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. Author manuscript; available in PMC: 2018 Oct 18.
Published in final edited form as: J Am Chem Soc. 2017 Oct 5;139(41):14638–14648. doi: 10.1021/jacs.7b07849

Figure 1. The Hydrogen Bonding Network and Primary Photoactivation Mechanism.

Figure 1

The isoalloxazine ring is surrounded by a conserved hydrogen bonded network that includes a Tyr Y21(8), Gln Q63(50) and Asn N45(32). Also shown is a Trp W104(91) which is shown hydrogen bonded to the Gln residue in the dark state but not in the light state. Movement of the Trp from a Trpin to a Trpout conformation is thought to be a central component of the BLUF photoactivation mechanism although we note that in the X-ray crystal structures of PixD in the dark (PDB: 2HFN)22 and a stable lit states (PDB: 3MZI),7 the W91 sidechain is rotated out of the binding pocket. Photoactivation involves rotation of the Gln side-chain, and in the above mechanism keto-enol tautomerism of the Gln side-chain is shown preceding side-chain rotation.10, 23–25 Residues are numbered based on the sequence of AppA with the residue number in PixD given in parentheses.