Paracoccus denitrificans: a genetically tractable model system for studying respiratory complex I.

Area of Science:

• Biochemistry
• Molecular Biology
• Bioenergetics

Background:

• Mitochondrial complex I (NADH:ubiquinone oxidoreductase) is vital for ATP synthesis.
• Its energy capture, proton pumping mechanisms, and pathways remain poorly understood.
• Current research is hindered by a lack of versatile model systems for comprehensive analysis.

Purpose of the Study:

• To establish a robust bacterial model system for mitochondrial complex I research.
• To enable combined functional, structural, and mutagenic analyses of complex I.
• To facilitate detailed investigation into the enzyme's catalytic and proton-translocating mechanisms.

Main Methods:

• Development of a purification protocol for active complex I using a His6-tag on the Nqo5 subunit in *Paracoccus denitrificans*.
• Optimization of enzyme reconstitution into liposomes to assess proton pumping activity.
• Engineering a *P. denitrificans* strain suitable for complex I mutagenesis, including creation of a catalytically inactive variant.

Main Results:

• A highly active form of mitochondrial complex I was successfully purified from *P. denitrificans*.
• Reconstitution assays confirmed the proton pumping activity of the purified enzyme.
• A genetically tractable *P. denitrificans* model was established, allowing for targeted mutagenesis of complex I subunits.

Conclusions:

• The developed *Paracoccus denitrificans* model system is suitable for studying mitochondrial complex I.
• This model integrates mutagenesis with biophysical measurements for comprehensive mechanistic studies.
• It offers new avenues to elucidate the complex mechanisms of energy transduction in complex I.