BioPHYSICAL chemistry

with the nonheme iron located directly on
the symmetry axis. The bacteriochlorophyll
monomer and bacteriopheophytin that serve
as intermediate electron acceptors are located
on the A branch. The spectroscopic studies
coupled with mutagenesis studies have
established that electron transfer follows
essentially exclusively along the A branch.
The advantage of the two branches is that
the quinones can serve different roles. The
primary quinone serves an intermediate
electron acceptor that shuttles the electrons
to the secondary quinone. The electron stays
on the secondary quinone until the second
electron arrives and the quinone can leave
with two electrons and two protons to be
replaced with an unreduced quinone from
the membrane pool.
The cofactors of the B branch of reac-
tion centers have no direct role in the
electron transfer in purple bacteria but
may participate in the related complex
photosystem I. Photosystem I is a much
larger complex with at least 10 protein
subunits in cyanobacteria and several more
in eukaryotic organisms. These complexes
have approximately 100 chlorophylls that
mostly serve as antenna molecules to
transfer the energy to the protein core. The
general architecture of the protein core is
similar to that of the reaction center,
with two core protein subunits and two
branches of cofactors (Figure 20.10). A
chlorophyll dimer near the luminal side of
the protein serves as the primary electron
donor P700. Four additional chlorophylls
and two quinones are also present. A unique
feature is the presence of three iron–sulfur
clusters that function as electron acceptors. One of the clusters, named
FX, is located along the central symmetry axis, as was found for the
nonheme iron of the reaction center. The other two iron–sulfur clusters,
called FAand FB, are bound to a small protein subunitattached to the
core subunits.
The rate of electron transfer from the excited state, P700*, is very fast
at 2 ps but the observed excited-state lifetime is significantly longer, at

CHAPTER 20 PHOTOSYNTHESIS 429

A QA

Branch

B

Branch

HA

BA BB

P865

HB

Fe QB

Figure 20.9Three-dimensional structure of the

bacterial reaction center.