Takehiko Tosha

In all kingdoms of life, the regulation of membrane-bound enzyme function via oligomerization is a fundamental aspect of cell physiology. Often, the mechanistic role of oligomerization is unclear, due to a lack of structure-function comparisons between constituent forms of the enzyme. Here, we elucidate the structural underpinnings of enzyme regulation and oligomerization in the quinol-dependent nitric oxide reductase (qNOR) from Neisseria meningitidis, by high-resolution structural analyses of the less active monomeric form (2.25 Å) and the highly active dimeric form (1.89 Å). The comparison revealed that broad helical flexibility near the dimer interface of the monomer causes a conformational change in a critical amino acid near the active site, located apart from the dimer interface. We demonstrate that the crosstalk between the dimer interface and catalytic site in qNOR allows enhanced activation of the enzyme via dimerization. Given Neisseria meningitidis' dependence on qNOR to detoxify the host's immune response of nitric oxide, our results pave a way for new strategies to combat bacterial infections, via the inactivation of qNOR by monomerization. More broadly, this provides new insights into the role of membrane protein oligomerization and its influence on regulating activity.

Metalloproteins represent a major fraction of the protein kingdom and often exploit the redox chemistry of transition metals to drive key biological events involving proton and electron transfer. Copper is one of the most widely used transition metals whose redox properties are utilised in both electron transfer and catalysis of chemical substrates. Copper nitrite reductases (CuNiRs) utilise two types of copper centres and have become a model system for studying complex biological events that underpin the reaction mechanisms of redox enzymes, including proton-coupled electron transfer and substrate gating. We utilised the higher X-ray energy (13 keV) available at the SACLA X-ray Free Electron Laser (XFEL) and SHELXL refinement to obtain accurate atomic resolution structures of CuNiRs at ~1 Å from three organisms - in the oxidised (low and high pH), reduced and substrate-bound states. A consistent picture now emerges with the observation of a pentacoordinated oxidised catalytic type-2 Cu (T2Cu2+) centre in all cases. A tetracoordinated reduced T2Cu+ site with a single solvent ligand has also been captured, giving structural support to the random-sequential scheme with ordered pathway being dominant.

Protein dynamics play a crucial role in various physiological functions, including enzyme catalysis. To explore conformational changes during enzyme catalysis, we conduct mix-and-inject serial crystallography, an advanced technique to capture time-resolved protein structures in real time, using the microcrystals of bacterial copper amine oxidase containing a protein-derived quinone cofactor. Within 50 ms of mixing the microcrystals (<4 μm) with a preferred substrate (2-phenylethylamine) under anaerobic conditions (reductive half-reaction), we observe domain movements associated with substrate binding and formation of a metastable reaction intermediate, a product Schiff-base of the quinone cofactor. At 100-1000 ms after mixing, conformational transition from aminoresorcinol to the semiquinone radical forms of the reduced cofactor progresses gradually, likely depending on the replacement of the product aldehyde by the next-cycle amine substrate that triggers the cofactor conformational change. Overall, this study provides structural insight into enzyme catalysis accompanying the active-site conformational changes that are hardly scrutinized by studies in solution.

Modern insulin still depends on phenol and zinc to keep the hormone stable in vials and pumps, yet both additives slow absorption and raise safety concerns. We therefore asked a simple, clinically driven question: Can we stabilize the fast-acting T-state of insulin without phenol/zinc by exploiting pH-dependent water and anion binding? Using high-resolution synchrotron crystallography (1.4-1.76 Å), we solved novel designer and acid-stable cubic insulin structures from pH 2 to 6 in citrate-sulfate buffers and mapped solvent/anion contacts onto computational analyses. Across the acidic range, we uncovered a conserved 'water-anion clamp' centered on the Phe1ᴮ-Asn3ᴮ pocket that locks insulin in its bioactive T-conformation while neutralizing the protein's positive charge. This clamp: (i) removes the need for phenolic ligands, and (ii) keeps monomers soluble at high concentration. The structural blueprint we provide can guide formulation of phenol- and zinc-free, ultra-rapid insulin for subcutaneous pumps and high-strength cartridges, addressing unmet needs in intensive diabetes management. By clarifying how simple buffer anions and structured water can replace traditional preservatives, our work may link atomic-level detail to a practical therapeutic goal: faster, safer insulin delivery.