Expression and characterization of cytochrome c553 from Heliobacterium modesticaldum

Area of Science:

• Biochemistry
• Photosynthesis Research
• Protein Engineering

Background:

• Cytochrome c553 is essential for electron transfer to the heliobacterial reaction center (HbRC) in Heliobacterium modesticaldum.
• The native cytochrome c553 is membrane-anchored, limiting its study in vitro.
• Understanding this electron transfer is key to deciphering bacterial photosynthesis.

Purpose of the Study:

• To create a soluble, recombinant form of cytochrome c553 for detailed biochemical analysis.
• To characterize the biophysical and spectral properties of the recombinant cytochrome c553.
• To investigate the kinetics and mechanism of electron transfer between recombinant cytochrome c553 and the HbRC.

Main Methods:

• Cloning and expression of the petJ gene in Escherichia coli with a hexahistidine tag.
• Spectroscopic analysis (UV-Vis, EPR) to characterize the recombinant cytochrome.
• Electrochemical measurements to determine midpoint potential.
• Kinetic studies of P800 re-reduction by the recombinant cytochrome.

Main Results:

• Successfully produced soluble, recombinant cytochrome c553 with typical c-type cytochrome spectral and EPR characteristics.
• Determined a midpoint potential of +217 ± 10 mV for the recombinant cytochrome.
• Observed accelerated P800 re-reduction kinetics, indicating efficient electron transfer with a second-order rate of 1.53 × 10^4 M⁻¹s⁻¹.
• Calculated a high activation energy (91 kJ mol⁻¹), similar to the native protein, suggesting a comparable interaction mechanism.

Conclusions:

• The recombinant cytochrome c553 is a functional and soluble analog of the native protein.
• The electron transfer kinetics and energy requirements are similar between the recombinant and native forms.
• The findings suggest that the heliobacterial reaction center may not possess a specific electrostatic binding site for cytochrome c553.