Correlating kinetic and structural data on ubiquinone binding and reduction by respiratory complex I

Area of Science:

• Biochemistry
• Molecular Biology
• Cellular Respiration

Background:

• Respiratory complex I (NADH:ubiquinone oxidoreductase) is essential for ATP synthesis in mammalian cells.
• Ubiquinone-10's hydrophobic nature and its binding site, located above the mitochondrial membrane, present a challenge for efficient catalysis.
• The mechanism of ubiquinone/ubiquinol exchange and its role in coupling electron transfer to proton translocation remain unclear.

Purpose of the Study:

• To investigate the kinetics of complex I catalysis with ubiquinones of varying isoprenoid chain lengths.
• To elucidate the role of the ubiquinone binding channel in substrate/product exchange and energy conversion.

Main Methods:

• Utilized proteoliposomes reconstituted with complex I and a quinol oxidase.
• Determined the kinetics of complex I catalysis using ubiquinones with isoprenoid chains of lengths 1 to 10.
• Interpreted kinetic data in conjunction with structural information of the ubiquinone binding channel.

Main Results:

• The hydrophobic channel in complex I features a charged region that influences ubiquinone binding and dissociation.
• Ubiquinol-10 dissociation is not rate-limiting, and ubiquinone-10 exhibits the highest binding affinity and fastest binding rate.
• The channel's structure, including a charged region and chain directionality, facilitates rapid product dissociation and short transit times for long-chain ubiquinones.

Conclusions:

• The intricate structure of the ubiquinone binding channel in complex I is optimized for efficient catalysis with physiological ubiquinone-10.
• Product dissociation can become rate-limiting for short-chain ubiquinones, highlighting the specificity of the channel.
• Ubiquinone/ubiquinol binding and dissociation likely play significant roles in the energy conversion mechanism of complex I.