Structure of a functional obligate complex III₂IV₂ respiratory supercomplex from Mycobacterium smegmatis

Area of Science:

• Biochemistry and Molecular Biology
• Microbiology
• Structural Biology

Background:

• The mycobacterial electron-transport chain (ETC) is vital for respiration and ATP synthesis.
• Respiratory complex III (CIII) transfers electrons to complex IV (CIV), reducing oxygen.
• Proton translocation across the membrane by the ETC establishes the electrochemical gradient.

Purpose of the Study:

• To isolate, characterize, and determine the structure of the obligate CIII2IV2 supercomplex from Mycobacterium smegmatis.
• To understand the functional role of this supercomplex in mycobacterial respiration.
• To investigate the mechanism of electron transfer and proton translocation within the supercomplex.

Main Methods:

• Isolation and biochemical characterization of the CIII2IV2 supercomplex.
• Determination of the supercomplex's three-dimensional structure.
• Enzyme activity assays and substrate binding studies (menaquinone).

Main Results:

• The M. smegmatis CIII2IV2 supercomplex exhibits quinol:O2 oxidoreductase activity without requiring exogenous cytochrome c.
• A superoxide dismutase subunit is integrated into the supercomplex, potentially detoxifying reactive oxygen species.
• Menaquinone was found bound at both Q_o and Q_i sites of CIII, and the intrinsic diheme cytochrome cc subunit shows conformational flexibility, acting as an electronic switch.

Conclusions:

• The CIII2IV2 supercomplex is a functional unit in mycobacteria, replacing canonical cytochrome c1 and soluble cytochrome c.
• The supercomplex's structure supports efficient electron transfer to oxygen and proton pumping.
• The conformational plasticity of the diheme cytochrome cc subunit suggests a regulatory role in respiration.