Covalent cofactor attachment to proteins: cytochrome c biogenesis

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Science

Background:

• Heme (Fe-protoporphyrin IX) is a vital cofactor in numerous proteins, mediating functions like electron transfer and catalysis.
• While most heme cofactors bind non-covalently, c-type cytochromes feature covalent attachment, essential for their function.
• Heme attachment is a complex post-translational modification occurring in the bacterial periplasm, involving multiple proteins.

Purpose of the Study:

• To investigate the unexpected covalent heme binding by the heme chaperone CcmE.
• To elucidate the role of CcmE in the post-translational modification and maturation of c-type cytochromes.
• To understand the mechanism of heme transfer from CcmE to apocytochromes.

Main Methods:

• In vitro studies to analyze heme binding to CcmE.
• In vivo experiments in Escherichia coli to study cytochrome c biogenesis.
• Biochemical assays to characterize the covalent bond formation between heme and CcmE.

Main Results:

• CcmE, a heme chaperone, was found to bind heme covalently via a single bond between a heme vinyl group and a histidine residue.
• This covalent binding occurs before CcmE transfers heme to apocytochromes.
• The study provides insights into the essential role of CcmE in the complex process of cytochrome c maturation.

Conclusions:

• CcmE plays a dual role in cytochrome c biogenesis, acting as both a chaperone and a temporary covalent heme binder.
• The covalent attachment of heme to CcmE is a critical step in ensuring efficient heme delivery for c-type cytochrome formation.
• Understanding this mechanism offers valuable insights into protein post-translational modification and cofactor insertion pathways.