Evolutionary models of the photosynthetic reaction centre in the purple bacteria propose
that an an ancestral protein (a putative homodimer, X2 ) may have had two
parallel electron transfer pathways, each of them capable of reducing a quinone molecule.
Over time, the two identical sequences are thought to have diverged and created a
heterodimeric structure, wherein the two quinone binding sites became more specialized and
the two quinones (QA and QB) accepted electrons in series.
The recent discovery of apparently homodimeric reaction-centre complexes in the non-purple
bacteria Heliobacillus mobilis and Chlorobium limicola, combined with
the fact that the two quinones in some of the purple bacteria are chemically identical but
nevertheless functionally different, raises the possibility that evolutionary
selection for more efficient electron transfer led to a sequence divergence within the
purple bacterial reaction centre; eventually forming the heterodimeric L- and M-subunit
core complex.