Component of the cytochrome bc complex which donates electrons to the photosynthetic reaction center. Binds 2 heme groups. One heme group is bound covalently by a single cysteine link, the other one non-covalently. The subunits of the cytochrome bc complex are a Rieske Fe-S protein (PetC), cytochrome b6 (PetB), subunit IV (PetD), and a diheme cytochrome c (PetX). Heme 1 (or BH or b566) is high-potential and absorbs at about 566 nm, and heme 2 (or BL or b562) is low-potential and absorbs at about 562 nm. Belongs to the cytochrome b family. PetB subfamily.