Biology subjects

Saura, P.

Publications and source records attributed to Saura, P..

6 recordsLinked to original sources

bioRxiv2026

Electrostatic control of conformational dynamics drives proton translocation in respiratory Complex I

Complex I is a large membrane protein that transduces redox energy into a proton motive force across a biological membrane, powering cellular respiration. Yet, despite significant experimental and computational efforts, its long-range proton translocation mechanism remains poorly understood. Here, we combine nuclear magnetic resonance spectroscopy and site-directed mutagenesis experiments with multiscale classical and hybrid quantum/classical (QM/MM) simulations to probe how conformational dynamics is linked to proton transfer reactions in the terminal antiporter module (Nqo12/NuoL/ND5) of Complex I. We show that protonation of a conserved proton-loading site (Lys329/His241Nqo12) in the terminal antiporter-like module gives rise to conformational changes that result in opening of the proton pathway. We further show that this protonation reaction is controlled by buried ion-pairs that modulate the barrier for proton translocation by intrinsic electric field effects. On a general level, our combined findings reveal how bioenergetic machines combine conformational dynamics with hydration transitions to establish efficient redox-driven proton pumps.

biophysics↗

bioRxiv2026

Redox-Triggered Coupling Network Mediates Long-Range Energy Trans-duction in Respiratory Complex I

Complex I is a gigantic redox-driven proton pump that powers oxidative phosphorylation by a unique long-range (>200 [A]) proton-coupled electron transfer process. To elucidate the molecular principles underlying this intricate action-at-a-distance effect, we combine here multiscale quantum/classical (QM/MM) simulations with site-directed mutagenesis, proteoliposome experiments, and cryo-electron microscopy (cryo-EM). We find that quinol binding at a distinct site within a local membrane cavity, triggers a long-range protonation cascade over water-mediated proton wires along a conserved carboxylate pathway (E-channel). We identify a central mechanical switch point, comprising the conserved Tyr156H, the mutation of which impedes conformational changes along conserved loops, but not the proton transfer reaction itself. Using our integrative multi-disciplinary approach, we reveal central coupling sites along the redox-driven proton transport process mediating energy conversion in Complex I, and illustrate the power of theory-guided experiments.

biophysics↗

bioRxiv2026

A Carboxylate Switch Point Controls Long-Range Energy Transduction in Respiratory Complex I

Complex I is a highly intricate membrane-bound protein that powers the cellular energy metabolism by a long-range (>300 [A]) proton-coupled electron transfer (PCET) reaction. Here, we investigate the unknown coupling mechanism of Complex I by probing the charge transfer reaction along its functionally central carboxylate pathway (E-channel). By combining biophysical and site-directed mutagenesis experiments with high-resolution (2.6-2.7 [A]) cryo-electron microscopy (cryo-EM) and multiscale simulations, we identify a conserved carboxylate switch point (D79NuoA) that mediates proton transfer by establishing a kinetic gate that couples the redox chemistry to proton pumping. We find that mutation of the identified site, as found in patients suffering from severe neurodegenerative disorders, perturbs the charge transfer mechanism, and results in a drastic (>80%) reduction of the long-range PCET activity. Our combined findings illustrate mechanistic principles of molecular gates underlying long-range charge transfer reactions, and show how disease mutations perturb the function of conserved switch points in energy transduction.

biophysics↗

bioRxiv2025

Atomic resolution structure of spinach rubisco reveals protons and dynamics

Photosynthetic organisms sustain life on Earth by storing solar energy in biomass. Central to this process is rubisco, the enzyme that catalyses the fixation of CO2 to ribulose-1,5-bisphosphate, providing the primary gateway for inorganic carbon into the biosphere. Rubiscos catalytic efficiency is a major determinant of crop productivity and global carbon flux, making it a longstanding target for protein engineering.1-4 Yet, attempts to enhance its performance through rational design have met limited success due to an incomplete understanding of rubiscos catalytic mechanism. Here, we report an atomic resolution (1.25 [A]) cryo-EM structure of spinach rubisco in complex with the transition-state analogue 2-carboxyarabinitol-1,5-bisphosphate. Supported by large-scale quantum/classical (QM/MM) calculations, our structural analysis reveals protonation equilibria within the active site and unexpected structural flexibility across large protein regions despite the exceptionally tight ligand binding. Our findings provide new insight into the complex interplay of protonation equilibria and conformational sampling, suggesting a novel basis for rubiscos rational redesign utilizing strategies that rely on a combination of dynamic and electrostatic control.

biochemistry↗

bioRxiv2024

Molecular principles of redox-coupled sodium pumping of the ancient Rnf machinery

The Rnf complex is the primary respiratory enzyme of several anaerobic prokaryotes that transfers electrons from ferredoxin to NAD+ and pumps sodium ions (Na+) across a membrane, powering ATP synthesis. Rnf is widespread in primordial organisms and the evolutionary predecessor of the Na+-pumping NADH-quinone oxidoreductase (Nqr)1. By running in reverse, Rnf reduces ferredoxin with NADH as reductant at the expense of the transmembrane electrochemical ion gradient and provides low potential electrons for nitrogenases as well as CO2 reductases. Yet, the molecular principles that couple the long-range electron transfer to the Na+ translocation across the membrane remain elusive. Here we resolve key functional states along the electron transfer pathway using redox-controlled cryo-electron microscopy (cryo-EM) that, in combination with biochemical functional assays and atomistic molecular simulations, provide key insight into the redox-driven Na+ pumping mechanism. We show that the reduction of the unique membrane-embedded [2Fe2S] cluster in the vestibule between the RnfA/E subunits electrostatically attracts the sodium ions, and in turn, triggers an inward/outward transition with alternating membrane access driving the Na+ pump and the reduction of NAD+. Our study unveils an ancient mechanism for redox-driven ion pumping, and provides key understanding of the fundamental principles governing energy conversion in biological systems.

biochemistry↗

bioRxiv2022

Structural basis of mammalian Complex IV inhibition by steroids

The mitochondrial electron transport chain maintains the proton motive force that powers ATP synthesis. The energy for this process comes from oxidation of NADH and succinate, with the electrons from this oxidation passed via intermediate carriers to oxygen. Complex IV (CIV), the terminal oxidase, transfers electrons from the intermediate electron carrier cytochrome c to oxygen, contributing to the proton motive force in the process. Within CIV, protons move through the K- and D-pathways during turnover. The former is responsible for transferring two protons to the enzymes catalytic site upon reduction of the site, where they eventually combine with oxygen and electrons to form water. CIV is the main site for respiratory regulation, and although previous studies showed that steroid-binding can regulate CIV activity little is known about how this regulation occurs. Here we characterize the interaction between CIV and steroids using a combination of kinetic experiments, structure determination, and molecular simulations. We show that molecules with a sterol moiety, such as glyco-diosgenin and cholesteryl hemisuccinate, reversibly inhibit CIV. Flash photolysis experiments probing the high-speed equilibration of electrons within CIV demonstrate that binding of these molecules inhibits proton uptake through the K-pathway. Single particle cryo-EM of CIV with glyco-diosgenin reveals a previously undescribed steroid-binding site adjacent to the K-pathway, and molecular simulations suggest that the steroid binding modulates the conformational dynamics of key residues and proton transfer kinetics within this pathway. The binding pose of the sterol group sheds light on possible structural gating mechanisms in the CIV catalytic cycle. SIGNIFICANCE STATEMENTMammalian complex IV (CIV), the final complex of the mitochondrial electron transport chain, uses electrons from cytochrome c to reduce oxygen to water, driving aerobic life. Although CIV functions as the main site for respiratory regulation, there is little structural or biochemical information on how this regulation occurs. Previous studies provided evidence of CIV regulation by steroids, but the steroid binding site and regulatory mechanism remain unclear. Using single particle cryogenic electron microscopy, we discover the binding site of the steroid-derived detergent, glyco-diosgenin. Results from flash photolysis kinetic experiments with CIV in the presence of glyco-diosgenin and cholesterol hemisuccinate are combined with cryo-EM and molecular simulations to elucidate how steroid binding limits proton uptake by the complex.

biophysics↗