Expression and characterization of cytochrome c 553 from Heliobacterium modesticaldum

Cytochrome c₅₅₃ of Heliobacterium modesticaldum is the donor to P₈₀₀ ⁺, the primary electron donor of the heliobacterial reaction center (HbRC). It is a membrane-anchored 14-kDa cytochrome that accomplishes electron transfer from the cytochrome bc complex to the HbRC. The petJ gene encoding cyt c ₅₅₃ was cloned and expressed in Escherichia coli with a hexahistidine tag replacing the lipid attachment site to create a soluble donor that could be made in a preparative scale. The recombinant cytochrome had spectral characteristics typical of a c-type cytochrome, including an asymmetric α-band, and a slightly red-shifted Soret band when reduced. The EPR spectrum of the oxidized protein was characteristic of a low-spin cytochrome. The midpoint potential of the recombinant cytochrome was +217 ± 10 mV. The interaction between soluble recombinant cytochrome c ₅₅₃ and the HbRC was also studied. Re-reduction of photooxidized P₈₀₀ ⁺ was accelerated by addition of reduced cytochrome c ₅₅₃. The kinetics were characteristic of a bimolecular reaction with a second order rate of 1.53 × 10⁴ M^?1 s^?1 at room temperature. The rate manifested a steep temperature dependence, with a calculated activation energy of 91 kJ mol^?1, similar to that of the native protein in Heliobacillus gestii cells. These data demonstrate that the recombinant soluble cytochrome is comparable to the native protein, and likely lacks a discrete electrostatic binding site on the HbRC.