A conserved haem redox and trafficking pathway for cofactor attachment.

Area of Science:

• Biochemistry
• Molecular Biology
• Cell Biology

Background:

• The cytochrome c maturation (Ccm) pathway is essential for synthesizing functional cytochrome c in bacteria, archaea, and eukaryotic mitochondria.
• This pathway involves eight membrane proteins (CcmABCDEFGH), with CcmABCDE proposed to deliver heme to the CcmF/H synthetase complex.
• The precise molecular mechanisms of heme binding, trafficking, and redox control within the Ccm pathway remain largely unknown.

Purpose of the Study:

• To elucidate the molecular mechanisms underlying heme binding, trafficking, and redox control in the Ccm pathway.
• To identify the specific roles of Ccm pathway components in handling heme.
• To understand how the Ccm pathway ensures heme availability and appropriate redox state for cytochrome c synthesis.

Main Methods:

• Purification of Ccm pathway complexes with trapped heme.
• In vitro biochemical assays to characterize protein-heme interactions and enzymatic activities.
• Spectroscopic analysis to determine heme redox states during trafficking.

Main Results:

• Demonstrated an early step in heme trafficking involving oxidation to Fe(3+), contrasting with the final attachment requirement for reduced Fe(2+) heme.
• Identified CcmF as a novel cytochrome b containing a previously unrecognized heme group.
• Established that CcmF functions as a quinol:heme oxidoreductase in vitro, actively reducing heme.

Conclusions:

• The Ccm pathway employs conserved and orchestrated mechanisms for heme trafficking, storage, and reduction.
• These mechanisms ensure efficient cytochrome c synthesis even under conditions of limited heme availability (iron limitation).
• The pathway actively reduces heme in oxidizing environments, highlighting sophisticated redox control for essential biological functions.