Nucleotide-processing enzymes are key players in biological processes. They often operate through high substrate specificity for catalysis. How such specificity is achieved is unclear. Here, we dealt with this question by investigating all-α dimeric deoxyuridine triphosphate nucleotidohydrolases (dUTPases). Typically, these dUTPases hydrolyze either dUTP or deoxyuridine diphosphate (dUDP) substrates. However, the dUTPase enzyme DR2231 from Deinococcus radiodurans selectively hydrolyzes dUTP only, and not dUDP. By means of extended classical molecular dynamics simulations and quantum chemical calculations, we show that DR2231 achieves this specificity for dUTP via second-shell basic residues that, together with the two catalytic magnesium ions, contribute to properly orienting the γ-phosphate of dUTP in a prereactive state. This allows a nucleophilic water to be correctly placed and activated in order to perform substrate hydrolysis. We show that this enzymatic mechanism is not viable when dUDP is bound to DR2231. Importantly, in several other dUTPases capable of hydrolyzing either dUTP or dUDP, we detected that active site second-shell basic residues are more in number, anchoring the β-phosphate of the nucleotide substrate too, in contrast to what is observed in DR2231. Thus, strategically located basic second-shell residues mediate precise reactant positioning at the catalytic site, determining substrate specificity in dUTPases and possibly in other structurally similar nucleotide-processing metalloenzymes.Nucleotide-processing enzymes are key players in biological processes. They often operate through high substrate specificity for catalysis. How such specificity is achieved is unclear. Here, we dealt with this question by investigating all-α dimeric deoxyuridine triphosphate nucleotidohydrolases (dUTPases). Typically, these dUTPases hydrolyze either dUTP or deoxyuridine diphosphate (dUDP) substrates. However, the dUTPase enzyme DR2231 from Deinococcus radiodurans selectively hydrolyzes dUTP only, and not dUDP. By means of extended classical molecular dynamics simulations and quantum chemical calculations, we show that DR2231 achieves this specificity for dUTP via second-shell basic residues that, together with the two catalytic magnesium ions, contribute to properly orienting the γ-phosphate of dUTP in a prereactive state. This allows a nucleophilic water to be correctly placed and activated in order to perform substrate hydrolysis. We show that this enzymatic mechanism is not viable when dUDP is bound to DR2231. Importantly, in several other dUTPases capable of hydrolyzing either dUTP or dUDP, we detected that active site second-shell basic residues are more in number, anchoring the β-phosphate of the nucleotide substrate too, in contrast to what is observed in DR2231. Thus, strategically located basic second-shell residues mediate precise reactant positioning at the catalytic site, determining substrate specificity in dUTPases and possibly in other structurally similar nucleotide-processing metalloenzymes.

Metal-dependent formate dehydrogenases (Fdh) catalyze the reversible conversion of CO2 to formate, with unrivalled efficiency and selectivity. However, the key catalytic aspects of these enzymes remain unknown, preventing us from fully benefiting from their capabilities in terms of biotechnological applications. Here, we report a time-resolved characterization by X-ray crystallography of the Desulfovibrio vulgaris Hildenborough SeCys/W-Fdh during formate oxidation. The results allowed us to model five different intermediate structures and to chronologically map the changes occurring during enzyme reduction. Formate molecules were assigned for the first time to populate the catalytic pocket of a Fdh. Finally, the redox reversibility of DvFdhAB in crystals was confirmed by reduction and reoxidation structural studies.

Metal-dependent formate dehydrogenases are very promising targets for enzyme optimization and design of bio-inspired catalysts for CO2 reduction, towards novel strategies for climate change mitigation. For effective application of these enzymes, the catalytic mechanism must be fully understood, and the molecular determinants clarified. Despite numerous studies, several doubts persist, namely regarding the role played by the possible dissociation of the SeCys ligand from the Mo/W active site. Additionally, the O2 sensitivity of these enzymes must also be understood as it poses an important obstacle for biotechnological applications. Here we present a combined biochemical, spectroscopic, and structural characterization of Desulfovibrio vulgaris FdhAB (DvFdhAB) when exposed to oxygen in the presence of a substrate (formate or CO2). This study reveals that O2 inactivation is promoted by the presence of either substrate and involves forming a new species in the active site, captured in the crystal structures, where the SeCys ligand is displaced from tungsten coordination and replaced by a dioxygen or peroxide molecule. This new form was reproducibly obtained and supports the conclusion that, although W-DvFdhAB can catalyze the oxidation of formate in the presence of oxygen for some minutes, it gets irreversibly inactivated after prolonged O2 exposure in the presence of either substrate. These results reveal that oxidative inactivation does not require reduction of the metal, as widely assumed, as it can also occur in the oxidized state in the presence of CO2.Competing Interest StatementThe authors have declared no competing interest.AORAldehyde Oxido-reductaseDTTDithiothreitolDvDesulfovibrio vulgarisEPRElectron Paramagnetic ResonanceFdhFormate dehydrogenaseHPHigh PressureMGDMolybdopterin Guanine DinucleotidesNDNew dropROSReactive Oxygen SpeciesSODSuperoxide dismutaseTSAThermal Shift Assay