Takeshi Hiromoto

Human MTH1, a Nudix enzyme, hydrolyzes several oxidized nucleotides such as 8-oxo-dGTP and 2-oxo-dATP, owing to its broad substrate specificity. MTH1 has also attracted attention as an anticancer target, and its substrate recognition is of biological and medical interest. Previous studies have suggested that MTH1 exhibits broad substrate specificity by changing the protonation states of Asp119 and Asp120 with high p K _a . However, the recognition mechanism remains unclear, owing to the difficulty in directly observing hydrogen atoms. Furthermore, a recent time-resolved X-ray study has proposed that Nudix hydrolases catalyze reactions through a new three-metal-ion mechanism. To understand the substrate recognition and catalytic mechanisms of human MTH1, we performed neutron and time-resolved X-ray crystallography. The neutron structures of MTH1 complexed with 8-oxo-dGTP and 2-oxo-dATP revealed the protonation states of the active-site residues, substrates, and water molecules, crucial for substrate binding and catalysis, providing direct experimental evidence that changes in the protonation states of Asp119 and Asp120 enable broad substrate recognition of MTH1. Time-resolved X-ray crystallography was used to visualize the entire reaction process through Mn ²⁺ ion. The combination of neutron and time-resolved X-ray crystallography led to the proposal of a reaction mechanism for MTH1 via three metal-binding sites, including the conformational dynamics of a loop region, nucleophilic substitution, and a potential deprotonation pathway. Overall, the mechanism involving three metal-binding sites may be a general feature in the catalysis of Nudix hydrolases.

Significance: DNA damage causes loss of or alterations in genetic information, resulting in cell death or mutations. Ionizing radiations produce local, multiple DNA damage sites called clustered DNA damage. In this study, a complete protocol was established to analyze the damage complexity of clustered DNA damage, wherein damage-containing genomic DNA fragments were selectively concentrated via pulldown, and clustered DNA damage was visualized by atomic force microscopy. It was found that X-rays and Fe ion beams caused clustered DNA damage. Fe ion beams also produced clustered DNA damage with high complexity. Fe ion beam–induced complex DNA double-strand breaks (DSBs) containing one or more base lesion(s) near the DSB end were refractory to repair, implying their lethal effects.

T4 phage lysozyme is an inverting glycoside hydrolase that degrades the murein of bacterial cell walls by cleaving the β‐1,4‐glycosidic bond. The substitution of the catalytic Thr26 residue to a histidine converts the wild type from an inverting to a retaining enzyme, which implies that the original general acid Glu11 can also act as an acid/base catalyst in the hydrolysis. Here, we have determined the neutron structure of the perdeuterated T26H mutant to clarify the protonation states of Glu11 and the substituted His26, which are key in the retaining reaction. The 2.09‐Å resolution structure shows that the imidazole group of His26 is in its singly protonated form in the active site, suggesting that the deprotonated Nɛ2 atom of His26 can attack the anomeric carbon of bound substrate as a nucleophile. The carboxyl group of Glu11 is partially protonated and interacts with the unusual neutral state of the guanidine moiety of Arg145, as well as two heavy water molecules. Considering that one of the water‐binding sites has the potential to be occupied by a hydronium ion, the bulk solvent could be the source for the protonation of Glu11. The respective protonation states of Glu11 and His26 are consistent with the bond lengths determined by an unrestrained refinement of the high‐resolution X‐ray structure of T26H at 1.04‐Å resolution. The detail structural information, including the coordinates of the deuterium atoms in the active site, provides insight into the distinctively different catalytic activities of the mutant and wild type enzymes.