sites, are widely separated by an eight-turn, solvent- 
exposed, central a helix. 
Second, the laboratory has also determined and 
refined (to 2.4 A) the structure of CaM bound to a 
20-residue peptide analogue of the CaM-binding re- 
gion of a target enzyme, smooth muscle myosin 
light-chain kinase. In the largest conformational 
change ever observed for a protein, the binding of 
CaM to the target peptide causes a five-residue turn 
of the central a helix to unwind and expand into a 
bend, such that the two domains converge and wrap 
around the a-helical peptide. Going from the struc- 
ture of the unbound CaM to that of the CaM-peptide 
complex amounts to an ~100° bend and ~120° 
twist between the two domains. 
The close association of the two domains creates a 
continuous hydrophobic patch that encompasses 
the shallow hydrophobic pocket in each domain. 
This patch interfaces with the hydrophobic side of 
the a-helical peptide. Of the ~ 185 contacts (<4 A) 
formed between CaM and the peptide, 80% are van 
der Waals contacts, 15% hydrogen bonds, and 5% 
salt links. The overwhelming involvement of van 
der Waals forces helps explain why CaM forms com- 
plexes with so many target proteins that show little 
sequence similarity in their CaM-binding domains. 
Adenosine Deaminase: Mimics 
of Pretransition and Transition States 
Adenosine deaminase (ADA; = 40,000), a key 
enzyme in purine metabolism, catalyzes the irrevers- 
ible hydrolysis of adenosine or deoxyadenosine to 
their respective inosine product and ammonia. It 
has a central role in maintaining immune compe- 
tence; lack of the enzyme is associated with severe 
combined immunodeficiency disease (SCID). 
The determination of the x-ray structure of adeno- 
sine deaminase, reported last year, led to the discov- 
eries that it contains a zinc cofactor and that the 
bound ligand is 6/?-hydroxyl-l ,6-dihydropurine ri- 
bonucleoside (HDPR) , a nearly ideal transition-state 
analogue, rather than purine ribonucleoside, which 
cocrystallized with the enzyme. 
The structure of ADA in complex with 1- 
deazaadenosine, an almost perfect substrate or 
ground-state analogue, has recently been deter- 
mined and refined at 2. 4-A resolution. This complex 
is considered to be a mimic of the pretransition 
state. The structure of the complex revealed the 
presence of a zinc-activated water or hydroxide. 
These structures, and the complex with the HDPR 
transition-state analogue, have provided a detailed 
anatomy of the chemical reaction catalyzed by ADA. 
The requirement of a zinc-bound water molecule is 
firmly established. Also, there is little doubt that a 
zinc-activated water (incipient hydroxide) is the 
nucleophile attacking the C6 of adenosine. 
The structure determination of the complex of 
ADA with inosine, the product of the enzyme- 
catalyzed reaction, is almost finished. 
Antibody Recognition of the O- Antigen 
of Shigella flexneri Polysaccharide 
The laboratory has determined and refined at 2.5 
A the structures of the Fab fragment of the antibody, 
alone and in complexes with two oligosaccharide 
antigens: the trisaccharide a-Rha(l-3)a-Rha(l-3)|8- 
GlcNAc and the pentasccharide a-Rha(l-2)a- 
Rha(l-3)a-Rha(l-3)/3-GlcNAc(l-2)a-Rha. The anti- 
genic site is designed to recognize only a 
trisaccharide. This structural work is in line with the 
interest of the laboratory in protein-carbohydrate 
interactions. Moreover, this work is relevant, as oli- 
gosaccharide epitopes of bacterial and tumor cell 
surface are considered to be disease markers and tar- 
gets for therapeutic antibodies. 
Signal Transduction in Periplasmic 
Receptors for Active Transport 
and Chemotaxis 
Ligand-induced conformational change of the 
periplasmic receptors of bacterial cells plays two 
important roles. It enables ligand recognition and 
affinity to be fully achieved and at the same time 
confers the precise geometry for productive 
docking of the periplasmic receptors with the 
membrane-bound components. Determining by x- 
ray crystallography the structures of both the ligand- 
free and ligand-bound structures of the receptor for 
maltodextrins has provided a precise understanding 
of the nature of this conformational change. Binding 
of maltodextrins to the receptor causes a large 
hinge-bending and a small twist motion between the 
two domains, bringing them together and enclosing 
the ligand bound in the cleft between them. It is this 
closed liganded form that is presumably preferen- 
tially recognized by the membrane-bound compo- 
nents, thus triggering the signaling mechanism that 
initiates transport or chemotaxis. 
Dr. Quiocho is also Professor of Biochemistry 
and Structural Biology and of Molecular Physiol- 
ogy and Biophysics at Baylor College of Medicine. 
Articles 
Jacobson, B.L., He, J.J. , Lemon, D.D., and Quiocho, 
F.A. 1992. Interdomain salt bridges modulate 
ligand-induced domain motion of the sulfate re- 
ceptor protein for active transport. / Mol Biol 
223:27-30. 
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