Solution structure and characterization of the heme chaperone CcmE

Area of Science:

• Biochemistry
• Molecular Biology
• Structural Biology

Background:

• C-type cytochromes are essential proteins requiring covalent heme attachment.
• Heme delivery in Gram-negative bacteria involves multiple proteins, including the CcmE protein.
• Understanding CcmE's role is key to elucidating the complex heme attachment pathway.

Purpose of the Study:

• To determine the solution structure of the soluble domain of apo-CcmE from Shewanella putrefaciens.
• To investigate potential heme-binding sites and interactions of CcmE.
• To gain insights into the heme delivery mechanism in Gram-negative bacteria.

Main Methods:

• Nuclear Magnetic Resonance (NMR) spectroscopy on 13C,15N-labeled apo-CcmE.
• Analysis of protein structure and identification of conserved residues.
• In vitro electronic spectroscopy and NMR to detect CcmE-heme interactions.

Main Results:

• The solution structure of apo-CcmE reveals a compact core with beta structure and unstructured termini, similar to the OB-fold.
• Conserved aromatic residues near the putative heme-binding residue His131 suggest a potential role in heme interaction.
• No direct in vitro interaction was detected between CcmE and heme or holocytochrome c.

Conclusions:

• CcmE adopts an OB-fold structure, indicating a conserved protein fold for heme-related functions.
• The identified structural features suggest a role for CcmE in the heme transfer pathway, possibly through interaction with other proteins.
• The heme transfer process likely involves a multi-protein complex and tight enzymatic control, rather than direct CcmE-heme binding.