Three-Dimensional Structures of Biological 
Macromolecules 
Johann Deisenhofer, Ph.D. — Investigator 
Dr. Deisenhofer is also Regental Professor of Biochemistry and holds the Virginia and Edward Linthicum 
Distinguished Chair in Biomolecular Science at the University of Texas Southwestern Medical Center at 
Dallas. He was born and educated in Germany. His Ph.D. research in protein crystallography was done 
at the Max- Planck- Institute for Biochemistry, Martinsried, and at the Technical University of Munich. As 
a postdoctoral fellow and as a staff scientist at the Max-Planck-Institute, he continued his structural 
analysis of biological macromolecules by x-ray crystallography. He has received many honors for his 
structure analysis of a photosynthetic reaction center, including the 1988 Nobel Prize in chemistry (with 
Hartmut Michel and Robert Huber). 
WE determine and study the three-dimen- 
sional structures of proteins in order to un- 
derstand their folding, structural stability, and 
function. We are particularly interested in pro- 
tein-pigment complexes catalyzing photochemi- 
cal energy conversion, energy transfer, and elec- 
tron transfer. Some of the most fascinating 
members of this group are membrane-spanning 
and membrane-associated proteins. 
One of the major obstacles on the way to a 
structure analysis by x-ray diffraction is the ne- 
cessity to grow big, well-ordered single crystals 
from pure samples of the macromolecule under 
investigation. Although numerous proteins have 
been crystallized, the crystallization process is 
still not well understood. Successful crystalliza- 
tions can provide guidelines, but every different 
protein presents a completely new problem, and 
there is no guarantee that good crystals will grow. 
This is especially true for proteins that are em- 
bedded in biological membranes. For crystalliza- 
tion they have to be removed from their natural 
environment with detergents. The choice of de- 
tergent is critical; moreover, the most successful 
crystallizations of membrane proteins were per- 
formed in the presence of a detergent mixed with 
a so-called small amphiphile. Thus, when trying 
to crystallize membrane proteins, we have to ma- 
nipulate a complex mixture of aqueous buffer, 
detergents, small amphiphiles, precipitant, and 
our protein. This complexity is one of the reasons 
the three-dimensional analysis of membrane pro- 
teins is progressing so slowly. 
Cytochrome bcj Complexes 
The chemical reactions during photosynthesis 
can be divided into "light" and "dark," with the 
former reactions capturing light energy and stor- 
ing it, mainly in the form of a proton gradient 
across a photosynthetic membrane. This gradient 
is then used for making, e.g., ATP, the universal 
"energy currency" of living cells. 
The photosynthetic light reactions in purple 
bacteria require three types of molecules: photo- 
synthetic reaction centers (RCs), cytochrome be, 
complexes, and cytochrome C2. The first two are 
integral membrane proteins, and the third is a 
small soluble electron-transport protein. 
The RCs perform the first electron transfer step 
of the light reactions. A pair of chlorophyll mole- 
cules absorb a photon and subsequently release 
one of its electrons, which moves within the RC 
through the membrane to a quinone molecule. 
This electron transfer process must happen twice 
in succession so that the quinone receives two 
electrons. The charged quinone picks up two 
protons and dissociates from the RC as a quinol, 
which migrates along the membrane to the cy- 
tochrome be, complex. 
This complex removes the electrons and pro- 
tons from the quinol and transfers them, with two 
additional protons from the cytoplasm, back 
through the membrane. The electrons are shut- 
tled back to the RC, where they can again enter 
the cyclic light-driven transfer process; the pro- 
tons accumulate on the outside of the membrane, 
building up the desired proton gradient. The 
structure analysis at 2.3 A resolution of the RC 
from the purple bacterium Rhodopseudomonas 
viridis has contributed significantly to our un- 
derstanding of the first half of this cyclic process. 
Determination of the three-dimensional structure 
of a cytochrome bcj complex would similarly 
elucidate the second half. 
Complexes of the bcj type play a crucial role 
not only in photosynthesis but also in cell respira- 
tion. For example, the mitochondria of all higher 
organisms have such complexes in their inner 
membrane. The photosynthetic purple bacteria 
have the simplest bcj complexes, consisting of 
only three different protein subunits, with three 
heme groups and an iron-sulfur cluster as cofac- 
tors. They occur in the bacterial inner membrane 
at a concentration significantly lower than that of 
RCs, and are therefore more difficult to isolate. 
The research group of David Knaff at Texas Tech- 
nical University, Lubbock, recently succeeded in 
isolating the be, complex from Rhodospirillum 
109 
