Protein Structures, Molecular Recognitions, 
and Functions 
Florante A. Quiocho, Ph.D. — Investigator 
Dr. Quiocho is also Professor of Biochemistry and Structural Biology and of Molecular Physiology and 
Biophysics at Baylor College of Medicine. He obtained his Ph.D. degree in biochemistry at Yale University 
and then did postdoctoral research in chemistry at Harvard University. He was a member of the Rice 
University faculty before joining the Baylor faculty. Dr. Quiocho has been a visiting research scientist at 
Oxford University, a research fellow of the European Molecular Biology Organization (EMBO), and a 
Guggenheim fellow. 
LIGAND specificity and the activity of pro- 
teins are derived from their precise three- 
dimensional structures. Using mainly x-ray 
crystallographic techniques, our laboratory is en- 
gaged in atomic-level elucidation of the struc- 
tures and functions of several proteins (including 
enzymes) involved in biologically important pro- 
cesses. To complement our work, w^e also employ 
biochemical and recombinant DNA techniques. 
Adenosine Deaminase 
Adenosine deaminase (ADA) is one of the major 
enzymes in purine metabolism, catalyzing the ir- 
reversible hydrolysis of adenosine or deoxyaden- 
osine to the respective inosine product and am- 
monia. The enzyme is found in nearly all 
mammalian cells and plays a central role in main- 
taining competency of the immune system, 
among several other functions. Lack or deficiency 
of ADA is associated with severe combined immu- 
nodeficiency disease (SCID), a genetically in- 
herited disorder usually fatal within two years of 
birth if left untreated. 
Last year we determined the crystal structure of 
ADA with bound 6-i?-hydroxyl-l,6-dihydropu- 
rine ribonucleoside (HDPR) , a nearly ideal tran- 
sition-state analogue inhibitor. We have since 
elucidated the structures of complexes of the en- 
zyme with the following ligands: 1 -deazaadeno- 
sine, a substrate analogue; 2'-i?-deoxycoformycin, 
a potent transition-state analogue and a chemo- 
therapeutic agent for the treatment of hairy cell 
leukemia; and inosine, the product of the deami- 
nation of adenosine. All these structures have 
provided us with a molecular anatomy of the 
various steps associated with ADA's catalytic 
activity. 
All of the four crystal structures of the deami- 
nase indicated above have been determined and 
refined at pH 4.2, where the enzyme is only 20 
percent active. We have also carried out the re- 
finement of the structure of ADA complexed with 
HDPR at pH 6, where the enzyme is fully active. 
The structures at either pH are essentially the 
same. 
Antibody- Antigen Interactions 
Because monoclonal antibodies against extra- 
cellular polysaccharide antigens exhibit very 
stringent specificity, they have been used in 
blood-grouping and in differentiating bacterial 
serogroups and serotypes. We previously ob- 
tained crystals of the Fab fragment of the antibody 
raised against the surface polysaccharide O-anti- 
gen of Shigella flexneri. In the past year we de- 
termined and refined the structures of the Fab 
and its complexes with a trisaccharide, a-Rha(l- 
3)Q!-Rha(l-3);8-GlcNAc, and a pentasaccharide, 
a-Rha(l-2)a-Rha(l-3)a-Rha(l-3)i3-GlcNAc-(l- 
2)a;-Rha. Both oligosaccharides contain determi- 
nants of the O-antigen serotype of the bacteria. 
This structural work is in line with our interest 
in protein-carbohydrate interactions. Moreover it 
is relevant to clinical problems, as oligosaccha- 
ride epitopes of bacterial and tumor cell surfaces 
are considered to be disease markers and targets 
for therapeutic antibodies. 
Aldose Reductase 
Aldose reductase catalyzes the NADPH-depen- 
dent reduction of a wide variety of carbonyl- 
containing compounds to their corresponding al- 
cohols, with a broad range of catalytic efficien- 
cies. Steroids are the best substrates and sugars 
the least favorable. Although the enzyme is found 
in a variety of cells, its physiological function has 
not been firmly established. A role in reducing 
the hyperglycemia of diabetes mellitus has been 
linked to diabetic complications aff"ecting the 
lens, retina, peripheral nerves, and kidney. Drugs 
designed to control these complications have not 
been clinically successful to date because of lack 
of specificity or inefficacy. 
In collaboration with Kurt Bohren and Kenneth 
Gabbay of the Baylor College of Medicine, we 
have obtained excellent diffracting crystals of re- 
combinant aldose reductase (from human pla- 
centa) with bound NAD PH. We have determined 
and refined the enzyme's three-dimensional, 
1.65-A resolution structure. The enzyme has a 
parallel jS/a-barrel motif, with eight central /?- 
strands connected by eight peripheral a-helices. 
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