Monoheme cytochrome which is the immediate electron donor to P840 of the photosynthetic reaction center complex. E(0) is +188 mV for the C-terminal domain (117-206) of cytochrome c (at pH 7.0). Monomer (PubMed:20156447). Component of the photosynthetic reaction center composed of protein subunits PscA, PscC, PscB and PscD. The reaction center interacts with FmoA (which forms the Fenna-Matthews-Olson (FMO) complex). The reaction center/FmoA complex has two PscA subunits, one PscB and one PscD subunit, probably two FmoA complexes and at least one PscC subunit (PubMed:10398586). Binds 1 heme group per subunit.