Protein Structures, Molecular Recognitions, 
and Functions 
Plorante A. Quiocho, Ph.D. — Investigator 
Dr. Quiocho is also Professor of Biochemistry and of Molecular Physiology and Biophysics at Baylor Col- 
lege of Medicine. He obtained his Ph.D. degree in biochemistry at Yale University and then did postdoctoral 
research in chemistry at Harvard University. He joined the Rice University faculty as an assistant professor 
of biochemistry. Leaving Rice as a full professor, he joined the faculty of Baylor. Dr. Quiocho has been a 
visiting research scientist at Oxford University, a research fellow of the European Molecular Biology Or- 
ganization (EMBO), and a Guggenheim fellow. 
FORMATION of complexes between proteins 
and their ligands is the basis of biological 
specificity and activity. Our long-term goal is to 
elucidate the atomic interactions between pro- 
teins and ligands in a variety of biological sys- 
tems. While x-ray crystallography is our primary 
approach, we also do correlative studies em- 
ploying biochemical and recombinant DNA 
techniques. 
Adenosine Deaminase 
Adenosine deaminase (ADA), present in vir- 
tually all mammalian cells, catalyzes the irrevers- 
ible deamination of adenosine to inosine. The 
enzyme has a central role in the normal develop- 
ment of the immune response, and its genetic ab- 
sence leads to severe combined immunodefi- 
ciency disease (SCID). It is also specifically 
involved, or its levels are changed, in a variety of 
diseases, including AIDS (acquired immune defi- 
ciency syndrome) , anemia, and various lympho- 
mas and leukemias. It also appears to modulate 
synaptic transmission. 
We have previously obtained crystals of ADA in 
the presence of purine ribonucleoside, a sub- 
strate analogue. Employing x-ray crystallographic 
techniques, we have recently determined and re- 
fined the three-dimensional structure of ADA at 
2.4 A resolution. The deaminase has a parallel 
(ai8)8 barrel structure. In the course of the struc- 
ture refinement, we discovered that ADA contains 
a zinc cofactor and that the bound ligand is 6-hy- 
droxyl-l,6-dihydropurine ribonucleoside (HDPR), 
a nearly ideal transition-state analogue, instead of 
the substrate analogue. The zinc and HDPR are 
bound and sequestered in a deep pocket located 
at the carboxyl-terminal end of the |8 barrel. 
The potent transition-state analogue (inhibi- 
tion constant estimated at 10"'^ M) is held tightly 
in place by the coordination of the 6-hydroxyl to 
the zinc ion and by the formation of nine hydro- 
gen bonds with seven residues. The enzyme pref- 
erentially recognizes the 6R distereomer of the 
ribonucleoside. 
The discovery of the bound zinc and transition- 
state analogue has enabled us to understand in 
great detail the mode of action of ADA. We have 
also been able to provide a molecular explana- 
tion of the mutations of the enzyme that result in 
SCID. Inherited deficiency of ADA accounts for 
about one-third of SCID cases. The sequences of 
eight point mutations of ADA from five cell lines 
derived from affected patients have been re- 
ported. All but two of these mutations occur 
close to an active-site pocket or to peptide seg- 
ments that line it. Consequently, the deleterious 
effects of the six mutations can be ascribed to 
structural changes that affect the active-site 
geometry. 
Knowledge of the ADA structure paves the way 
for determining the structures of mutants, some 
mimicking those related to deaminase defi- 
ciency, and other complexes with various ana- 
logues and inhibitors. We have also obtained 
crystals of the deaminase in the presence of two 
other analogues (one currently being used for 
chemotherapy in certain leukemias) and seven 
compounds with various therapeutic uses, 
such as sedatives, anxiolytics, analgesics, and 
relaxants. 
The High Specificity of a Phosphate 
Transport System 
Phosphorus in the form of orthophosphate is 
probably one of the most essential nutrients for 
cell life. The transport of phosphate into cells 
and organelles (mitochondria, chloroplasts, etc.) 
is highly specific. Obtaining the extremely well- 
refined 1.7-A structure of the liganded form of 
phosphate-binding protein has revealed for the 
first time the atomic features responsible for the 
exquisite high selectivity for phosphate and the 
exclusion of sulfate. The phosphate, which is to- 
tally dehydrated and buried in the cleft between 
the two domains of the protein, is held in place 
by a total of 1 2 hydrogen bonds formed with pro- 
tein groups, one of which is the carboxylate 
group of an Asp residue. This residue is responsi- 
ble for the recognition of a proton on the 
phosphate. 
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