bims-cytox1 2026-01-18 papers

J Phys Chem B. 2026 Jan 10.

Conformational Gating in Electron Transfer from Cytochrome c to Cytochrome c Oxidase.

Wataru Sato, Kowit Hengphasatporn, Atsuhiro Shimada, Takeshi Uchida, Shinya Yoshikawa, Yasuteru Shigeta, Koichiro Ishimori.

Understanding the molecular basis of interprotein electron transfer (ET) is essential for elucidating the mechanisms of bioenergetic processes. In this study, we characterize the ET kinetics between cytochrome c (Cyt c) and cytochrome c oxidase (CcO) by determining the ET rate constant (kET) within their complex using temperature-dependent flow-flash spectroscopy. The measured kET was on the order of 104 s-1, corresponding to an ET distance of ∼13 Å, as estimated via Marcus theory, significantly shorter than the ∼23 Å distance inferred for the ES complex based on docking simulations using Michaelis constants (KM). These results provide strong evidence that ET does not occur within the canonical ES complex but rather within a distinct ET-active complex characterized by a shorter ET distance. Docking simulations further support the existence of this ET-active complex by identifying configurations with restricted ET distances. Importantly, the observed kET is approximately 80 times faster than the catalytic constant (kcat), indicating that ET is not the rate-limiting step in the overall Cyt c-CcO reaction. Given that kcat reflects a millisecond-scale conformational transition from the ES complex to the ET-active complex, it is likely governed by the structural fluctuation of the proteins. These findings support a conformational gating mechanism, wherein thermal fluctuations of protein structure critically regulate ET efficiency. This study advances our understanding of protein-protein ET from Cyt c to CcO by highlighting the role of dynamic structural transitions in modulating the reaction kinetics.

DOI: https://doi.org/10.1021/acs.jpcb.5c06274

Chem Commun (Camb). 2026 Jan 12.

Electronic coupling in a synthetic model of the Cu-cofactor unit of the PM state in cytochrome c oxidase.

Sang Tae Lee, Christopher G Bailey, Daniel A Santos, Mohan Bhadbhade, Heejin Kim, Dong Jun Kim.

A hydroquinone-imidazole-Cu(II) complex reproduces the Tyr-His-Cu unit of cytochrome c oxidase. Laser-induced EPR and DFT reveal a semiquinone radical electronically isolated from Cu(II) by orthogonal orbital alignment. The weak magnetic exchange underscores how geometry governs coupling, providing insight into transient states of metalloenzymes and their synthetic analogues.

DOI: https://doi.org/10.1039/d5cc05355b

Nat Commun. 2026 Jan 10.

Structural basis for late maturation steps of mitochondrial respiratory chain complex IV within the human respirasome.

Minh Duc Nguyen, Ana Sierra-Magro, Vivek Singh, Anas Khawaja, Alba Timón-Gómez, Antoni Barrientos, Joanna Rorbach.

The mitochondrial respiratory chain comprises four multimeric complexes (CI-CIV) that drive oxidative phosphorylation by transferring electrons to oxygen and generating the proton gradient required for ATP synthesis. These complexes can associate into supercomplexes (SCs), such as the CI + CIII₂ + CIV respirasome, but how SCs form, by joining preassembled complexes or by engaging partially assembled intermediates, remains unresolved. Here, we use cryo-electron microscopy to determine high-resolution structures of native human CI + CIII₂ + CIV late-assembly intermediates. Together with biochemical analyses, these structures show that respirasome biogenesis concludes with the final maturation of CIV while it is associated with fully assembled CI and CIII₂. We identify HIGD2A as a placeholder factor within isolated and supercomplexed CIV that is replaced by subunit NDUFA4 during the last step of CIV and respirasome assembly. This mechanism suggests that placeholders such as HIGD2A act as molecular timers, preventing premature incorporation of NDUFA4 or its isoforms and ensuring the orderly progression of pre-SC particles into functional respirasomes. Since defects in CIV assembly, including NDUFA4 deficiencies, cause severe encephalomyopathies and neurodegenerative disorders, understanding the molecular architecture and assembly pathways of isolated and supercomplexed CIV offers insight into the pathogenic mechanisms underlying these conditions.

DOI: https://doi.org/10.1038/s41467-025-68274-3