, charge-transfer relay in NarG. Three water molecules are conserved across a range of NarGHI structures, including those described by Protein Data Bank codes 1Q16, 1R27, 1YZ4 and 1Y5I (25, 60, 61). These waters are linked to the pyran oxygen of the bicyclic distal pyranopterin by NarG-His¹¹⁸⁴ and NarG-His¹¹⁶³. The OG oxygen of NarG-Ser⁷¹⁹ is also within H-bonding distance of the pyran oxygen. The image was generated using the PyMol molecular visualization package (version 1.4.1, Schrödinger LLC). B, mechanism of charge-transfer between the pyran oxygen and the conserved water molecules. The indicated charge-transfer results in protonation of the alkoxide anion form of the pyran oxygen. The image was generated using the MarvinSketch software package (www.chemaxon.com). In both panels, some predicted hydrogens have been added for clarity.