(20.4)
Electron transfer proceeds rapidly from the excited state to a bacterio-
chlorophyll monomer in 3 ps, and a bacteriopheophytin in 1 ps, the
primary quinone with slower transfer to the two quinones (Figure 20.8).
Since the rapid transfer from the bacteriochlorophyll monomer to bacterio-
pheophytin is faster than the transfer to the bacteriochlorophyll, the elec-
trons are immediately depleted from the bacteriochlorophyll monomer,
and a reduced state of the bacteriochlorophyll monomer is present in only
very low proportions and is very difficult to observed experimentally. Since
each step is energetically favorable with rapid forward electron-transfer
rates, the yield of the transfer, termed the quantum yield, is nearly one,
meaning that for every excitation of P865 an electron is transferred to the
secondary quinone. The use of multiple electron acceptors does result in
the loss of most of the initial excitation energy but the desired outcome
is simply the transfer of an electron and subsequent proton transfer that
can be used to make energy-rich compounds (Chapter 8).
A critical step in our understanding of the properties of reaction centers
was the determination of their structures in the late 1980s. While pro-
tein crystallography was being used to solve the structures of thousands
of water-soluble proteins (Chapter 15), the structures of integral mem-
brane proteins proved to be elusive due to the difficulties in biochemically
preparing and crystallizing these proteins in the presence of detergents.
The first structure of an integral membrane protein was the reaction
center from Rhodopseudomonas 9 iridis(since renamed Blastochloris 9 iridis) by
Johann Deisenhofer, Hartmut Michel, and Robert Huber and coworkers
(Deisenhofer et al. 1985). This structure was a landmark (awarded the
Nobel Prize in Chemistry in 1988) as it demonstrated the feasibility of
obtaining crystals of membrane proteins that were suitable for X-ray
diffraction studies and has been followed, albeit somewhat slowly, by the
elucidation of a number of other structures including the reaction center
from Rhodobacter sphaeroides as well as unrelated proteins, including
bacteriorhodopsin and ion channels (Chapters 17 and 18).
The reaction-center structure has a number of features that are con-
served among all bacteria. Although the total number of protein subunits
varies among the different bacterial species, there is always a central core
pairing of two subunits with five transmembrane helices each (Figure 20.9).
These two subunits are structurally related to each other by a pseudo
2-fold symmetry axis that runs down the center of the protein. The same
symmetry axis also divides the cofactors into two branches that span
most of the membrane from the periplasmic to the cytoplasmic side. The
bacteriochlorophyll dimer, P870, is at one end, near the periplasmic side of
the protein, and the primary and secondary quinones are at the other end
Eh
hc (. )(.
===
××−
ν
λ
6 62 10^24 Js 2 99 10^8 mss
m
JeV
−
−
−
×
=× =
1
7
9
865 10
23 10 14
)
.
..
428 PART 3 UNDERSTANDING BIOLOGICAL SYSTEMS USING PHYSICAL CHEMISTRY
