  A new computational scheme to analyze electron transfer (ET) pathways in
large biomolecules is presented with applications to ETs in bacterial
photosynthetic reaction center. It consists of a linear combination of fragment
molecular orbitals and an electron tunneling current analysis, which enables an
efficient first-principles analysis of ET pathways in large biomolecules. The
scheme has been applied to the ET from menaquinone to ubiquinone via nonheme
iron complex in bacterial photosynthetic reaction center. It has revealed that
not only the central Fe$^{2+}$ ion but also particular histidine ligands are
involved in the ET pathways in such a way to mitigate perturbations that can be
caused by metal ion substitution and depletion, which elucidates the
experimentally observed insensitivity of the ET rate to these perturbations.
