Masahide Hikita

Nickel‐chelatase CfbA, unlike descendant chelatases, is an ancestral class II chelatase with a symmetric active site architecture. CfbA utilizes sirohydrochlorin (SHC) as a physiological substrate in the biosynthesis of coenzyme F430. CbiX^S, a structural analog of CfbA, can use uroporphyrin III (UPIII) and uroporphyrin I (UPI) as non‐physiological substrates. Owing to the broad tetrapyrrole specificity of the unique active site of ancestral class II chelatases, the substrate recognition mechanism of CfbA has garnered interest. Herein, we conducted an X‐ray crystallographic analysis of CfbA in complex with UPIII and UPI. Interestingly, the binding sites for UPIII and UPI were distinct. UPI was bound at the entrance of the active site, whereas UPIII was bound deep inside the active site cavity in a manner similar to SHC. Despite the difference in the binding positions of UPIII and UPI, Ser11 at the active site provided critical polar interactions for recognizing UPIII and UPI. Several CfbA variants with a Ser11 mutation were studied to confirm the significance of Ser11's position in the context of tetrapyrrole recognition. The CfbA S11T variant showed Ni²⁺‐chelatase activity against coproporphyrin I (CPI), which is a more hydrophobic tetrapyrrole than UPIII and UPI. Using a CPI‐docked model of the S11T variant, we proposed that balancing the hydrophobic/polar interactions at residue 11 could alter substrate selectivity. The structural and mutational analyses reported here highlight the importance of polar and hydrophobic interactions at the entry region of the active site for substrate tetrapyrrole recognition by ancestral and descendant class II chelatases.

While native SAD phasing is a promising method for next-generation macromolecular crystallography, it requires the collection of high-quality diffraction data using long-wavelength X-rays. The crystal itself and the noncrystalline medium around the crystal can cause background noise during long-wavelength X-ray data collection, hampering native SAD phasing. Optimizing the crystal size and shape or removing noncrystalline sample portions have thus been considered to be effective means of improving the data quality. A crystal-processing machine that uses a deep-UV laser has been developed. The machine utilizes the pulsed UV laser soft ablation (PULSA) technique, which generates less heat than methods using infrared or visible lasers. Since protein crystals are sensitive to heat damage, PULSA is an appropriate method to process them. Integration of a high-speed Galvano scanner and a high-precision goniometer enables protein crystals to be shaped precisely and efficiently. Application of this crystal-processing machine to a long-wavelength X-ray diffraction experiment significantly improved the diffraction data quality and thereby increased the success rate in experimental phasing using anomalous diffraction from atoms.

Applying long wavelength X-ray （ca. longer than 2 Å） in macromolecular crystallography has been limited due to the lack of dedicated measurement environment. Photon Factory BL-1A, as an MX beamline optimized for diffraction experiment using the wavelength from 2.7 to 3.3 Å, offers the opportunity to routinely perform data collection for Native SAD phasing or light atoms identification with much sophisticated manner. Sample optimization is another key for successful experiment at the wavelength where severe absorption is inevitable. We describe how the difficulties in use of long wavelength X-ray were overcome at the beamline and the auxiliary facility, as well as the current results of Native SAD phasing.